Wednesday, November 23, 2011

RS422 / Sony P2 protocol and serial stuff!

When running P2 protocol over RS422 (i.e. Sony VTR remotes) there is no hardware handshaking so RTS and CTS (Request To Send and Clear To Send) aren’t used; a bit like the old 3-wire XModem/YModem/Kermit protocols used in RS232 (remember RS422 is just a balanced version of RS232).

We base our RS422 wiring on the Quartz remote standard (Quartz were one of the first firms to use RJ45s & cat5 for RS422 remotes):

However – I know for certain that Probel use a different standard and many places are wired to whatever the local standard is; remember – until ten years ago most places wired ‘422 on star-quad cable rather than cat5e/6. I don’t know if current model Evertz routers have maintained the Quartz standard – I bet they have given they bought Quartz for its router business.

Whatever wiring standard is used always make sure that pins 2 & 7 are a twisted pair and likewise 3 & 8 otherwise you lose all the advantage of common-mode noise rejection that balanced RS422 brings.

Finally you need to be certain if a place is wired for chassis earth (pin 1 on a 9-pin) or signal earth (pins 4 & 6 on 9-pin). Signal earth is best as there is always a chance of earth-hum between areas when you tie chassis earths together but hopefully properly designed kit with balanced lines have the signal earth floating WRT to power/chassis ground. BUT, you have to stick with the local standard; if the engineer has wired only chassis earths you need to continue using pin 1 or even shorting pins 1, 4 & 6 at the remote end.

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