Poking around internals of a retired Canon Pixma MX340 multi-function inkjet, I decided its control panel circuit board was within my skill level to decipher. After following a set of five wires from control board LCD screen to the biggest chip on the board, I think the next effort is to trace out the relationship between that chip and all components on this circuit board. I started doing this by eye but the traces are thin, the contrast is low, and my eyesight isn’t what it used to be. If I will be tackling many more similar projects, I might invest in a magnifier or microscope designed for circuit board work. But for today, I’ll use what I already have on hand: taking pictures with my camera + macro lens then stitch them together.

In order to keep the circuit board traces as well as surface component markings in focus, I need to narrow the aperture for greater depth of field. I also wanted to use my polarizing filter to eliminate glare. Both decisions reduced the amount of light reaching my camera sensor. To compensate, I can increase sensitivity, but that adds noise and I don’t want a noisy picture when looking at fine copper traces. The alternate solution is to increase shutter time, which meant setting up my tripod to keep my camera steady during long duration shots. I ended up using an 8 second shutter time in order to stay at low noise ISO 100 sensitivity.

For each picture, I wanted to fill my imaging area with circuit board. The aspect ratio of the circuit board meant I could cover the entire width with four pictures. However, that means a seam somewhere in the middle where all the important bits (main board connector, LCD connector) are located so I decided to take five pictures. One center shot with all the important bits together in one shot, then two pictures on either side.

Once I had the pictures, I brought them into my photo editor putting each image into its own layer. I brought the center image to the foreground, the edge most images to the back, and intermediate images between them. I then scaled image in each layer so all the copper circuit traces lined up at the edges. Looks like I didn’t manage to maintain distance between camera and circuit board, because this resulted in a slightly trapezoidal shape.

After the traces were all lined up, I adjusted image color levels to bring out contrast of copper circuit traces. Doing this meant blowing out bright parts and dark details got crushed, but I decided that’s not important for this project. I also decided not to worry about the fact seams are very noticeable, because color balance changed between pictures.

I didn’t use panorama photo stitching software here because they have different goals. They fix color balance, warp perspective, and blend edges to create a pretty picture. For this project, I’m not looking for pretty, I’m looking for accurate. When I’m following a copper trace across the board, I want to be confident I’m following real untouched data and not misled by something that had been modified to look pretty.

Here is a low-resolution version of the result. If anyone else wants a look, the full resolution version (and original set of five images) are up on my GitHub as they are too large to be hosted here. Obviously I’ll use the full resolution version for further circuit board exploration.

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This teardown ran far longer than I originally thought it would. Click here for the starting point.

I’m examining the control panel circuit board from a Canon Pixma MX340 multi-function inkjet to see what I can learn. After identifying all but five LCD screen connector pins, I’m obviously inclined to follow those five pins. Again, I took the best picture I could and edited the photo to highlight traces and make them easier to follow. The good news is that all five wires traveled together to a conspicuous feature on this board: a row of five resistors marked 331 for 330 ohms.

I noticed this row of resistors in my initial cursory examination and had been curious to their purpose, now they’ve been moved to the top of the queue. One bit of oddity that caught my attention was the fact their marking numbers didn’t all align the same way: three were installed one direction, and two the other. I’m pretty sure they would have been all in the same orientation from the product reel, yet the pick-and-place machine decided on different orientations. While the orientation doesn’t matter for resistor functionality, there must have been a reason to not do the same thing five times and I wonder what it was.

Between each resistor and the LCD connector, each wire also has a capacitor to ground. (C123-C127 inclusive.) C124 is also accompanied by R136, which looks like a pull-down resistor. There were no other features I could see, aside from test points labeled with the CP prefix. “Check Point”, maybe? On the other side of the resistor, all five wires were routed under a few zero ohm resistors labeled with the JP prefix (probably meaning “jumper”) and went under the biggest (only?) chip on this circuit board.

The chip is marked NEC K13988 1022MM1H. A search for this designation found several electronic component distributors offering to sell some to me. I checked two of those links: neither actually had the chip in stock, and both used a placeholder image instead of a picture of the actual chip. More importantly, while both pages had a “Datasheet” link, they didn’t link to a PDF. Instead, they were links to different datasheet dump sites, both of which helpfully offer “Here are some datasheets for components starting with K139…” none of which was K13988 and none were made by NEC.

Without a datasheet, I’ll have to infer this chip’s functionality. This is a 30-pin part mounted on a single-layer board serving a known purpose, so this should be a tractable problem. In addition to normal power and ground wires, there are the five pins for communication with the LCD. A few pins would be routed to the cable connector to communicate with the main board. Some of the pins are likely used as outputs to indicator LEDs, but there’s a small chance the main board directly handle all LEDs. (There are only a few of them.) The rest would be used to read button status, likely with a row/column matrix of wires.

