I started looking for an inexpensive electronic indicator with digital output port, and ended up splurging for a genuine Mitutoyo. Sure it is over five times the cost of the Harbor Freight alternative, but I thought it would be worth the price for two reasons. One: Mitutoyo is known for high quality precision instruments, and two: they are popular enough that the data output port should be documented somewhere online.

The second point turned out to be moot because the data output port was actually documented by pamphlet in the box, no need to go hunting online. But I went online anyway to get a second opinion, and found this project on Instructables. Most of the information matched up, but the wiring pinout specifically did not. Their cable schematic had a few apparent inconsistencies. (Example: one end of the cable had two ground pins and the other end did not.) They also had a “Menu” button that I lacked. These may just be the result of different products, but in any case it is information on the internet to be taken with a grain of salt. I took a meter to my own cable to ensure I have the pinout described by the pamphlet in my own instrument.

Their Arduino code matched the pamphlet description, so I took that code as a starting point. I then released my derivative publicly on GitHub with the following changes:

• Calculate distance within numeric domain instead of converting to string and back.
• Decimal point placement with a single math expression instead of a list of six if statements.
• Their code didn’t output if value is inch or millimeter, I added units.

A limitation of their code (that I did not fix) is a recovery path, should the Arduino falls out of sync. The Mitutoyo protocol was designed with a recovery provision: If the communication gets out of sync, we can sync back up using the opening 0xFFFF. But since there’s no code watching for that situation, if it falls out of sync our code would just be permanently confused until reset by the user.

For debugging I added the capability to output in raw hex. I was going to remove it once I had the distance calculation and decimal point code figured out, but I left it in place as a compile-time parameter just in case that would become handy in the future. Sending just hexadecimal data and skipping conversion to human-readable text would allow faster loops.

Note that this Mitutoyo Statistical Process Control (SPC) protocol has no interactive control — it just reports whatever is on the display. Switching units, switching direction, zeroing, all such functions are done through device buttons.

Once it all appears to work on the prototyping breadboard, I again soldered up a compact version and put it inside a custom 3D printed enclosure.

Oh yeah, now we're talking!
Mitutoyo SPC data translated to USB serial with Arduino code I've published to https://t.co/BDOfrAFVmu
Here USB serial data is plotted by a Node-RED dashboard chart widget. pic.twitter.com/xnL1Lmf6nr

— Roger Cheng (@Regorlas) July 31, 2020