0
\$\begingroup\$

In my company I'm often using RS232 connections between PCs and old peripherals (sub-D 9 pin). I'm only needing RX & TX signals, so I usually use audio-like coaxial stereo cable, to keep it simple. I use both signal cables for RX & TX signals and connect the screen to ground pin. It works fine, but I'm not sure if it is the best practice and the possible caveats... I suppose it will be better to use 3 wire cable (RX, TX & ground) and then connect the screen to the connector frames, but then the cable will be bigger, more expensive and difficult to find. So, when using 2 wire screened cable... Is the noise protection assured? There may be ground issues?

\$\endgroup\$
  • \$\begingroup\$ shielded pair shud be fine. There may be ground difference noise which adds jitter to the start bit edge used for clock sync, which you can certainly measure. Consider adding Odd Parity detection \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 7 '16 at 7:04
  • \$\begingroup\$ Where I worked, we used shielded #22 twisted pair (Belden 8451) for RS-232 connections. I don't recall any problems that were related to the use of the shield as the signal ground. The usual RS-232 problem was exchanging TX and Rx (if it doesn't work, swap pins 2 and 3 :-) ) \$\endgroup\$ – Peter Bennett Nov 7 '16 at 7:20
2
\$\begingroup\$

This depends on the application. The signal ground fills two purposes:

  • Even out voltage potential differences = fundamental electronics.
  • Act as a clean signal reference, as opposed to the "dirty" supply ground.

Some applications are less picky than others, and then you can "cheat" by not using a signal ground, but let the supply ground act as reference. This might work ok if the supply is decent and there are no high currents in the system. But for industrial/automotive applications, a dedicated signal ground is almost certainly needed.

The shield is there for another purpose entirely, namely to protect against immunity/emissions. It should only be connected on one side, to a ground different from the signal ground (for example supply ground, chassis etc). Depending on baudrate, environment etc, a shield may or may not be needed.

\$\endgroup\$
  • \$\begingroup\$ Thanks for the clarification. I'll keep it in mind. \$\endgroup\$ – Adam Calvet Bohl Nov 13 '18 at 4:44
0
\$\begingroup\$

After some time I decided to answer my own question. As all the serial connections I made using the way stated in the question were working fine, I suppose there is no real risk in using "audio-like" cable and connecting the screen to ground pin.

Also, an electrical engineering PhD from an outsourced company validated some of this setups, so I assume it is really Ok.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.