> >/RING0 on the DB9 is at 0V, and shows positive-going activity
>
> On pin 9 of the DB9 connector on the black plastic box?

Yes

> I'd expect that to be a bipolar (RS-232 style) signal

> Is it possible that the negative voltage generator on the
> MAX232-like chip inside the box is not working?

I'm sure it's fine. Voltages on the DB9 pins are -10, +10 or 0
with nothing connected. These reduce to +/- 7 with the PC or
PIC (via the MAX232) connected

I've studied the serial waveform very carefully with an analogue
scope, DSO and logic analyser. There is no breakdown in the
signal. Characters recorded by the PIC are those actually sent
from the MC35. I need to get more information from Siemens,
hopefully in the next couple of days, about this particular unit

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