To find these answers, I’ll need to follow copper traces over much longer distances, possibly across the entire width of this control panel circuit board. I couldn’t do this by eye because the contrast is too poor for me to follow narrow traces. I need the photo editing contrast enhancement. But if I pull my camera back far enough to encompass the entire board, those narrow traces become too blurry. I’ll have to take multiple pictures and stitch them together.

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This teardown ran far longer than I originally thought it would. Click here for the starting point.

I have the front control panel of a Canon Pixma MX340 multi-function inkjet and I will try to understand of how it worked. I’m starting with this 12-pin connector for the ribbon cable leading to the main board, because all input and output will necessarily go through this connector. Luckily this circuit board only has a single layer of copper traces. Making it much easier to glean information just by looking at the connector and its immediate surroundings.

The first piece of good news is the fact Canon engineers had drawn an arrow with a number 1 in white silkscreen on the board, sparing me the agony of trying to guess which pin is #1. Using that numbering system, the first circuit observation is that pins 4 and 8 are visibly connected together immediately adjacent to the connector. Pins 5, 6, 7 must go under the connector out the other side, something I’ll track down later.

Having multiple pins connected together with large sections of copper (instead of thin traces) usually indicate multiple pins connected to a common ground. Probing with my multi-meter I confirmed those two pins showed only about a single ohm of resistance to the metal chassis. But the “ground wires are big” is only a rule of thumb, because I probed the remaining pins and found pin 10 — a thin trace typical of data signals — is also connected to ground. I assume it is part of a circuit with low power draw so it doesn’t need a fat low resistance ground wire.

The next circuit observation are the capacitors right next to the connector, sitting between the just-identified ground wires and three other wires. This is typical pattern for decoupling capacitors and imply those other wires — pins 3, 9, and 12 — are power supply wires. Probing with my multi-meter while the printer is turned on, pins 3 and 9 read 3.3V and 5.5V for pin 12. I then repeated the probe with the printer in standby (“turned off”) state, and saw pin 9 still had 3.3V while the other two has dropped to zero. Implying certain parts of the board stay active even in standby state while the rest of the board is powered down.

Pin 3 has a resistor adjacent to its decoupling capacitor. Marked with “471” indicating 470 Ohms. That is a popular value to use as LED current-limiting resistor when connected to 5V DC. This wire gets 3.3V instead of 5V but that should still result in more than enough current to illuminate a modern LED. When I trace through the circuit later, I will find out if this is a LED illuminated directly by the main board or if the familiar 470 Ohm value is only a coincidence.

With this basic exploration, I already have tentative information on half of the pins. [UPDATE: This later post has filled in the rest of the table.] This is a very encouraging start and motivated me to go look at the other connector on this board, for its LCD screen.

Pin Number	Tentative Guess	Printer On	Printer Standby (“Off”)
1	?
2	?
3	Maybe LED power?	3.3V DC	0V DC
4	Ground
5	?
6	?
7	?
8	Ground
9	Always-on power	3.3V DC	3.3V DC
10	Ground
11	?
12	Power	5.5V DC	0V DC

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This teardown ran far longer than I originally thought it would. Click here for the starting point.

After looking at the paper feed roller shaft position encoders of this Canon Pixma MX340 multi-function inkjet, I don’t have too many other circuit boards to probe. But the remaining boards are significantly larger like the user control panel. I took a quick look at this earlier during mechanical disassembly, now I want to look at it electrically.

Thankfully, it is large mostly because it spaces things out for user-friendliness.

The circuit board only has a single layer of copper traces. Most of the components on the non-copper side are push buttons. There are a few LEDs, and a few jumper wires to help route signals over backside copper traces.

Electrically speaking, this board needs to read the state of all the push buttons and report them back to the main board. It is also home to a bitmap LCD screen, displaying information on command from the main board. Plus the on/off state of those LEDs. All main board communication occurs over a small ribbon cable with 12 conductors.

Functionally, this serves a similar purpose to the front face plate of a car audio system. I’ve looked at two units earlier, one from a Toyota and another from a Honda. Both of them used chips from Sanyo designed to perform both tasks (button matrix scanning and driving the LCD) on behalf of a main board via an electrical connector with roughly a dozen or two of conductors. Like those face plates, only a single significant looking chip resides on this printer control board. Perhaps it serves a similar purpose.

Before I dive in to that chip, though, I thought I would first explore around it. This is another lesson learned from those car faceplate teardowns: it is common for a few electrical conductors to be reserved for direct main board control bypassing that chip. If such direct connection wires are among the 12 conductors of this cable, knowing where they are would make later analysis easier. I’ll look at that connector first.

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This teardown ran far longer than I originally thought it would. Click here for the starting point.

