Cheap chinese digital calipers and you

Some time ago I bought a bunch of cheap Chinese digital calipers and I was surprised to find that there is a port on almost all of them out the top. Now having looked up the format for both Starrett and Mitutoyo SPC formats, I have determined that it matches neither.

A cheap chinese vernierThis one happens to be a Mastercraft one but you can buy one off Ebay for like 8 bucks delivered. You can see the connector is already installed with solder onto the first three pins.

the pinout is as follows, from left to right looking at the face:

  1. Ground
  2. Data
  3. Clock
  4. +1.5v (or whatever the battery happens to be)

Here’s a bit of a closeup:

A look at the pinouts
A look at the pinouts, hastily soldered.

So I hooked it up to my oscilloscope and determined that there is both a data line and a clock. Here are the specs:

  • Data is sent in a packet of 6 nybbles.
  • Data is sent every 100ms
  • Data is LSB first
  • The numbers only seem to represent what’s on the screen. It represents units of the smallest graduation in the current display mode (inch/mm). For example, if values are converted to decimal: 1.5055″ = 3011 units or 3.45mm = 345 units
virgin saleae logic
A look at my virgin Saleae logic analyser

And yes, I bought a Saleae Logic Analyser. Unfortunately it does not support synchronous serial analysis. This would’ve been a great tool, had the software supported it.

Anyways, I tried to get the output to work on the logic analyser but the signal was really spotty. I determined it was because it only output 1.5v. I quickly whipped up a breadboarded circuit with a 7014D op-amp setup like a comparator to amplify the singal to 5V

the board to amplify signals
the board to amplify signals

Simple simple. This allowed me to run the signal onto my logic analyzer. It worked great except for the software not being able to decode the data. Wasn’t an issue though, I decoded it mentally.

Anyways, I hope this is useful to somebody. I can’t guarantee anything is correct but these device might make for interesting low-cost DRO units or something. There are some caveats however:

  • The cheap verniers are exactly that, cheap!
  • They don’t handle any kind of moisture, even a humid day is enough to screw them up.
  • The update rate is a paltry 10Hz
  • You can’t trigger the output when you want to, unlike SPC outputs of other manufacturers.
  • Battery life sucks
  • You can’t feed higher voltages to the battery compartment, no likey!

Again, hope this was helpful to somebody out there on the interwebs.