30 May 2015

Protek 608 RS-232 details

Protek 608 RS-232 login analyser
This post is a continuation of the Protek 608 DMM software project.

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:

Protek 608 RS-232 wiring diagram

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

DTEData Terminal Equipment such as a computer terminal
DCEData Communication Equipment such as a modem
DCDData Carrier Detect (DCE is connected to telephone line)
DTRData Terminal Ready (indicates presence of DTE to DCE)
DSRData Set Ready (DCE is ready to receive commands data)
RTSRequest To Send (DTE requests the DCE prepare to receive data)
CTSClear To Send (indicates DCE is ready to accept data)

Serial data properties

Here are the serial properties from the manual.

Baud rate9600
Data bits7
Stop bits1
Flow control'RTS/CTS' or 'DTR/DSR' not 'None' or 'XON/OFF'

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.

RS-232 serial cable wires

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.

Protek 608 RS-232 login analyser

Here are a few interesting captures from the logic analyser software

Protek 608 RS-232 connect
Connection to serial port
Protek 608 RS-232 disconnect
Disconnection from serial port
Protek 608 RS-232 full packet
Receiving a full packet from the DMM
Protek 608 RS-232 packet start
Zoom-on of the start of the received packet
Protek 608 RS-232 TXD hold command
Sending the HOLD command
Protek 608 RS-232 TXD interrupting RXD. The packet continues after a 30ms delay.
How sending data interrupts a packet being received

Further reading

About the Author:

Hardware and software engineer with experience in product development and building automation. Director at Cabot Technologies and Product Manager at NEX Data Management Systems.


  1. Jack

    I shit You not…. I installed the windows 98 version on windows 10 and it worked!!! installed; I dont have the IR interface so cant test buttttt INSTALLED

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