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Plessey PM-837 (Omnibus card)

gnupublic

gnupublic

Experienced Member
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Jan 23, 2019
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198
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Germany, Berlin
While waiting for some parts to change the fans of my 8/e desktop, I took this card to look after it.

IMG_7651.jpg


I got it two years ago and did not do anything with it. I even was not able to get any information about it. The numbers 700292-100 or 700233-10 did not help. Until now my friend reminded me that he showed me this foto once:


Plessey_PM-837.jpg

So this was clear then. Looking at the card I find some interesting markings on the ICs:


IMG_7662.jpg

IMG_7661.jpg

I tested the capacitors OK, so I put it in my 8/e. And it did not smoke. Manually I'm able to switch the core fields. But trying to boot OS/8 fails with this lights on:

IMG_7664.JPG

So now I have some questions, is there anybody having the shematics of this card? Some more input about it? Stories? They seem to be seldom, because I do not find that with bing or google.

Have fun,
Volker
 
Have you tried running the extended memory maindecs on it?

The following is copied from https://www.bernhard-baehr.de/pdp8e/MAINDECs.html

KM8-E Memory Extension​


MAINDEC-8E-D1BB-PB KM8-E 4K Extended Memory Checkerboard (Binary)
MAINDEC-8E-D1BB-D KM8-E 4K Extended Memory Checkerboard (Documentation)

The original documentation is not available. Set SR9-11 to the number of memory fields to check. After each successful cycle, the digit “5” is printed on the teletype. Set SR5 or SR4 to halt the test and set SR3 to avoid relocating of the program.

MAINDEC-8E-D1FB-PB PDP-8/E Extended Memory Address Test (Binary)
MAINDEC-8E-D1FB-D PDP-8/E Extended Memory Address Test (Documentation)
​


MAINDEC-8E-D1HA-PB PDP-8/E Memory Extension and Timeshare Test (Binary)
MAINDEC-8E-D1HA-D PDP-8/E Memory Extension and Timeshare Test (Documentation)
 
The IC placement of the Plessey PM-837 board looks very similar to the original (M837 photo copied from so-much-stuff.com). I wonder if the schematic of the Plessey board is also very similar, or maybe even identical.
1705698461784.png
 
Yes it looks very similar! Next week I can put the cards side by side and see what types of IC's are used.
I did no further tests with the plessey cards yet. Need more time...
 
It looks more than similar...

The ICs and resistors appear to be all in the same positions. The decoupling capacitors look to have been replaced by the bus bars (that usually contain integrated decoupling capacitors).

I would be highly surprised if the ICs were not a carbon copy of the original...

Dave
 
I agree that the ICs and placement seem way too similar. It is clear that the PCB is routed differently so not a direct copy of the PCB artwork.

Since you can't expand memory beyond 4k without an M837 (or an M8317 in an 8/a), was this one of those situations where DEC would not sell an M837 unless you also bought some memory with it? I could see this happening which would have forced Plessey into cloning the M837. All that is just conjecture. I don't remember anything like that but then I never bought a computer from DEC. Anyone know if something like that was DEC policy? It is also possible that Plessey didn't want you talking to DEC at all if you were buying a memory upgrade so they made an M837 clone for that initial bump beyond 4k.

I find it interesting that in the photo of the 8/e trying to boot OS/8 it was sitting on address 17645. If the address was 07645 it would be sitting in the SYS handler, probably in an IOT wait loop. 17645 is ijust before the field 1 resident portion of OS/8. What device were you trying to boot from? The boot sector contains three pieces of code. The first is the secondary boot loader. The second is the field 1 OS/8 resident code that resides from 17647 to 17777 and the third is the field 0 OS/8 resident code most of which is the SYS handler that ends up from 07600 to 07777. The secondary boot will place the OS/8 resident pieces in their appropriate places and then do a return to OS/8. There are several ways the secondary bootloader did this depending on the device. If a data break device then the whole boot block would end up in memory and the boot loader would need to copy the pieces into their correct locations. If something like the RX01 then the sectors could be placed directly where they are supposed to be. It does not seem right to me that it should be looping at 17645 on a fresh OS/8 boot since no OS/8 code should be there yet.

Run the diags and see what they say about it. You don't need to run the portion for the Time Share part unless you are going to run TSS. I believe that requires you to change a jumper.
 
DougIngraham said:
Run the diags and see what they say about it. You don't need to run the portion for the Time Share part unless you are going to run TSS. I believe that requires you to change a jumper.
Click to expand...
I would also look at the USER status light and make sure the M837 clone doesn't think you are in user mode. That's actually a failure mode for the M837 that I've seen before.
 
I made a comparison of the chips and yes, the IC positions are the same. IC types are:
Plessey DEC
74H00 N7400A
74H10 N7410A
74H20 N7420A
74H74 N7474A
DM74H04N N7404A
N8235B 8235B DEC
N8251B 8251B DEC
N8271B 8271 DEC
N8881A 8881 DEC
N8T380A 6380A DEC
RC8266MP DEC 8266
SP314A 384A DEC
SP384A 384A DEC

Maindecs hopefully tomorrow.....
 
Meanwhile I'm a few steps further, but still not ready. (I'm too slow)
PM837:
The Plessey PM837 was able to change the fields, but it was not able to do a RIB (Read Interrupt Buffer). A simple loop LAS-RIB-JMP showed that not all bits come into AC (AC5-11). This looked different with a working M837. The IC was E-3 a Signetics 8271. I changed this.
But the PM837 still did not work. The MAINDEC. 8E-D1HA-PB even did not halt at 3651. It looped somewhere. After checking suddely the behaviour changed and now it stops at different places when starting. From the MAINDEC pdf this means there is something wrong in the Time Sharing Part of the card.

M837:
I also have a further defect M837. It was not able to change any field. I found E-53 7410 to be the reason. After that It was possible to change fields. When starting the MAINDEC it stops at 3651, but after pressing CONT it needs some seconds, than it loops in field 5: 7745 - 6262 (very strange)
I was unsure where to locate the problem, so I filmed the PC when pressing CONT in the MAINDEC. So I found the problem starts in TEST 02. Stepping through the code showed:
After changing a DATA Field, a first indirect DCA leads to loading the datafield number into EMA. This does not happen with a working M837. E-1 ist the 8235, putting the Data on the EMA lines.

need more time...
 
Last time I bring that thread to the top, yesterday I spend some hours and found a second bad IC. An 74H04 (E27). It did not invert a signal, that lead to some strange things.

With this replaced, the MAINDEC-8E-D1HA runs fine. Both with and without the jumper for timeshare. Other than the DEC M837, the standard position is without jumper in timeshare position. I soldered in a jumper to test without timeshare.

With the maindec there is a strange thing, if you start it with fewer memory fields than available, it runs into errors. After I changed the start SR9-11 from 2 to 7, to test all available fields, it runs without errors and stops at 1565 (End Of Test) when SR1=1.

Happy to have this card running again, OS/8 starts well from TU-56 (TD8E) in my 8/m!
 
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