27 Jun
2006
27 Jun
'06
3:12 p.m.
I've got some equipment here that has RS422 serial on it - i.e. it uses
differential signal pairs rather than RS232's reference against ground.
Question is, can I wire up a cable so that I can use an RS232 terminal with
this RS422 equipment? (running over a short distance)
This somewhat depends on the drivers/receivers used. The 26LS3x ones
generally will work as you want them to. To do the job properly, I'd use
similar chips to convert the signals back to TTL, then add a MAX232 or
similar.
Looking at the schematics, I think it's possible, with signals mapping to:
RS422 RS232
RXD- GND
RXD+ RX
TXD- no connection?
TXD+ TX
CTS- GND
CTS+ CTS
BSY- no connection
BSY+ RTS
... but I don't know if I should ground TXD-, or indeed one web site I saw
No, leave the 'unused' TxD output unconnected.
suggested that TXD- should be used as the signal and
TXD+ is actually the
reference. I suspect that one or other should be left floating and the other
pin of the pair forms the 'real' TXD in RS232-speak.
Which one you use depends on the convention for labelling '+' and '-'
here. Find the one that's -ve when the unit is idle, and use that.
(the actual comms chip is a 6551A, then the receiver for RXD and CTS is a
26LS32 and the driver for TXD and BSY is 26LS30)
If all else fails, disconnect the RS422 stuff and link a MAX232 or
similar to the right pins on the 6551...
-tony