The Long Tale of the Caliper: Part 1

You’ve seen them before—and probably even have one in your toolkit. Those inexpensive digital calipers of questionable design lineage that make measuring things fun, instead of a pain in the Vernier.

Mine is a General Tools 147 came in a nice, sturdy case, runs one one button-cell battery, and can display either millimeters or decimal inches (the newer equivalent model can display fractional inches to the nearest 1⁄64^th).

Its coolest feature is hiding under a sliding cover on the top: four electrical contacts on the PC board that the instruction sheet describes as a “SPC RS-232 serial port”. Yet I have never, until this week, been able to locate a serial cable for the unit.

Actually, I long-ago discovered the truth—that it’s not a real serial port. No, not your typical serial interface trying to masquerade as RS-232 using ±5 V DC instead of ±10−12—this thing ain’t even close, using 1.5 V DC signaling, with an oddball data format.

But getting this thing to talk to computers turned out to be the perfect foil for a work project (and I might even be able to share the results somewhere down the line), so I’ve spent more than a few hours this week researching this funky caliper interface and its battery usage.

The reasons behind the funky design of this unit couldn’t be found anywhere on the Net until two months ago—when the original design engineer posted a couple of paragraphs on a message forum. And what I discovered in my research is interesting and relevant enough for product design that this will take more than one post.

When I finally found a cable that will fit, it was one that ends in a 4-pin mini-DIN plug—just like your old-fashioned S-Video cables use—and costs between 1/2 and nearly double the cost of the calipers it fits (depending on which supplier you choose. No, this is not an old Macintosh serial connector (that’s another mini-DIN, and maybe not even a standard one). Nope; not a serial cable. It’s meant to connect the caliper to a DRO (Digital Read-Out) for use on milling machines.

You see, this caliper design is a cheap Chinese knock-off of a Swiss-made caliper—from 1982, when computers with serial ports weren’t all that common in the average hobby’sts garage or even in machine shops and engineering firms.

Because of the scarcity of this cable/connector, all of the hacks I’ve seen to interface it with a PC’s serial port wind up soldering wires directly to the PC board or carving out the plastic case and soldering a small 4-pin connector never meant for repetitive use to the caliper.

Beyond that, the caliper’s output is not quite to scale, and is fairly difficult to decode. In a blog post comment from March of this year, original design engineer Hans U. Meyer writes:

Here’s why the Chinese Caliper’s binary output is not to scale: I designed it for Sylvac in 1982, when the small number of on-chip transistors limited the options. Hence a scale pitch of 5.08mm=0.2inch, easy to subdivide in both metric and inch with serial binary (LSB first) arithmetic logic. The same logic also calculated the serial BCD output in mm or inch for the display. The serial binary signal, common to both units, was output as an afterthought: only few people wanted it, to which Sylvac simply sold binary-to-BCD-to-RS232 adapters.

Elsewhere in that thread, and in other discussions sprinkled across the net I discover that among all of the code examples of how to read these calipers, nobody has quite done it correctly, because they typically ignore the last bit of data that is “too jittery”, instead of averaging it to extract all the valid data available.

Yuck.

There are so many things wrong with this picture that I’m not quite where to start. So, in no particular order:

1. These companies essentially stole another company’s design, copying it verbatim with engineers that didn’t fully understand the technology.
2. Said companies are still making and selling a badly compromised design from 1982, having invested zero dollars or time in improving it.
3. These companies in the past have falsely (even if unknowingly) advertised these units as having a true RS-232 serial port.
4. We as consumers have blindly bought these units, unaware of their design and operational defects.
5. Very few people spend the time required to analyze and/or rectify these problems.

In a word, it’s all schlock. But why?

I fear that it’s become an inherent part of our culture—that executives, product managers, and engineers alike have misconstrued the concepts of “good enough”, “progress”, and “value”.

This has happened because our media and schools have taught us to learn by copying, instead of by analyzing and thinking. This is unacceptable, and as I go further down the tale of the Chinese Calipers, I’ll share with you what I believe should have happened at each step, as opposed to what did happen.

Author: Peter Sheerin

Peter Sheerin is best known for the decade he spent as the Technical Editor of CADENCE magazine, where he was the acknowledged expert in Computer-Aided Design hardware and software. He has a long-standing passion for improving usability of software, hardware, and everyday objects that is always interwoven in his articles. Peter is available for freelance technical writing and product reviews, and is exploring career opportunities in interaction design. His pet personal project is exploring the best ways to harmonize visual, tactile, and audible symbols for improving the effectiveness of alerting systems. View all posts by Peter Sheerin