PDP-8m Console Switch Problems - fixed!

[Allison]

>
>Subject: Re: PDP-8m Console Switch Problems - fixed!
>   From: Don North <ak6dn at mindspring.com>
>   Date: Thu, 07 Sep 2006 00:49:17 -0700
>     To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
>     Cc: cctech at classiccmp.org
>
>None of the databooks I have ever seen actually show the gate level logic
>for the internals of the 74x175 or 74x174; they are only black box diagrams.
>So it is not clear to me that the outputs are unbuffered/bidirectional.
>They certainly seem to be for Signetics (see below) but my lab tests cannot
>verify this for any other mfg.

I have old databooks sets that go to the gate and some cases 
transistor level.

>I have pulled totem-pole outputs forcibly high or low before, but
>have always used an external series current-limiting resistor to limit
>the override current to a 'safe' value.

For pullup a current limit is advised, for pull down it's not needed
as the pull up device is current limited already.

>Well, I am seeing something very different here. I built a breadboard
>with the circuit under question: 74x175 device, 470ohm to +5V on MR~,
>CLK and Dx inputs forced to ground. I connected voltmeters to the
>Q and Q~ outputs, and then tried forcing Q and Q~ alternately to ground
>to see if I could change the state of the device.
>
>I had a number of devices to test, here are the results:
>
>                DATE
>   MFG  DEVICE  CODE  RESULT
>   ---  ------  ----  ---------
>    TI  74175    87   FAIL, shorting Q~ to gnd never changes Q to high
>   SGS  74LS175  82   ditto
>    TI  74AS175  87   ditto
>   SIG  74S175   84   PASS, with 470ohm pullup to +5V on Q req'd for Q0-Q3
>   SIG  74S175   76   PASS, Q1-Q3 work w/ no resistor, Q0 requires 470ohm
>
>For the failed devices, I tried with no pullup, and 100, 470, 1K, 4.7K
>pullups to +5V on Qx. No value of pullup made any difference. Shorting
>a Q~ signal (at ~4V) to ground never changed Q (at ~0.4V) to a HIGH.

It the MR/ is not in the correct state (may need pullup) I'd conclude
you have some bad parts.  Especially the TI[I have the most data on those]!



>> I'd give the 7404 the hairy eyeball! A quick test is socket a '175
>> with the Q and /Q output pins floating and using a jumper to ground
>> make it flip [It WILL NOT IF MR/ is asserted, you can bend out the
>> MR/ pin to avoid that.]. Then test the '04 for input changes output.
>>
>>
>> Allison
>
>The 7404 on the output seems OK, as is the rest of the downstream logic
>(the priority encoder). I measured the input currents required on a
>suspect '04 input to set the output high and low and they are well within
>spec (about +20uA for input high, -0.7mA for input low). With the 74S175
>out of its socket I could set all the 7404 inputs H/L and observe the
>downstream priority encoder outputs were just as expected.

That's good.

>I have about 15 of the 1984 Signetics 74S175s, I tried all of them in
>the console board socket; none of them worked, even a little bit, with
>no pullups added.

I'd pulse them slow with Q/ connected to D and see if they toggle.
I suspect you have a bad run of old chips.  I just tossed a few tubes
of mid 80s NOS parts as they apprently died of silicon rust [moisture gets 
into the plastc and they die].

>So I added 470ohm pullups to +5V on the 74S175 Q outputs to 7404 inputs.
>Everything started working as would be expected. The switch decode logic
>is now 100% functional.

You have something really messed up with those 175s your testing.

>My thought is that the output pullups on the 74S175 Q pins are trying to
>pull those outputs high fairly strongly (10mA load) but the Q output can
>still drive a valid low (it is a 20mA schottky driver). This pullup
>'preloads' the output, so that a kick on the QB~ (by shorting to gnd)
>gets the Q output moving high, the resistor keeps it moving high, able to
>override some smaller internal driver trying to keep the output low.
>At least that is the only rational explanation I can think of right now.

Sounds like you have some '175s with the upper device fried in the totem
pole outputs.

>Just the 7404 by itself is a -1mA low, +40uA high load; not very much.

Unless it has an input with high leakage to Vcc or ground, i've seen both.


>In any event, the fix is simple (three resistors really, but I'll add one
>on each of the six used outputs). Turns out all the original logic chips
>appear to be good (I had removed the original 74S175 intact for testing).
>
>Thanks to everyone for all the suggestions and helpful hints.

Put all the parts on a header and plug it in rather than mess the board up.


Allison