I’m poking around inside a Canon Pixma MX340 multi-function inkjet and was surprised when my oscilloscope showed it rapidly cycled between forward and backward power to its two DC motors. I’m sure it is related to the fact they’re part of a closed-loop control system: the printer has high-resolution encoders telling the mainboard how much each axis has moved, so the control system can adjust the forward/back power ratio.

I can see the encoder strip used for the print carriage, but the encoder itself is not easily accessible. The data wires would be among print cartridge control wires somewhere in the trio of cables between print carriage and main board.

In contrast, the paper feed encoder is a standalone unit with only four wires already conveniently labeled 1 through 4 from left to right in this picture.

It looks like the encoder has some sort of physical pad to keep the encoder disc properly aligned in the channel. It has worn a groove into the plastic encoder disc and will eventually wear through and destroy the disc. But it lasted long enough for me to retire the printer first, so “It’s not just good, it’s good enough!

Since physical position and alignment is clearly important here, I didn’t want to remove the module while I still want to probe the system in a functioning state. It looks like a pretty simple one-layer circuit board. Aside from the cable with four wires (1 through 4 from right to left, now that we’re looking on the back side) and the encoder sensor itself, there are two surface mount components. One looks like a capacitor, and other looks like a small resistor marked with either 820 (82 Ohms) or 82D (629K as per Digi-Key tool).

It was a tight fit to get my soldering iron into that space, but at least the four wires were generously spaced apart so I could solder wires for attachment to oscilloscope probes.

This is again from the power-up sequence, when this motor is running backwards. This matches the wave form I expected to see from an incremental encoder. Unlike the photo interrupter sensors that ran at 5.5V DC, this encoder runs at 3.3V DC. The shape of the curve, tapering off as rose up to 3.3V DC and a sharp drop with no taper when it goes to ground, tells me there’s a pull-up resistor on each line and the sensor itself pulls to ground.

For comparison, here’s the plot from later in the power-up sequence, when the motor is running forward but at a slower speed. The red/blue ordering is reversed, and the pulses are wider apart. I’m curious about the slight voltage drop I see in one color when the other signal is pulled to ground. Is this caused by interaction between the two pull-up resistors or something else?

The pinout for this sensor can be determined from these oscilloscope plots. As per numbers printed on the board: pin 1 is 3.3V DC, pin 4 is ground, and A/B phase signals live between them.

That was a nice simple circuit board to decipher, the next project is a little more complex.

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This teardown ran far longer than I originally thought it would. Click here for the starting point.

When I started taking apart my old Canon Pixma MX340 multi-function inkjet, I didn’t expect to make heads or tails of the sensor bar. So I was very proud of myself for getting far enough with a reasonable hypothesis on how to work with it, and jotted down some potential approaches potentially scaled down to my skill level. They are potential projects for later. Right now I want to move on with observing other parts of the machine. Today’s subjects: the motors used to move the print carriage horizontally (“X-axis”, in my mind) and a separate but similar motor responsible for feeding sheets of paper (“Y-axis”).

There are two wires on each motor, characteristic of inexpensive commodity brushed DC motors. I was pretty confident I knew what I’ll see on my oscilloscope screen: 0V ground on one wire and 24V DC from the power supply on the other wire in a PWM pattern depending on desired speed. It turns out I was both right and wrong.

There’s definitely PWM going on, but in both directions! Here’s the oscilloscope plot for the print carriage motor upon startup, moving the print carriage out of its parking spot across the width of the printer. For slow motion I expected a low PWM cycle on one pin and the other pin held kept at ground, but the printer actually alternates between turning the motor forward and reverse with no de-energized states between them. (Not counting the very brief MOSFET ramp-down during the switch.) I guess this is similar to how when we want to make precise motions with our limbs, we keep some tension on muscles on both sides of the joint. The motor speed is thus dictated by the ratio of time spent in each direction.

Continuing the startup sequence: After the print head moved across the entire width of the printer, it moves back towards its parking position at the same speed. Changing the edge trigger channel made it easy to see the same ratio was used except with the polarity reversed.

The paper feed motor is driven in a similar manner. This was also taken during the printer startup sequence, at the moment the motor is spinning in a direction backwards from normal paper feed direction during printing. I don’t know what this motion accomplishes yet, I hope to find out later in this teardown.

Shortly after that action, the motor turned in the normal paper feed direction for a short distance. (Not far enough to pop open the paper output tray door.) It also turns at a slightly slower speed. The most obvious difference between these two plots are the different in voltage for opposite direction, and a less obvious difference is the power energized ratio between blue and green wires. It is slightly closer to 50/50 when the motor is turning slower.

