This is following on from my previous post introducing the Protek 608 multimeter and my software project. In this post I detail the communications hardware, an important consideration for future development.

The Protek 608 uses an RS-232 connection to allow for interfacing with a PC. This is a very common connection that most DMMs used. These days many DMMs have a direct USB cable, so communications will be TTL, but I suspect the protocols are much the same. I use a RS-232 to USB converter cable. I have a few of these, and they are easy to find at your local electronics or computer store, or online of course.

RS-232 connection

Here’s a wiring diagram from the Protek 608 manual:

If you’re not familiar with RS-232, this is a description of the signal lines labelled:

Serial data properties

Here are the serial properties from the manual.

When I was first trying to read data from the multimeter I didn’t RTFM and assumed 8 data bits, as is quite common. I tried different serial programs, different USB converter, until I finally realised my mistake. 😛

RS-232 signals

In order to measure the RS-232 signals, I cut into a standard serial cable, and tinned each wire with solder. I belled-out each pin signal with each wire colour.

This gave me a convenient connection point for each of the 9 wires. I’m using a Intronix 34-channel Logicport logic analyser. I have a bunch of little clips that I originally purchased with the logic analyser, but found that I was able to gingerly plug each signal wire to the serial cable wires directly.

Here are a few interesting captures from the logic analyser software

Further reading

• A teardown of the Protek 608 RS-232 serial cable
• A detailed look at how the Protek 608 protocol works