MCT Serial board mystery - Can you crack the pinout code?

tony duell ard at
Sun Nov 16 00:06:02 CST 2014

> Ok, I think I've made some progress..
> Header pins 1 & 7 are both common to opt #2 of the op-amp. The inputs of
> op-amp #2 are as follows - the +IN seems to be tied to a voltage ref, but
> the -IN is tied to the SEROUT (serial output) of the UART.
> Sooo.. header pins 1 & 7 are both serial outputs, though one is a 'direct'
> out (via a 1K2) and the other is via the collector of a 3906 PNP. So if you
> view this with the pin 4 & 6 situation in mind, it does seem that this
> board provides both 'plain' (RS-232?) outputs as well as current-loop
> outputs, PLUS 'plain' and current-loop inputs - on the same header.

I think so. The current loop connection is the collector of a PNP transistor (2N3906) which 
suggests that's the positive side of the loop (emitter to a +ve voltage). So my next guess is that that
pin with the 1k resistor to -12V is the -ve side of the loop. If the transistor emitter goes to +12V or something
like that you have a 20-odd mA loop which seems about right.

I still want to know more about the input circuitry...

> Am I getting that right??
> Also, I found more interesting info.. Header pin 9 is connected to opt #1
> of the op-amp. The +IN of #1 seems to again be tied to a voltage ref, but
> the -IN is tied to the DAV line (Data Available) of the UART. Apparently,
> DAV goes high when a full character has been received and is ready to be
> read-out of the UART buffer. So this must be the... I don't know.. a line
> that goes 'hi' when the UART can receive no more bits and must wait till
> the current char is read-out. So this must be something like the DSR / DTR
> line? Letting the other end know that it can't send more bits till the UART
> buffer has been read?

My guess, again, is that this is related to using the board with an ASR33. It was a common
modification to such teletypes to add a reader control relay, allowing the computer to start
and stop the paper tape reader. DEC, HP, Intel all did it (in much the same way) for their
machines. I am going to guess that this output is to operate such a relay so you can use the
paper tape reader (e.g. to load software) and not loose characters.


More information about the cctech mailing list