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Driving the motor in this manner is not very energy-inefficient, as most of the power is spent fighting power expended a few microseconds earlier. Only a fraction actually goes into motion, all the rest are turned into heat. Indeed these motors grew warm to the touch after some use. There’s probably an explanation for why this is the preferred approach, somewhere in an electrical engineering motor control theory textbook. Maybe this is the only way to reliably accomplish precise movements with inexpensive DC motors? I should experiment with this concept the next time I revisit my micro Sawppy project, perhaps it would allow the little thing crawl at a more rover-like speed. However, I think it’s likely this motor control scheme only works because it has encoder feedback for a closed-loop control system.

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This teardown ran far longer than I originally thought it would. Click here for the starting point.

I have a contact image sensor (CIS) from the scanner module of a Canon MX340 multi-function inkjet. Connecting its wires to my oscilloscope, I found a wire that I believe to convey image data.

The yellow line is probably the clock signal, and the green line fluctuates in rough correlation with sensor light levels. When the sensor bar is facing down, the signal hovers near lower voltage levels. When it is facing up towards the light, more of the signal line shows at a higher voltage. I think this is the “analog” output format for Canon contact image sensors!

If the yellow line is indeed the clock signal for each sensor pixel, and the voltage level is an analog representation of the brightness of that pixel, there is still one missing piece of data: how would we know when a scan line had completed and another begins? There must be a signal on one of the other wires, and I know there’s something that occasionally flashes past this screen. Time to chase that down.

Playing around with the oscilloscope trigger, I eventually pinned the flashing culprit down to the red wire. Its voltage is raised high for a single clock cycle at a rate of 425.543Hz. Dividing the frequency of the yellow clock signal (2.37538MHz) by 425.543 yields 5582 yellow clock cycles per red cycle. The scanner bed is 8.5 inches wide, so 5582/8.5 ~= 656. This would make sense for a 600dpi image scanner bar that is slightly wider than the scan bed.

This oscilloscope plot also shows the red signal wire voltage looks wavy, but not when compared against the blue signal wire. I think these two voltage values are intended to be evaluated together: the “new scan line” signal should be evaluated as red voltage minus blue voltage.

Keeping the oscilloscope trigger on the red wire, I changed horizontal scale to see a longer time duration. Lengthening the time scale 5000 times (from 100ns/grid to 500us/grid) showed scan lines represented by analog voltage. This particular plot shows three scan lines (one and a half scan line on either side of the trigger in the middle) and there’s a dip in the middle of each scan line. This is with the sensor bar looking up at the room ceiling so my finger is casting a shadow in the middle. I got excited that I could see some correlation between oscilloscope plot and light level, getting very close to understanding this sensor. But there’s a big problem: The shadow part looks pretty consistent, but the bright parts jump around wildly. Why is the data so noisy?

I lengthened the time scale again, by ten times to 5ms/grid so I could see about 30 scan lines at once. There’s definitely a pattern here, what is it?

Perhaps the fluctuation is normal? Since I just learned red wire voltage level appears to be relative to blue wire voltage level, maybe the image signal analog voltage is relative to another baseline. I moved the blue wire over to the pin adjacent to the signal wire to see if they fluctuate together in sync.

The good news is the baseline voltage hypothesis appears confirmed, with dark pixels dropping to the level of the newly relocated blue wire. These three scan lines show me holding my hand over the sensor bar, with several fingers casting shadows. The bad news is the non-shadowed areas are still fluctuating wildly.

I went back to staring at the earlier oscilloscope plot capture, trying to decipher a pattern to its repetition. The breakthrough came when I counted the period of repetition: at 5ms/grid, this is repeating every three grids plus a little more. So period of a little over 15ms, or a frequency somewhere under 66Hz.

Ah-Ha!

That was literally a light bulb moment: the image sensor bar is not only picking up the overhead lights, it’s picking up the 60Hz cycle of household AC power behind those lights. To test this hypothesis, I turned off my room’s overhead lights and switched to a battery-powered LED flashlight. The fluctuations disappeared, hypothesis confirmed! Now I feel pretty good about my understanding of this image sensor.

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This teardown ran far longer than I originally thought it would. Click here for the starting point.

I want to see how much I can understand of the scanner image sensor bar I pulled out of a Canon MX340 multi-function inkjet. A little research side quest helped me feel better prepared for the task of figuring out what happens over the twelve wires connecting the sensor bar to the printer main board.

Looking at the copper traces on the circuit board, I can see some of them are directly wired to the pins for LED illumination. Verifying continuity between those connector pins and LED pins identified the left-most five of twelve wires. Probing for continuity with chassis ground, I found one more ground wire. Measuring the remaining wires while the system is powered up, I found one pin that held steady at 3.3V. A good power source candidate for image sensor logic.

That left five unknowns whose voltages fluctuate. Too quickly for my volt meter to catch what’s happening, hinting at data signals. My oscilloscope has four channels so I need to choose four to solder wires to and leave one for later.

