Help with a busted MSV11-L?

Noel Chiappa jnc at
Sun Dec 7 07:22:01 CST 2014

Hi, all, I'm trying to fix a broken MSV11-L card (M8059-KF). The symptom is
that it always reads back with the 020 bit set, but otherwise appears to be
fine. (E.g. you can write a number in 0, and a different number in 020, and
each reads back OK, with the exception that the 020 bit in each is always
set; i.e. the addressing is working fine, as is the QBUS generally.)

So poking around with my 'scope, I find that the latched D04 signal ("LAT D04
H") is always around 2V - not good! From the prints:

it appears to be generated on page 6 of the prints (page 10 of that file), as
the wired-or output of two octal 3-state-output latches (E57 and E64, an
'LS373 and an 'S373 respectively). The only place it appears to go is to the
quad bus transceiver (E62, an 8641), on page 2.

My question is: any guesses as to which of the three is causing this? Or do I
just have to start replacing them until I find the guilty party? :-) If so,
what's the best order to do so in? FWIW, the two octal latches are both TI
Malaysia parts (date code 8231), and the transceiver is a Motorola (date code

Thanks in advance for any help!


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