Thursday, January 09, 2014

Chassis vs Signal earth on RS422 remotes

Grounding is essential to reliable operation of any RS422 connections. It is also the most overlooked and least understood. The easiest way to ground your RS422 equipment is to simply use "Earth" ground as your return path. Although easy this may not be the best method for grounding your application, because current leaking from equipment, electro-static discharge (ESD), and lightning all drive current through this path which results in high noise content. The reason for this increased noise level is due to the fact that "Earth" ground presents a relatively high resistance. RS422 is designed to operate normally with a ground potential difference of +/- 12 Volts. During normal operations this is typically not a problem, however during fault conditions or lightning strikes even within ½ mile the ground potential difference can reach hundreds and in some cases thousands of volts. This will most likely result in damage or failure of one or more devices on the RS422 router.

In TV facilities I most often come across three methods of earthing;
  1. No earth - assume that the mains return is good (all equipment is class-1 and bolted into it's bay and that signal and chassis earths will be close)
  2. Use pins 1 and/or 9 on the 9-pin D-type to couple the chassis earths together; please, don't get me started about intentionally connecting mains earths between different areas! Do you like dealing with induced hum between different areas or buildings?!
  3. The best way; using pins 4 & 6 - the signal screens (it's what they're there for).
The job I'm finishing at the moment had a problem with Digital Rapids workstation which wouldn't run through the RS422 router although patching around the router worked - the PC could control a VTR. My first port of call was to test the cabling/router patch by sticking an old Sony RM450 edit controller at the back of the Rapids and pretend the RM450 was the workstation - all good; VTR control and timecode return worked fine (so Tx and Rx doing their things). 
So my first thought was to measure the impedance between the signal ground on the router and the mains earth - high Z so no return patch for the RS422 via the router if it was relying on the mains earth (scenarios 1 & 2 above) and since the router is optically isolated on it's data inputs I wasn't surprised. That is the way it should be done.
Now then; most PCs that are running video apps and have to control a piece of broadcast kit use an RS232 port with an external RS232-422 adaptor. These essentially just balance the Tx and Rx pins and there are several models. None of them (in my experience) actually use a pair of rep coils to properly balance rather they use a pair of op amps in a differential input configuration. This is fine but doesn't have the noise immunity that you get with coils (common mode rejection). What it does mean is that all of the noise immunity of the circuit comes from the electrostatic shielding of the earth and so you better get it right!
I cracked open the cheap'n'cheerful '232-422 adaptors supplied with the rapids and they had the screen connected to the shield of the 9-pin on the RS422 side (so relying on scenario 1 above). Moving that to pins 4 & 6 (scenario 3) fixed the problem. 

As an aside the Adenda "Rosetta Stone" adaptors that Avid supply do the right thing!

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