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

drlegendre . drlegendre at
Sat Nov 15 13:52:41 CST 2014

Oh, and to add to my confusion.. I've read that the early Altair serial
boards used a 20mA  current-loop transmission standard.. which as far as I
know, is +not+ standard RS-232 stuff. So should I actually expect this
board to be a standard RS-232 device, or is it something else.?

On Sat, Nov 15, 2014 at 1:49 PM, drlegendre . <drlegendre at> wrote:

> Howdy,
> On Sat, Nov 15, 2014 at 9:10 AM, Tothwolf <tothwolf at> wrote:
>>> Standard numbering scheme? Surely you jest? ;)
>> Do you mean something like this?
>> 2 4 6 8 10
>> . . . . .
>> . . . . .
>> 1 3 5 7 9
> No joke, you got it right. That's the standard scheme for IDS (I said DIP,
> which is the wrong term I think) headers.. across-and-back, as you wrote
> it.
>> It sounds like you've just about got it figured out though. If you have
>> the datasheet for the 1458 (aren't there 1459s on the board too?) then you
>> can follow the signals through the level converters back to the UART, and
>> its datasheet should then tell you exactly which signals are which.
> Don't see any 1459s on there, just the pair of 1458s up near the IDS I/O
> headers
> But yes, you have a point.. I could probably have continued, tracing back
> from the 1458 inputs, through the various logic gates, to the 1014 / 1015
> UARTs.  But I stopped where I did as it was already pretty tedious and I
> figured I had enough info for you in-the-know types to figure it out at a
> glance.. ;-)
> That, and I am confused by a couple of my findings, particularly the Pin 4
> - Pin 6 relationship. Why are these pins essentially common to each other,
> and to the XNOR inputs? One pin has a 47R in series to the XNOR gate, the
> other has a 150R in series to that same gate. It's as if the line with the
> 150R could be asserted 'high' by some line and then the line with the 47R
> could re-assert it 'low' - but not all the way to zero, more like to 1/3 of
> voltage. See, that makes no sense to me,.
> Ditto for the Pin 1 / Pin 7 relationship. Both of these pins are
> essentially outputs from the same op-amp output - one is basically a
> direct-out from the op-amp #2, the other is via the collector of a PNP
> (3906) transistor - the base of which is also tied to op-amp output #2.
> This is the stuff that's really confusing me.. is the 3906 acting as an
> inverter? If so, why?
> To be clear, J1 (10-pin I/O header) has two pins (4 & 6) both tied to the
> selfsame +input+ point, and two pins (1 & 7) tied to the selfsame +output+
> point. J2 is identical, but it uses a different op-amp and a different XNOR
> input. Otherwise, J1 and J2 are carbon-copies of each other, electronically
> speaking.
>> It makes sense that pin 10 would be an extra ground too, since it is
>> quite possible they originally used a 9-conductor ribbon cable and wouldn't
>> have connected that pin. You may also find than the signals come out on the
>> correct pins of a DE9 connected to a 10-pin IDS connector with some
>> 9-conductor ribbon cable.
> Or to a 25-pin version of the same. Sure wish I had one! But I tossed /
> gave all that away years ago, I think.. =/

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