DEC PDP-11/53: New CPU Board

The chap who sold me some of the PDP parts was kind enough to let me try another KDJ11-D CPU board that he had, since I thought that the RAM was dead on the first one.  Here it is:

New CPU Board

This one works!  Well, in reality it also has issues ... if I try to use all 1.5 Mb of RAM it also fails the self-test on startup.  So I tried setting the W25 jumper as documented here.  With that jumper in place it seems to work fine, although it means that I'm limited to 512k of memory.  So I imagine that the problem must be in the upper megabyte.  It would seem that these old RAM chips have a tendency to die.  However, it should be good enough for me to continue moving forward.  Although at some point it would be nice to have a fully working KDJ11-D (M7554) in my machine.  I must keep an eye out for that.

microPDP-11/53 - you win some, you lose some

After some fiddling and consulting of the KDJ11-D (CPU card) manual I've come to the conclusion that the message I was getting is nothing to do with boot failure, but a failure of the RAM chips on the board.  If I start the machine with the HALT button pressed I can get into ODT (Octal Debugging Technique) and this should let me set values in memory directly.  You just type the memory location and a forward slash / and the system tells you what is in memory there.  You can then enter a new value.  Typing the same location again with a slash should give you the value you have just entered.  However, with my machine the values I put in seem to get garbled.  This is a real setback, since my soldering skills are not good enough to replace all these RAM chips (there are 54 of them).  But still I have found out a lot about ODT, and that is a good thing.

However, I can ignore these memory errors by pressing <CTRL+O>4<Enter>, this allows you to carry on regardless.  So I installed the RQDX3 disk controller in slot two of the backplane and started the machine again, this time ignoring the RAM error and attempting to boot, but without a disk drive installed.  This time you would expect a different message - I imagine that it should find the disk controller but complain that there is no disk drive.  Here is the transcript of what I got:

KDJ11-D/S   4.55
Error, see troubleshooting section in Owner's manual for assistance
RAM    VPC=024454  PA=17604454  00000000/104676 <> 177776

KDJ11-D/S> 4

Commands are Help, Boot, List, Map, Test and Wrap.
Type a command then press the RETURN key: BOOT DU0

KDJ11-D/S   E.11
Drive error

Commands are Help, Boot, List, Map, Test and Wrap.
Type a command then press the RETURN key:

Not bad!  It looks like it does indeed find the RQDX3 card and attempt to boot.  If I install my RX33 floppy disk drive, I wonder if I'll get a slightly different message?  That's got to be the next thing...