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PDP-8/M on eBay

You are going to want to keep your eye out for one of these:

http://www.ebay.com/itm/DIGITAL-EQU...t=BI_Control_Systems_PLCs&hash=item4613ce4722

or else you are going to get very tired toggling in the bootstrap over and over again when it gets wiped out.

Don

PS I have two fully working PDP-8m, each with 16K core, RX-02 drive, and EAE option.
Each 20 slot backplane is basically full of cards (18 of 20 slots populated).
 
Thanks! I will share many pictures when I get a chance, here and/or on my web page. I might be able to snap a few cell phone pics of the cards before I head home from work this evening.
 
AK6DN said:
You are going to want to keep your eye out for one of these:

http://www.ebay.com/itm/DIGITAL-EQU...t=BI_Control_Systems_PLCs&hash=item4613ce4722

or else you are going to get very tired toggling in the bootstrap over and over again when it gets wiped out.

Don

PS I have two fully working PDP-8m, each with 16K core, RX-02 drive, and EAE option.
Each 20 slot backplane is basically full of cards (18 of 20 slots populated).
Click to expand...

Yes the novelty of toggling in RIM fades pretty quickly, mind you that one is a bit pricey, I just missed one on a buy it now EBAY UK for £40

($65) half an hour ago, would have been handy for a RX01 boot.
Dave H
 
I have my MI8E setup with the RX01 bootstrap. I worked it all out on paper and then reworked all the diodes and jumpers. To save anyone the time in the future, I can take a photo and the interested party can merely duplicate the configuration.

And Mark, go ahead and start describing the problems. I am surely not the only one here who has been deep into the 8/e central processor.

Lou
 
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Hi All;
Lou, Its Pretty Amazing what anyone can do with a couple of handfulls of Diodes.. Especially If You do not have the Proms.. And for short word length, it still beats Fat Fingering in..
THANK YOU Marty
 
Marty,

Well, it took me a few hours to rework all those handfuls of diodes. But by now I do believe that from a time standpoint, the effort has paid for itself.

Fat fingering is not all that bad. It's hardwired into my brain now since I've been doing it for over 30 years. Similarly when checking my work, looking at the lights and translating back to octal is second nature.

Lou
 
Lou - N2MIY said:
I have my MI8E setup with the RX01 bootstrap. I worked it all out on paper and then reworked all the diodes and jumpers. To save anyone the time in the future, I can take a photo and the interested party can merely duplicate the configuration.

And Mark, go ahead and start describing the problems. I am surely not the only one here who has been deep into the 8/e central processor.

Lou
Click to expand...
Lou, I will take you up on that, I was going to work it out, done it before for a RIM but dont mind "cribbing" from you:D , and yes toggling in short programs is not to bad though I find the m switches much nicer than the e, and Mark , you will do quite a bit of this during the test /repair period.
Dave H
 
By the way: I wonder if anybody has ever bothered to make an MI8E equivalent that's programmable by DIP switches instead of soldered diodes?
 
NF6X said:
By the way: I wonder if anybody has ever bothered to make an MI8E equivalent that's programmable by DIP switches instead of soldered diodes?
Click to expand...
Well you could do it, just put a switch in series with each diode but its a lot of switches.
Dave H
 
Lou - N2MIY said:
I have my MI8E setup with the RX01 bootstrap. I worked it all out on paper and then reworked all the diodes and jumpers. To save anyone the time in the future, I can take a photo and the interested party can merely duplicate the configuration.
Click to expand...

Feel free to share the photo in this thread if you would like to!

Lou - N2MIY said:
And Mark, go ahead and start describing the problems. I am surely not the only one here who has been deep into the 8/e central processor.
Click to expand...

I'll need to fiddle with it some more before I describe the problems in detail. I'm at work now, so I've only been able to fiddle with it a little bit. Examining memory doesn't seem to advance the address, the LSB of the address looks like it may be stuck on when I load it from the front panel, the accumulator doesn't fully clear, etc. Just cleaning out the machine and introducing the toggle switches to Mr. DeOxIt (from a needle applicator, not the awful spray) may help a lot. I also want to measure the power supply outputs, look for any of those dreaded metallized paper dielectric EMI caps that I've learned to replace before they explode, etc.

