Date   : Wed, 18 Jul 2012 21:38:39 +0100
From   : walker_matt@... (Matthew Walker)
Subject: RS423 won't transmit. Dead 6850?

>Normally with serial ports its the TX/RX chips that go, which in the
>case of the BBC micro are IC74(RX) (DS88LS120N) IC75(TX) (DS3691N)

Ah that makes sense, I missed that when I was probing things. I thought
I'd start at the DIN socket and work back with the probe, but when I
probed IC75, I probed pin 2 (RTS) rather than 7. Oops.
I also should have noticed that IC75 is getting mad hot. 
Now I feel a bit of an idiot.

Thanks for the advice Peter!