RetroHacker_
Veteran Member
I have a PDP-11/05 which won't deposit into memory from the front panel. I can examine, and I can run code already in memory, but I can't deposit. Toggling the deposit switch will display the correct value on the lights, and will increment the address, but nothing ever actually gets written to memory. If you go back and check, the memory location is unchanged.
Fortunately, I have a friend who recently acquired a working machine. So, the problem has been narrowed down to the Control board (M7261) - one half of the CPU. The memory and the front panel itself is fine. The computer works properly with the good control board from the other machine, and the other machine exhibits the same problem when the bad control board is installed.
I can install the good control board in this machine, toggle in a program, shut it off, switch the bad board back in, fire it back up, and run the program with the bad board. This has proven that the computer *can* write to memory with the bad board installed, from program control. It just can't do it from the front panel. And, from what I can tell, instructions all appear to work with this board.
Attempts to troubleshoot the bad control board haven't been terribly fruitful so far. Lacking card extenders, it's very difficult to probe or test signals. Thanks to some creative cabling and a logic probe, I've been able to verify that the basic circuitry driven by the deposit switch - a couple flip flops and an inverter (E97-99 on the board) all seems to work. Further than that though, I'm not really sure. The actual console switch register is on the data path board - not this one.
Anyone have any suggestions or pointers? I've got a headache from trying to comprehend the nomenclature on the DEC schematics - and from trying to read the many-generation photocopy that was scanned for bitsavers. I'm not really sure where some of these signals go, and I don't fully understand what's supposed to happen when that deposit switch gets flipped. What signal actually triggers copying the console switch register into memory?
And, of course, failing repair, does anyone happen to have an extra M7261?
-Ian
Fortunately, I have a friend who recently acquired a working machine. So, the problem has been narrowed down to the Control board (M7261) - one half of the CPU. The memory and the front panel itself is fine. The computer works properly with the good control board from the other machine, and the other machine exhibits the same problem when the bad control board is installed.
I can install the good control board in this machine, toggle in a program, shut it off, switch the bad board back in, fire it back up, and run the program with the bad board. This has proven that the computer *can* write to memory with the bad board installed, from program control. It just can't do it from the front panel. And, from what I can tell, instructions all appear to work with this board.
Attempts to troubleshoot the bad control board haven't been terribly fruitful so far. Lacking card extenders, it's very difficult to probe or test signals. Thanks to some creative cabling and a logic probe, I've been able to verify that the basic circuitry driven by the deposit switch - a couple flip flops and an inverter (E97-99 on the board) all seems to work. Further than that though, I'm not really sure. The actual console switch register is on the data path board - not this one.
Anyone have any suggestions or pointers? I've got a headache from trying to comprehend the nomenclature on the DEC schematics - and from trying to read the many-generation photocopy that was scanned for bitsavers. I'm not really sure where some of these signals go, and I don't fully understand what's supposed to happen when that deposit switch gets flipped. What signal actually triggers copying the console switch register into memory?
And, of course, failing repair, does anyone happen to have an extra M7261?
-Ian