This connector also has 1.0mm pitch and this time I managed to solder wires without creating bridges. Luck or improving skill I don’t know yet, either way I expect my MX340 teardown project will give me many more practice opportunities.

I turned on the MX340 and saw this on my oscilloscope screen. The yellow wire is attached to a pretty steady wave at 2.375MHz, possibly a clock signal. Its adjacent red and blue wires seem to track pretty closely to each other, the green wire looks much the same but at roughly 1V higher.

Unfortunately, that’s pretty much everything I got. I see this pattern as soon as the printer powers up, and it looks much the same during the homing sequence and afterwards. I waved my hand and shined a light at the sensor bar and saw no significant change in these signals. If I were looking at LVDS, according to Wikipedia I should see a 350mV change, but I don’t.

That said, there is something that occasionally flashes past this screen, but I didn’t know how to isolate it yet. Whatever it was, it occurred too infrequently to be image data. (Later I would determine it is a line sync signal on the red wire.)

Could the magical data wire be the fifth and final wire I had yet to examine? On paper it was a 20% chance I missed the most important wire, but Murphy’s Law raises those odds. Besides, I’m so close and it’d be silly to give up now. I turned off the printer and oscilloscope and turned on the soldering iron. The green wire was moved one pin over, and I tried again.

Jackpot! I have a candidate image sensor data signal.

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This teardown ran far longer than I originally thought it would. Click here for the starting point.

I’m looking over the scanner module of a Canon Pixma MX340 multi-function inkjet. Its stepper motor got the oscilloscope treatment first, now onward to the image sensor bar.

When I took apart the scanner module, I noticed there were a few through-hole pins at the end of the image sensor bar and wondered what components they might be attached to. As I explored the scanner further to determine its startup behavior, I stared at the bar and saw the answer.

This end of the image sensor bar housed illumination light sources. This close-up shot showed the red, green, and blue LEDs working together to light the bar white.

From that I would expect three pins, one for each color, and a fourth pin as either a common anode or a common cathode. In actuality, I see five pins in this row. What did I miss? Starting with the basics, I probed the pins for continuity with chassis ground and found one, topmost in this picture.

I soldered wires to the four unknowns so I can see what they look like during printer startup, when the light bar is illuminated white looking for the homing marker.

Looks like the green wire is supplying 5.5V DC, possibly the same voltage plane powering the ADF paper sensors. The remaining three wires show a pattern that switches between two voltage levels and repeats less than 2.5 milliseconds. That works out to be less than 400Hz, too slow to be some sort of LED PWM scheme. Something else is going on. But first, I want to figure out if these wires are the positive anode (LED illuminates at higher voltage) or negative anode (LED illuminates at lower voltage).

I pulled out my LED tester and determined it is the latter: the LED illuminates from the 5.5V rail to the lower voltage shown on oscilloscope. I guess the ground pin is present merely for chassis ground and not actively used in this circuit. I also determined which wire corresponded to which colors, which is unfortunately now very confusing because I soldered these wires without knowing beforehand. Here is the mapping table:

Oscilloscope Wire Color	LED Color
Yellow	Red
Red	Green
Blue	Blue

At least I got blue right.

Using this chart and looking back at the oscilloscope plot, I see each of the three colors are pulled low in non-overlapping time windows. The blue line (for blue LED) has the shortest time. Then the yellow line (red LED) with a slightly longer duration, and finally the red line (for green LED) for almost as long as the other two combined.

Why would the scanner light up one LED at a time and cycle through them? My hypothesis is this allows a monochrome image sensor to scan in color. Light up blue, scan. Light up red, scan again. Light up green, scan a third time. Assemble them together for full RGB color information. If so, the difference in LED illumination duration may be proportional to the image sensor’s sensitivity to those colors.

I am very happy with this discovery. It is already much further into understanding the scanner sensor bar than I had originally expected to get. Encouraged by this success, I want to see how much I can decipher of the rest of this image sensor. Which meant a quick side quest for contact image sensor research.

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This teardown ran far longer than I originally thought it would. Click here for the starting point.

I’m exploring the inner workings of a Canon Pixma MX340 multi-function inkjet as I take it apart. After coming up with a hypothesis on how the scanner bar finds its way to its home position, I had some fun confirming that hypothesis and confusing that electronic brain. Next I wanted to look at the motor that drives scan head movement.

Canon engineers designed the scanner bar to be easily disassembled into its two major components. A slight sideways push was all it took to separate the motor gearbox motion-control assembly from the lighting and imaging sensor bar.

Flipping the motion control assembly on its side, I can take a close look at the connector to a long cable leading back to the printer main board. There are five conductors in this wire, one of which went immediately to a screw fastening it to the stamped sheet-metal structure. This would be the chassis ground. The other four pins likely led to a pair of coils inside a stepper motor.

