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Thread: PDP8/m fix before use...

  1. #1

    Default PDP8/m fix before use...

    It has been a year ago when I used my 8m so this weekend I've connected it again.
    Would it still run? Well, no. I could not get any data in or even select an memory address.
    Well, check the power first. I knew this was a weak point in this machine. The 5V was
    nicely 5V on the power but about 4.6V at the backplane.

    Okay that must be that weak and fried connector on the power supply. I've been a bit rude here...
    To prevent any problems again I've soldered an extra GND and 5V wire onto the power PCB and put that
    in parallel with the original wires to the backplane. The backplane already has two fastons for each
    power supply. Why not use both? Now the 5V is happy again!

    2.jpg 3.jpg

    But it didn't change a thing. Still not possible to toggle anything in.
    Even the address load did not work at all. The cause was the deposit switch.
    All switches were 0.1 Ohm in the on state, the deposit was in meg ohms...
    Dirt... After cleaning yess... It runs again...

    1.jpg

    Well, I got OS8 booting trough the OS8 disk server. But there is something strange here.
    All the text seems to be normal, but the dot is presented as three characters.
    And everything which I type in shows up as currupted data. But if I type DIR and press
    enter the directory listing shows up very nice. But that dot again is scrambled.

    4.jpg start OS8.jpg

    I really thought that it is a serial port setting which is not good.
    But why does all the other text show up normally? it is just set as
    2400 Baud, 8 bit No parity and 2 stop bits... I've used another PC, another
    terminal program, swapped the serial card... But it stays the same...
    Even put in the echo program and all characters show up nicely...

    I'm probably laughing about this in a week because I've overlooked
    something very stupid... But now I'm thinking about to run the maindecs...
    WTB: Dec TU56 / Case for Altair 8800

  2. #2
    Join Date
    Jun 2012
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    UK - Worcester
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    Default

    My guess would be memory or disk corruption somewhere. It looks like the prompt 'character' has become corrupt. I wonder if they use a string-based output subroutine to issue the prompt '.' And something has corrupted memory?

    Dave

  3. #3

    Default

    Hi Dave,

    I thought also in this direction. I've used another disk image on the
    OS8 server and swapped the addresses from the two 8K memory boards.
    But it stays the same. I swapped the CPU boards and timeshare PCB's as well.
    Reloaded OS8 and it stays the same. The only cards which I didn't change
    out are the serial port to the disk server and the busloads.

    Strange enough it is not only that dot which is damaged. Also the text
    which I type shows up corrupted on the screen...

    But it arrives good at the PDP because when I type DIR -enter- I get the directory listing.
    WTB: Dec TU56 / Case for Altair 8800

  4. #4

    Default

    So I've wiped off all the dust of my VT220 in the basement, set it to 2400 baud.
    And guess what? It works great on the PDP.

    So there is no problem with the machine but it is a compatibility problem between the
    three PC's I've tried and the PDP... Quite stupid, I've used the PDPs on a PC many times.

    Edit:

    That little bastard... Ascii was expanded with an extra bit from 7 to 8 bits.
    If I set the terminal program (tera term) to 7 bit the Text shows up normally. (just slice that bit off)
    the three weird characters are just like linefeed, carriage return and a dot, but with an extra bit set.

    I used to use GTTY but that won't run on Windows 10 any more.
    But even hyperterminal on my windows 98 laptop used that expanded ascii table.

    So my first guess was right, it was a serial communication problem.
    But I don't know how to fix this yet. it works for txt but I want to sent
    papertape files as well of course...
    Last edited by Roland Huisman; October 7th, 2018 at 04:07 AM.
    WTB: Dec TU56 / Case for Altair 8800

  5. #5
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
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    Default

    The old ones are the best ! Glad you're operational again though...

    Dave

  6. #6

    Default

    I use to set the terminal settings to 8N1 when I transfer paper tape files. And 7M1 when I use the PC as terminal (parity bit as MARK). And yes, I have done what you just did and spent hours to trouble shoot.

  7. #7

    Default

    Quote Originally Posted by anders_bzn View Post
    I use to set the terminal settings to 8N1 when I transfer paper tape files. And 7M1 when I use the PC as terminal (parity bit as MARK). And yes, I have done what you just did and spent hours to trouble shoot.
    Yeah it took me some time to figure it out too. And I'm also swapping between these settings now.

    The GTTY96a program which I used to use seems to do this in the right way already. But it is an old MS-dos program.
    I found the name of Jack Rubin in the source code and I've asked Jack if there is a new version of it.
    He tried to modify and compile it for Windows but the differences are a bit to big, between MS-Dos
    and Windows, to do this without too much work... He asked some people if they can upgrade it. I hope so...

    But still I think it is strange that the PDP sends it's txt in a different way then the command prompt...

    Regards, Roland
    WTB: Dec TU56 / Case for Altair 8800

  8. #8
    Join Date
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    Location
    Wilmette, IL (north of Chicago)
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    Default

    Just to be clear, Lyle Bickley wrote GTTY - glass teletype - using Dave Dunfield's micro-C compiler. It's a nice lightweight terminal program for the PDP-8 that does handy stuff like automatic upper-casing and swapping between 7M1 and 8N1 serial protocols as you move between terminal and binary uploads. It was written to run under DOS and isn't quite ready for the 64-bit world.

    Hopefully, a gcc version will be emerging soon(ish).

  9. #9

    Default

    If you don't find a happy compiler, I can most likely hack something together in Forth. I had to give up using my F-PC Forth, also a 16 bit based. I'm now using Win32-Forth. It still runs on a 64 bit machine. I have several terminal programs that could be easily modified to meet what you'd want for the 8m.
    Things like Enter to ????, BackSpace to ???? and how you'd like to enter file names ( text, binaries and possibly .HEX ). I can connect F keys to special functions as well.
    Of course, If someone would like to give me a 8m, I could make much faster progress
    Dwight

  10. #10

    Default

    Quote Originally Posted by Dwight Elvey View Post
    If you don't find a happy compiler, I can most likely hack something together in Forth. I had to give up using my F-PC Forth, also a 16 bit based. I'm now using Win32-Forth. It still runs on a 64 bit machine. I have several terminal programs that could be easily modified to meet what you'd want for the 8m.
    Things like Enter to ????, BackSpace to ???? and how you'd like to enter file names ( text, binaries and possibly .HEX ). I can connect F keys to special functions as well.
    To be honest, I had to run some main dec's tonight and I turned on an old computer to run GTTY.
    Some maindec's immediately give a text reply when the bin is loaded. Then it's nice that a terminal program
    like GTTY switches over immediately. I don't know of any other TTY program that can use
    a different setting for sending and receiving data on the same serial port.

    So a good terminal program on a modern Windows 64 bit machine dedicated for the PDP's would be very appreciated
    I'm more a low level programmer on small hardware. I've got no useful experience to make Windows programs.

    Quote Originally Posted by Dwight Elvey View Post
    Of course, If someone would like to give me a 8m, I could make much faster progress Dwight
    I thought every US collector had one or more PDP8 machines in their collections?
    WTB: Dec TU56 / Case for Altair 8800

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