PDP8/a Initial Power Up

Gordon Henderson gordon+cctalk at drogon.net
Fri Nov 2 12:43:40 CDT 2018

On Fri, 2 Nov 2018, Columbia Valley Maker Space via cctalk wrote:

> Hello everyone - my first post, so be easy on me!
> I have just acquired a PDP8/a and a Remex punch tape reader. The unit
> starts up and displays some data on the displays, and that is about all
> I can tell you.
> I am going to do some googling about this, but I am looking for basic
> initial tests .... something I can enter via the keypad.
> I learnt some basic programming in 1978 on a PDP8, but that was the last
> time I touched one, so if you are going to suggest some tests, I need
> complete instructions. I don't know how to modify a memory location, let
> alone enter and check a program. I will pick all this back up very
> quickly and I do use computers in my work a lot - I am also an
> electronic hobby guy and have been for years. My point is I am OK with
> component level measurements, I have a scope and probes, etc.
> So there you go - hope to hear back form you guys.

So: power on. Power key up, boot & panel-lock down.

Display memory:


You should see the MD LED light up.

Enter an address:


then LA then E-THIS

That will give you the value of location 0200.

Enter a number - e.g. 1234 then D-Next will enter that number into the 
displayed address and advance the program counter by one.

Simple program:

     2        0100          *100
     3 00100  0000  COUNTER,        0
     5        0200          *0200
     6 00200  7300          CLA CLL
     7 00201  3100          dca     COUNTER
     9              DELAY,
    10 00202  2100          ISZ     COUNTER
    11 00203  5202          JMP     DELAY
    13 00204  7001          iac
    14 00205  5202          JMP     DELAY

Numbers in the 3rd column are the ones you enter in.

So MD DISP, then:

   0000 LXA
   0200 LA
 	7300 D-Next
         3100 D-Next
 	2100 D-Next
 	5202 D-Next
 	7001 D-Next
 	5202 D-Next

Check with

   0200 LA
   D-This (Should see 7300), then D-Next, etc.


   0000 LXA
   0200 LA

then display the accumulator (AC DISP)

and you should see it counting in octal

It was probably controlling a CNC machine until recently - that 
combination was very popular and what mine was used for until it
was decomissioned only a few years back. There are still some
in-use today!



I don't have the tape drive, and I did a one-wire mod to the IO board to 
allow for TTL level serial out so I could connect it to a TTL serial USB 
adapter on a Pi.

Always check the display mode - make sure in MD mode when changing or 
looking at memory.

Have fun, and good luck!


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