I have since found the full print set on
vt100.net, so
I'm all set on
that front. I've started tracing back from the logic controlling the
LINE and CTS indicators but haven't found anything obviously wrong as
yet. I have however read some funny-looking voltages off the 7406 in
this part of the circuit: 11.something volts while the datasheet says
30. Could of course be down to my test equipment as I'm limited to a
basic digital multimeter at the moment.
The 7406 has what's called an 'open collector output'. The output pin is
connected to the colelctor of an NPN transsitor (the emitter being
grounded), but to nothing else inside the chip. What this means is that
the output pin is either low (transistor tuend on) or floating
(transistor turned off), it'll never be pulled high bu that chip.
The 7406 can handle and external voltage oof 30V on the output (that is,
you could connect a resistor between the output pin and any voltage less
than 30V, in shich case that supply voltage essentially becomes the
'high' output voltage). But there's no reason why you _have_ to use 30V.
Any lower voltage is fine. In particualr, using this chip (and its
non-inverting brother, the 7407) to convert between TTL and 12V levels is
quite common, ans what I suspect is happening here.
-tony