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Thread: Looking for PDP-11/05 CPU card M7261

  1. #1

    Default Looking for PDP-11/05 CPU card M7261

    I hauled out my old 11/05 last month, to use with a UNIBONE. Yesterday, it started refusing to store or retrieve anything in bit 0. I took out all the memory and wrote test programs using the registers - still can't access bit 0 (and it takes some effort to write test programs that don't use bit 0, since a lot of addressing modes work off of the PC, which has a 1 set in bit 0 in the destination field). I swapped out M7260 with a spare - no change. I swapped out M7261 with a spare - the spare worked worse than the first one - it screws up so radically I can't even describe its behavior.

    So, I'm thinking, M7261 is bad. Anyone seen this particular problem before? More important - anyone have an M7261 they'd be willing to sell?

  2. #2

    Default

    When you say you can't store or retrieve anything in bit 0, I assume you can load bit 0 in the address register? Is bit 0 bad in all the processor registers (177700 through 177707)?

    Probably not too hard to diagnose if you have a scope. I was able to find and replace a couple of bad transistors on my G110 using a simple dual input scope.

    Where are you located? I don't have a spare M7261, but I could loan you one if you happen to be in the bay area.

    --Jay

  3. #3

    Default

    Quote Originally Posted by leegleason View Post
    I hauled out my old 11/05 last month, to use with a UNIBONE. Yesterday, it started refusing to store or retrieve anything in bit 0. I took out all the memory and wrote test programs using the registers - still can't access bit 0 (and it takes some effort to write test programs that don't use bit 0, since a lot of addressing modes work off of the PC, which has a 1 set in bit 0 in the destination field). I swapped out M7260 with a spare - no change. I swapped out M7261 with a spare - the spare worked worse than the first one - it screws up so radically I can't even describe its behavior.
    The 11/05 is quite easy to diagnose. You would probably need extender boards to do that. If bit 0 is missing all the time you should be able to trace it along its way around the machine and find where it disappears. Bus receiver / transmitters is very good to check early on. The schematic and the tech manual are excellent in describing how the machine works.

    Quote Originally Posted by leegleason View Post
    So, I'm thinking, M7261 is bad. Anyone seen this particular problem before? More important - anyone have an M7261 they'd be willing to sell?
    This is not mine and maybe you are not is Europe. And the price is high, but maybe the seller would consider an offer. https://www.ebay.de/itm/184061082005

  4. #4

    Default

    Quote Originally Posted by leegleason View Post
    I hauled out my old 11/05 last month, to use with a UNIBONE. Yesterday, it started refusing to store or retrieve anything in bit 0. I took out all the memory and wrote test programs using the registers - still can't access bit 0 (and it takes some effort to write test programs that don't use bit 0, since a lot of addressing modes work off of the PC, which has a 1 set in bit 0 in the destination field). I swapped out M7260 with a spare - no change. I swapped out M7261 with a spare - the spare worked worse than the first one - it screws up so radically I can't even describe its behavior.

    So, I'm thinking, M7261 is bad. Anyone seen this particular problem before? More important - anyone have an M7261 they'd be willing to sell?
    Have also some dead '05 boards here, never found time to look at.
    Is bit 0 also dead on console EXAM/DEPOSIT with stopped CPU?
    A constant dead bit should be debuggable, if you microstep your test program or console operation.
    You'd need a KM11 maintenance panel, can be build from http://www.retrocmp.com/tools/dec-km...ntenance-panel
    or otherwise. We also made Extender PCBs here, only flip chip sockets are difficult to get.

    best regards,
    Joerg

  5. #5

    Default

    "When you say you can't store or retrieve anything in bit 0, I assume you can load bit 0 in the address register? Is bit 0 bad in all the processor registers (177700 through 177707)?"

    I can use the console to address odd addresses (well, the registers are the only places you can examine an odd address) fine - I can examine & deposit to R1 at 177001. However - bit 0 can't be set in the special register locations just like it can't in regular memory.

    "Where are you located? I don't have a spare M7261, but I could loan you one if you happen to be in the bay area." THanks for the offer, but I'm in Houston - too far to bother.

  6. #6

    Default

    "The 11/05 is quite easy to diagnose. You would probably need extender boards to do that. If bit 0 is missing all the time you should be able to trace it along its way around the machine and find where it disappears. Bus receiver / transmitters is very good to check early on. "

    Yes, I see a lot of scope time in my future - the few replacement cards I've seen are all too expensive for a hobby machine. An extender would make it easier, but I think I can get by with solder tacking wire wrap wire at likely locations - slow and messy, but, effective.

  7. #7
    Join Date
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    PM sent.

    Pete

  8. #8

    Default Joerg's KM11 Boards

    Quote Originally Posted by Joerg Hoppe View Post
    Have also some dead '05 boards here, never found time to look at.
    Is bit 0 also dead on console EXAM/DEPOSIT with stopped CPU?
    A constant dead bit should be debuggable, if you microstep your test program or console operation.
    You'd need a KM11 maintenance panel, can be build from http://www.retrocmp.com/tools/dec-km...ntenance-panel
    or otherwise. We also made Extender PCBs here, only flip chip sockets are difficult to get.

    best regards,
    Joerg
    I'm going to be ordering some of Joerg's KM11 maintenance panels from PCBCart in China.

    I'll post again when I get them in.

    Thank you!

    Todd

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