16 Nov
2014
16 Nov
'14
6:06 a.m.
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.
-tony