27 May
2012
27 May
'12
8:49 a.m.
Date: Sat, 26 May 2012 15:53:35 -0700 (PDT)
From: Mr Ian Primus <ian_primus at yahoo.com>
To: cctalk at classiccmp.org
Subject: PDP-11/05 Control (M7261) won't deposit
Message-ID:
<1338072815.56383.YahooMailClassic at web121606.mail.ne1.yahoo.com>
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Working on a PDP-11/05. The front panel appears to work, I can step through and examine
addresses in memory, and I can run the CPU and halt it. Deposit, however, does NOT work.
When the deposit switch is flipped, the LED's show the correct value, but examining
the address again shows no change to memory.
I know that this board is the problem, having swapped in a module from a working
computer. I can, with the good board, toggle a test program into core. Then, shut it off,
switch in the bad board, and the program (still in core) runs, and I can examine all areas
of memory and see it correctly.
I've only begun to glance at the schematics for the control board, but I would assume
there is some kind of write buffer that's not working correctly. I'm going to
continue to try to figure this out, but without a card extender, it's very difficult
to troubleshoot
Well, it is not likely to be the buffer (register) as this would require
16 bad FF's. Does the address
increment when you hit deposit? If so, that means the switch is being
sensed. So, the problem
is likely the write command is not getting to the memory. There are a
bunch of ways to
make a front panel work, but since this is a raw logic machine (no
microcode) they probably
worked hard to minimize the logic needed. So, probably the deposit and
exam logic
share a lot of functions, but there is probably a FF that is set for
deposit and cleared
for exam, and then feeds through some gates to perform the memory read
or write.
Something in that chain of logic is most likely where the problem is.
Jon