Probing those pins with a multi-meter, I found 15 Ohms of resistance between pairs of pins that I’ve labeled as +/- endpoints of two coils A and B.

This connector has 1mm spacing between pins (“pitch”) which is less than half of the 0.1″ (~2.54mm) pitch I’m familiar with. I’ve found this is roughly the limit of my current soldering skill, requiring two attempts before I could solder wires to them without accidental solder bridges. These wires were connected to my oscilloscope, measuring their voltage levels as the scanner went through its homing sequence.

This stepper motor is also run at 24V DC as delivered by the power supply, same as the ADF motor. The coils in this scanner motor measured 15 Ohms which calculates out to 1.6 Amps of current through each coil. Less than half of the power delivered to the ADF motor. And similar to the ADF motor, the scanner position motor does not need to hold position which means it avoids the high stress duty of having power continuously on a coil heating it up.

There’s a visibly repeating pattern in this oscilloscope snapshot, but it’s a bit difficult to make out each of the four wires when they’re all together on the same plot. Here they are individually:

Here are two ends of “A” coil, most of the time energized to one direction or another but there’s a brief period where both ends are at ground leaving the coil de-energized.

A similar pattern is visible in the B coil, offset by half a pattern period. I’m not sure why the motor cycle has this bit of de-energized coil time in between other energized patterns. Except for that small window, this pattern would be match what I expected from stepper motor operating theory. For a stepper motor spinning at a constant speed, I would expect something like this four-state cycle with equal duration per state:

A+	B+
A+	B-
A-	B-
A-	B+

But that’s not what I see in this oscilloscope trace. This is what I picked out, a slightly different pattern and some states are held for different duration.

A+	B+
A+	B-
A-	B-
A-	(off)
(off)	B+

This difference from textbook description serves a purpose I couldn’t determine. Perhaps this helps ensure the coils don’t overheat from constant power? Maybe this is to help reduce resonance problems? Perhaps this is synchronized to the imaging sensor for some data purpose? It’ll stay a mystery for now, as I proceed to take a closer look at said imaging sensor.

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This teardown ran far longer than I originally thought it would. Click here for the starting point.

I’m playing with a partially disassembled scanner module from a Canon Pixma MX340 multi-function inkjet. I’ve figured out how the scan head finds its way to its home position, and now I want to experiment with the marker used by the process. Back when I disassembled the scanner module, I had expected a mechanical homing switch but didn’t find one. I suspect the homing sequence uses a distinct pattern I found on the bottom of the bar separating the two windows.

Is this the homing marker? I will now confirm my suspicion.

The first experiment was to deliberately assemble the scanner module incorrectly. Instead of putting the lid (with glass windows) in its correct position, I placed it roughly 15cm offset in the positive direction. This was wider than the ADF window so the first test was a failure: the scan head didn’t move far enough before reversing direction. On the second test, I manually moved the scan head a bit further up, so the bar was in range of the initial positive scan. This time, the scan head homed to its new position. It also homed to the new position if I manually placed the scan head further in the positive direction: it would scan for a while, reverse direction, and find the relocated marker. Homing marker confirmed!

I then tried to see if I can spoof that marker. I measured the special pattern to start about 62mm from the document left edge. It is made of three black rectangles 20mm long and 3mm wide, separated by 20mm of white between them.

I drew up that pattern and printed it on paper. It took several attempts to actually match the measured dimensions because apparently “print at 100%” is a lie. I didn’t want to waste paper so my multiple attempts were printed on different ends and sides of the same sheet of paper.

Again I moved the scan head out of its homing position, and placed my fake homing marker paper down on the flatbed scan area. I turned on the printer and, when the scan head saw my fake marker, it acted as if it saw the real marker.

After this success, I realized that I had accidentally used one of the slightly smaller size fake markers. Retrying a few times with my different printing attempts, I found my 18mm and 19mm long markers worked just as well as the 20mm long markers. So the homing program has some tolerance for marker size.

If it is tolerant of less-than-perfect homing markers, perhaps I can spoof it with the pattern taped on a ruler? This worked when I put it on the flatbed glass like earlier experiments, but it failed if I skipped the flatbed glass and held the ruler on top of the sensor in midair. I can think of a few possible explanations:

1. Holding the ruler above the sensor by hand had improper alignment. (It’s crooked.)
2. Holding the ruler above the sensor by hand could not maintain the correct distance for proper image focus.
3. The scanner sensor bar requires a sheet of glass between it and the scan subject for proper optical behavior.

And naturally, the explanation might be none of the above. I don’t feel the need to dig deeper right now because I’ve already had my fun and ready to move on to look at the motor moving this scann head.

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This teardown ran far longer than I originally thought it would. Click here for the starting point.

