MCT Serial board mystery - Can you crack the pinout code?
drlegendre at gmail.com
Sat Nov 15 23:02:54 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.
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?
On Sat, Nov 15, 2014 at 10:18 PM, drlegendre . <drlegendre at gmail.com> wrote:
> Quick update - I made an error.
> Pin 3 (both i/o headers) isn't tied to the 1458 Vcc+ - they're tied to the
> 1458 Vcc-. So, they have a 1K pull-down to -12V rather than a pull-up to
> So why is there a pin (3) on each header that's essentially a -12V @ 12mA
> current source?
> On Sat, Nov 15, 2014 at 8:42 PM, drlegendre . <drlegendre at gmail.com>
>> On Sat, Nov 15, 2014 at 5:37 PM, Tothwolf <tothwolf at concentric.net>
>>>> What happened with the charred area between the uarts and the edge
>>> connector (hopefully not conductive)? Did that pair of tantalum capacitors
>>> burn up? Nichicon PW series low-esr aluminum electrolytics make for an
>>> excellent radial tantalum replacement.
>> Ah, that info must have been part of a different discussion.. and it
>> might also be a clue, since the board was (apparently?) operating in +some+
>> form prior to the repair..
>> Those charred areas locate the 12V zener diodes & dropping resistors that
>> form the +/- 12V supplies for the board. As received, the board was already
>> like this, and the offending components had been simply cut away - this all
>> happened long before my time with it.
>> Now obviously, I didn't initially know what was supposed to be there. But
>> I started tracing the board and found that one set of traces went to the
>> -12V supply input for the original 1013 UARTs as well as the GND / -Vcc
>> terminals of the 1458 op-amps. Now, the present UARTs no longer require a
>> -12V supply - that need was eliminated with the 1014/1015, so they would
>> have been fine without the zener reg.. But the op-amps most certainly do
>> require a bipolar supply (if they're going to do real RS-232 conversion,
>> So that tells me that whatever was going on previously wasn't using the
>> board as originally designed.. because the never bothered to replace the
>> +/- regs once they killed them. I went ahead and fixed them, this time with
>> 5W zeners and 1.2W resistors. Now I'm thinking.. I need to see if there's a
>> hack anywhere on that board to supply single-supply 8V or 5V power to the
>> op-amps.. but I think they need more than 5V, don't they? (time to look at
>> the datasheet)
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