I'm very happy with the machine. Even though it's not quite working right, I have seen some semi-reasonable responses to front panel controls, I've managed to get the run flip-flop turned on, etc., so it's showing real signs of life.
 
DaveH said:
Well you could do it, just put a switch in series with each diode but its a lot of switches.
Dave H
Click to expand...

34 or so 12-switch DIP switches at around $1.60 each by my estimate. With different control logic on a newly-made board, the diodes might not be necessary if a better way to scan the switches can be devised without totally ruining the ambiance by resorting to a small FPGA. :p
 
Or just craft in a couple of 32x8 PROMs and a 5bit address counter in place of the diode array.
Or a couple of 512x8 PROMs, an address counter, and four dip switches to set the high bits to select multiple different bootstraps.

Still in the same decade of logic availability.
 
You are going to want to pull your front panel and check the resistance of all the switches, and probably do some cleaning.

When I got my 8M's, the core memory drivers and the front panel gave me the most problems overall.

The DEC designer (I use that term loosely...) of the 8M front panel played some funky tricks with FFs to do the switch debouncing. Look at the schematic and you will see what I mean.

The circuit is very sensitive to the switch resistance. And also very sensitive to the exact vendor/date codes of the FF's used in the design.

Until you know all the front panel control signals into the OMNIBUS are working correctly debugging will be impossible.
 
Got it. Looking at the front panel was my natural first impulse anyway, simply because slide switches are easy to understand and likely to get grungy after 40ish years.
 
Hi All;
Lou, I think all of the Front Panel machines that I have worked with were/are Octal Machines..So it like you said comes second nature for me.. I never could get Hex down.. Octal make more sense to me, but that is me..
THANK YOU marty
 
AK6DN said:
The DEC designer (I use that term loosely...) of the 8M front panel played some funky tricks with FFs to do the switch debouncing. Look at the schematic and you will see what I mean.

.
Click to expand...

Hmm, so they do, id not seen that as on both my m's the switches worked ok, it was the led logic , the 7454's that I had problems with , one unit needed 7 of these replaced.

Just had a closer look at that circuit, its actually sort of elegant , I guess those FF's are simple cross coupled gates but it does look rather odd to set a latch by grounding the bar output !


Dave H
 
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DaveH said:
Just had a closer look at that circuit, its actually sort of elegant , I guess those FF's are simple cross coupled gates but it does look rather odd to set a latch by grounding the bar output !

Dave H
Click to expand...

Well I would call 'elegant' being very kind. In my mind it is an unjustified hack. But I'm biased, I had to debug the darn thing.

In repairing my panel I had several of those 74175 devices that failed. I replaced them (in sockets) and plugged in NOS devices of the same type.

The panel switch function did not work at all. Switches that were nonfunctional remained so. Plus other switches stopped working too.

Turns out only some vendor's devices will work in that circuit. Many do not respond correctly to that circuit hack at all, or had a power supply voltage dependency.

So I went to ACE Electronics locally here in silicon valley. They have a warehouse full of NOS tubes of every old semiconductor you can think of. I bought samples of all the variations of vendor and date code I could find.

Ultimately I was able to source some devices that made the panel function correctly again.

Elegant indeed. :mad:

Don

Found my old net posting from a while back:

Code: 
Subject: Re: PDP-8m Console Switch Problems - fixed!
From: Don North <ak6dn@mindspring.com>
Date: Sat, 09 Sep 2006 00:08:30 -0700
To: General Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org>
CC: General@priv-edtnaa06.telusplanet.net

Brent Hilpert wrote:
> I was curious ... pulled out a bunch of NOS 74x175s
> and did some tests. I noticed that the behaviour was
> sensitive to Vcc and ended up with the following table.
>
> The values 0<=n<=4 in the matrix indicate the number of
> flip-flops in the IC which worked as desired, so 4 is 'good'.
>
> <--> repeat across
> --> progression up
> <-- progression down
                                    Vcc
Unit MFG  DEVICE  DATE   4.1 4.3 4.5 4.7 4.9 5.1 5.3
---- ---  ------  ----  |---|---|---|---|---|---|---|

