Subject: Re: My second PDP8

From: [email-masked] (PDP11 Hacker .....)
Date: Mon, 2 May 1994 16:17:00 GMT

In article <[email-masked]>, [email-masked] (Douglas W. Jones,201H MLH,3193350740,3193382879) writes...
>From article <[email-masked]>,
>by [email-masked] (PDP11 Hacker .....):
>> 
>> Now, I've not even powered the machine up yet - is there anything I
>> should know before I do.
> 
>Welcome to the club!  My suggested sequence for bringing your new machine
>up:

Well, I've now powered it up, and after a little work, I've got it to run the
'Accumulator Incrementing test' in the front of the maintenance manual. I guess
that means that most of the machine is working.

> 
>1) Pull the boards from the backplane and dust it!  Vacuum cleaner,
>  compressed air, etc are all good.

Yes, did that. Also ripped off the decaying fan filter, on the grounds it would
add more dust than it removed.

> 
>2) Check the power supply output voltages, and while you're at it, check
>  that the cooling fans all spin under power.

Opened up PSU, reseated A1 and A2 control modules, checked wiring, connected
voltmeter to +5V line, powered up. +5V line spot on. Then checked the +15/-15
lines. Forgot at this point to check the Power OK line, see below.

> 
>3) Put in the bus terminator and front panel, then power it up and verify
>  that you've got minimal functionality (not much).  Basically, verify that
>  you have a good backplane between terminator and front panel, and that
>  the panel has some working lights and switches.

At this point I added the CPU (3 boards - timing, regs, reg control), Bus
loads, front panel. Found I could load address, but nothing else. BTW, my front
panel has LEDs (like an 8/m), not bulbs, but I have the H724 PSU. The machine
says PDP8/e on the front. So, I had to do some debugging. First place was on
the console board, where I checked the correct control signals were being
generated on an Examine. Still no luck - the signals were right, and the
address still did not increment. Then I put the timing board on an extender,
and looked at the run flip-flop signals with a logic probe. The Set input was
being pulsed, but there was no latching action, and the timing generator was
not starting. Aha, the clear input is stuck low. Why? Because the Power OK
signal from the PSU is low. (This was being done in the early hours of the
morning, and was nowhere near as quick as I'm making it out to be - I was
digging through a lot of manuals and logic). So, a PSU problem. Out came the
PSU, and I put the A2 board on a single extender from my PDP12 spares box
(:-)). A bit of work with a voltmeter revealed that pin 6 of the module was at
0V. So, I thought the fault was on the A1 module, and for some reason the -15V
supply was thought to be faulty (the -15V line was fine). Now, I put the A1
module on the extender, and spend a long time digging through the circuitry, to
find nothing (Incidentally, my copy of the 8e printset does not contain layouts
of the A1,A2 PSU modules - should it?). In the end, I checked with an ohmeter,
and found that pin 6 of the modules (The power fail link between them) was
shorted to ground. After removing the edge connectors, I found a trapped wire.
So, I repositioned it, reassembled the PSU module (The screws in the edge
connector assembly are impossible to get at !!!), and tested again. Power OK
now at 4.5V. Refitted PSU, tried again. Examine/deposit now increment address

> 
>4) Power down, add one memory module, and add the master CPU clock module.
>  You can now load and store in memory from the lights and switches.  Test
>  a bit of memory (can you store 7777 0000 5252 2525 in 4 successive
>  locations?)

At this point, I added all the memory - it was on a separate backplane,  so I
moved the Bus Loads into the rearmost slot, and fitted a couple of M935 (?)
jumpers.

> 
>5) Power down, add the CPU, then toggle in a tiny program, some classic
>  like LOOP,IAC; JMP LOOP.  It should run.

My CPU was already in, so I toggled in the CPU test (4 instructions, can't
remember what) from the maintenance manual. It worked!

> 
>6) Drop in an asynch port, add an asynch terminal, and toggle in some
>  code to echo what you type.

Added all my other modules, and checked that the program still ran. It did. No
tests on the other stuff (like serial ports, disks) yet.

> 
>7) etc.
> 
>On my surplus PDP-8/F, I had to start fixing things at step 2, 3, and 6.
>Good luck!
>				Doug Jones
>				[email-masked]

Well, my problems were mechanical(ish), but it now seems to work.

-tony

Bristol University takes no responsibility for the views expressed in this
posting. They are the personal views of the user concerned.

---

From: [email-masked] (Douglas W. Jones,201H MLH,3193350740,3193382879)
Date: 2 May 1994 03:32:50 GMT

7) etc.

---

From: [email-masked] (PDP11 Hacker .....)
Date: Fri, 29 Apr 1994 23:37:00 GMT

As mentioned in my previous posting, I could have obtained a PDP8e. Well, I've
now got it, and I'm in love.... No, I don't know much about it yet, but it came
with a lot of circuit diagrams (for the PDP8e CPU, Extended address option,
EAE, memory, serial port, RK8, etc. No, I don't have all that lot). What I seem
to have is :
PDP8e CPU, extended address, 3 serial ports, 32K(? 4 sets of I think 8K) Core,
printer(?) port, line clock, RK8. Also, spare RK8, and some other boards.
Manuals : printsets for most of that, PDP8e maintenance manual (all 3 volumes
!!), assorted user guides and... (Don't all jump on me at once) ... An almost
complete set of _PDP12_ maintenance manuals... (No, they're not up for grabs).

Now, I've not even powered the machine up yet - is there anything I should know
before I do. I was intending to test the PSU off-load before I connect any
logic to it, but is there anything else to be aware of?

-tony