I’m carefully taking apart my old Canon Pixma MX340 multi-function inkjet, exploring its workings as I go. I’ve recorded the electrical parameters of the automatic document feeder (ADF) motor during printer startup, completing this phase of ADF exploration. My next focus is the scanner module, starting with the scan head homing sequence.

During power-up, there is a programmed sequence to ensure the scanner head is at its home position, sitting under the bar separating the small glass window (used for ADF scanning) and the large glass window for flatbed scanning. After opening up the scanner module, I could manually place the scan head at various locations to observe its homing sequence. I think I now understand how it works and writing it down here.

For the purposes of this description, I’m arbitrarily labeling the home position as zero. Movement towards the small ADF scanner window is in the negative direction, and moving down the flatbed scanner page is in the positive direction.

Usually, the scan head is already at the home position. It moves a little bit in the positive direction to verify it can detect its homing marker (more about that later) then returns to its home position.

If the scan head is not already at the home position, it will keep moving in the positive direction for a short distance. I measured this distance to be the roughly the size of the ADF scanner window. So if the scan head was located somewhere under the ADF scanner window, this motion should bring the homing marker into view.

If that short positive distance fails to detect the homing marker, the scan head reverses direction and runs for a much longer distance. I think this is the size of the large flatbed scanner window. So if the scan head was located somewhere under the large window at power-up, it will scan a bit in the wrong direction (thinking it might be under the ADF window) then course-correct back towards the zero position.

If the homing marker is not found after scanning in the negative direction for the size of the large glass, the printer enters an error state. Both its power LED and the “Alarm” LED will blink, and the LCD screen alternates between two messages: “Printer error has occurred” and “5011” which I assume is the error code indicating scan head homing failure.

It was fun to figure out how the scanner homing sequence worked, but it was even more fun to spoof that homing marker.

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This teardown ran far longer than I originally thought it would. Click here for the starting point.

Working to understand the electronics of a Canon Pixa MX340 multi-function inkjet, I started with the automatic document feeder (ADF) on top of the machine and believe I’ve fully deciphered its paper presence sensors. Next objective is to measure the ADF motor’s electrical parameters.

Seeing four wires leading to the motor, I guessed it was a bipolar stepper motor. I disconnected its cable from the mainboard to measure resistance between each of these wires. I found two 8.8 Ohms pairs and open circuit in other combinations. This is consistent with the guess.

I didn’t find identifying labels around the perimeter. There might be something on the side facing the gearbox, but I didn’t want to disassemble it just yet.

The non-gearbox side is marked with QK1-6332 OXKT. Searching on that designation found a few eBay vendors selling replacement motors for a Canon MX340. This is oddly model-specific. I would have expected the same motor to be used across multiple devices. Perhaps this is not a designation for the motor but designation for a MX340 replacement part.

Taking advantage of the fact I still have a running printer, I soldered some wires onto the motor interface board to see their behavior during the printer’s startup sequence. The upper pair of wires (green and blue wires) is one of the 8.8-Ohm pairs, the lower pair (yellow and red) is the other pair. I expect to see four square waves alternating in a pattern consistent with driving bipolar stepper motors.

Here’s a snapshot of them during the startup sequence. Yep, bunch of square waves! I see the voltage level swings between 24V DC and ground, getting full power from the power supply. With 8.8 Ohm coils, straightforward Ohm’s Law calculation says the MX340 is driving this motor at 2.7A. Higher than I would have guessed, but within the range of power figures for a small stepper motor. It should also be noted that the ADF motor doesn’t seem to get powered on at a single coil for an extended period of time. Some stepper motor usage scenarios do this to hold position, but apparently not here. This is good because when power pulses are rapid and short in duration, the motor can tolerate higher amperage levels without burning out.

Isolating each of the wires, here’s the yellow-red coil. In the short time period of this snapshot, the coil is always energized one way or another.

And the green-blue coil doing the same. I’m not sure exactly what’s going on at the time of this particular snapshot, but it doesn’t have the regular pattern of a motor turning at a constant rate in a single direction. Perhaps I just happened to catch the motor as it switched direction? Digging for further details may be important if I am here to repair the document feeder. But as someone who only expects to repurpose the stepper motor in a future project, it is enough for me to confirm this looks reasonable for a bipolar stepper motor and knowing I can drive it at up to 2.7A at least for short periods.

That’s everything I wanted out of the ADF in this pass, I’m moving on to the scanner.

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This teardown ran far longer than I originally thought it would. Click here for the starting point.

I want to probe the electronic guts of this Canon Pixma MX340 multi-function inkjet and see what I can learn. Before I dive in, though, there were a few bits of preparatory work to be done.