1.   TI      175  7340    4   4   4   3   3   3   3
2.                 "      4          <-->         4
3.                 "      4   4   4   4   3   3   3
4.   TI      175  7624    0          <-->         0
5.   Hit     175  6G46    4   3      <--          0

6.   TI     S175  7340    4          <-->         4
7.                 "      1      -->      3   4   4
8.                 "      0           -->         3
9.                 "      2   3   4      <-->     4
10.  TI     S175  7936    4          <-->         4
11.                "      4          <-->         4
12.                "      4          <-->         4
13.                "      4          <-->         4
14.  NS     S175  8742    4          <-->         4

15.  NS    LS175  8332    0          <-->         0

                                    Vcc
Unit MFG  DEVICE  DATE   4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5   Notes
---- ---  ------  ----  |---|---|---|---|---|---|---|---|  -------------
16.  NS      175  7923    4   4   4   4   4   4   3   3   
17.  NS      175  8948    4   4   4   4   4   2   0   0    
18.  FAIR    175  8001    4   4   4   4   4   3   1   0
19.  SIG     175  8202    4   4   4   4   4   4   4   4    Works in PDP-8m console
20.  TI      175  8705    0   0   0   0   0   0   0   0
21.  SGS   LS175  8241    0   0   0   0   0   0   0   0
22.  TI     S175  345B    4   4   4   4   4   4   4   4    Works in PDP-8m console
23.  SIG    S175  8421    0   0   0   0   0   0   0   0    Fails w/ no pullups

> Observations:
> - behaviour may differ between IC units even within valid Vcc range,
> - a given IC unit may change its behaviour within or near valid Vcc range,
> - 175 class may pass at lower Vcc and fail at higher Vcc,
> - S175 class may pass at higher Vcc and fail at lower Vcc.
>
> Interesting the way 175 and S175 devices differ in their response to Vcc change.

I didn't see any of the fail at lower VCC on any S175 parts, but I only
tested one sample from each date code. I have more parts, but I got bored :-)

> The group of TI S175s from 1979 did seem to show reliable behaviour over the
> entire range.

I found this to be true of the TI S175 date code 345B (which I don't know how
to decode) and the Signetics 175 from '82. Both were rock solid in the test
circuit and work reliably in the PDP-8m console socket with no add'l rework
(ie, no pullups required as with the previous S175 SIG part).

> One way or the other, the DEC front panel is relying on unspecified behaviour
> of the device. I wonder if the designers were just relying on old habits of
> setting flip-flops via collector triggering from the discrete days, and just
> got lucky that it worked.

I agree, this circuit is more complex than it really needs to be. The two 175s
and the 04 hex inverter packs can be replaced by three 7400 quad nand packs (plus
six pullup resistors) that implement simple cross-coupled latch debouncers.
Sometimes a designer gets a bug in their head to implement a 'cute' circuit
when really doing something more simple and straightforward is the better solution.
Of course I would never do that :-) , I'm only reporting that I've seen it done.

Anyway, for now it appears replacing the existing flaky S175 dc '76 parts in my
PDP-8m console with SIG 175 dc '82 parts has fixed the problems, no add'l rework
or circuit hacks required.

Don
 
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@NF6X:
Congratulations on your purchase!
I'm in Chino Hills, so if you want to troubleshoot by board swapping, just let me know.
 
Hi All;
Don, I am really Amazed at the variation of what worked and what didn't, in the Circuit.. Since I haven't seen the Circuit in question, so I cannot make any generalizations.. It makes me wonder If DEC had any other problems in Later Designs, such as my 11/45 and my 11/40 or if by that time, they had pretty much worked out any circuit variations between vendors and dates..
Also is the vendor you mentioned still around and can He supply older ic's, I have mostly a good supply but there are some Ic's that I don't have..
THANK YOU Marty
 
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