There is a small switch in the device to detect if the cover (which includes automatic document feeder + scanner modules) has been lifted. This is usually a sign the user intends to either replace ink cartridges or clear a paper jam, so the printer goes into a non-printing standby state. It is spring-loaded to the open state. When the cover is closed, it presses on the lever and closes the circuit. In order to control state of this circuit without the weight of the cover pressing on the lever, I’m unsoldering its wires and moving them over to a large switch I salvaged from an earlier teardown.

I also wanted to see what’s going on with the power supply wires plugged into the mainboard. The label says 24V DC but there are three wires. From prior experience I expected them to be ground wire, power wire, and a signal wire to toggle low-power sleep mode. Thankfully, this connector exposed enough metal for my volt meter to reach them without fancy tools, and it was easy to probe the voltage level of each wire in the powered-up “ON” state and the standby “OFF” state. Looking on the circuit board silkscreen, I see this was designated CN701 and there’s an arrow pointing to pin 1, corresponding to the white wire.

Printer State	Pin 1 (White)	Pin 2 (Blue)	Pin 3 (Blue)
On	0.00	3.24	24.21
Standby/”Off”	0.00	0.00	8.50

Looks like pin 1 is ground, pin 3 delivers either 8.50V or 24.21V DC depending on the state of pin 2. The 3.24V I see on pin 2 in full-power mode is likely the logic high voltage, implying a circuit board primarily built around 3.3V DC components.

I also measured continuity (or at least minimal resistance) between pin 1 ground and the metal chassis. This will make electrical probing easier, because I can do things like putting oscilloscope probes’ ground reference alligator clip on the metal chassis instead of having to solder a separate wire to the circuit board ground plane. This was useful when I probed the automatic document feeder (ADF) sensors.

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This teardown ran far longer than I originally thought it would. Click here for the starting point.

I’m pausing mechanical disassembly of a Canon Pixma MX340 multi-function inkjet so I can explore its electrical aspects while everything is still running. I know I won’t understand everything and that is OK. Roughly in order of my self-confidence going in, the electrical subsystems I expect to find are:

Motors and Sensors: Yes

I expect to be able to understand all of the motors and mechanical sensors.

• There are two stepper motors, one for the automatic document feeder and one for the scanner. Determine open-circuit resistance for their coils, and monitor the voltage they see during printer startup sequence. That should give me a ballpark figure of their electrical current capacity.
• Determine pin-out of photo-interrupter sensors in this printer. This printer seem to use the same unit throughout, one that integrates the LED sender and the photosensitive receiver in a plastic package. Figure out which pins are power, ground, signal, and their voltage ranges.
• Determine the typical voltage sent to the two DC motors, one for paper feed and the other for print head carriage. Since they are used in a closed-loop control system for precise positioning, I don’t expect them to see full 24V DC from the power supply.
• Determine if pin-out of the paper feed motor’s encoder are power/ground/A/B as expected. Print carriage encoder is not currently accessible and will have to wait for later.

Control Panel: Hopefully Yes

I’ll take a look at the control panel, full of buttons and an integrated LCD. I managed to decipher the front panel of a Toyota tape deck and a Honda CD unit, and I’m optimistic that experience will help me understand what I will find on this front panel.

Scanner: Unlikely

While I’m pretty confident I can understand the stepper motor in the scanner, I’m less optimistic about the image capture side. A bit of web search found this Arduino forum thread, where I learned the imaging bar that moves across a scanner’s glass bed is called a contact image sensor (CIS). I haven’t found much in the way of publicly available documentation on any CIS modules. The prospects are even lower here because, as a worldwide leader in imaging technology, Canon probably produced this particular CIS for internal consumption. Meaning there’s even less reason for related documentation to be public.

I found a few YouTube videos that purport to cover all the things you can do to repurpose scanner components. They usually talk about the glass, the motor, the gears, the illumination light bar, etc. The sensor? “You’re not going to be able to reuse that.” Bah.

Maybe they’re right, but I’ll poke around anyway.

WLAN Module: Not Interested

I will ignore the Canon WiFi module, as I don’t foresee reusing that component. If I have a project idea that involves WiFi, I’m far more likely to pull out an ESP8266 or ESP32.

Ink Cartridges: Hell No

I will ignore the ink cartridges as well. I have a hard enough time understanding electronics without trying to untangle deliberate obfuscation. Ink cartridges are the profit center for inkjet manufacturers and encumbered with all sorts of trickery to discourage aftermarket ink cartridges. I’m going to steer clear of that mess.

Main Board: Examination Only

I’m interested in the motherboard only as far as looking it over for generalities. Try to figure out where the power handling areas are, identify peripherals like the stepper motor and DC motor control chips, stuff like that. Most of the components will be too small for me to reuse, and I’m not interested in trying to modify the firmware or running my own.

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I think that plan is within my current capabilities, and I’ll start with the simplest things: a switch and power input.

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This teardown ran far longer than I originally thought it would. Click here for the starting point.