BDV11 'mis-feature'

Noel Chiappa jnc at
Fri Feb 19 10:08:31 CST 2016

So I recently discovered that the LTC feature on the BDV11 bootstrap board
has a minor issue: the latch that stores the 'LTC enabled' bit is _not_
cleared by INIT (unlike every other PDP-11 device I've ever heard of), but
only by a direct store into the LTC CSR, _or_ power cycling (BDCOK, to be

This means that once you turn the LTC on using the BDV11, neither an INIT
instruction, nor a 'Start' command to ODT, will disable it! Needless to say,
it tends to scramble the booting process when an LTC interrupt shows up
before the software is ready for one...

I _was_ going to ECO the board, to connect BINIT to the clear input of the
latch, but... I now think I know why DEC did this. There is no bus receiver
on BINIT! (And there is, alas, no empty DIP space I could put one in.)

I'm seriously tempted to connect the 'reset' circuitry on the BDV11 to BINIT,
instead of BDCOK. That would mean that hitting the reset switch on the BDV11
wouldn't emulate a power cycle any more, but maybe I could live with that - I
never use that switch anyway.

And it would be _so_ nice to have 'Start' disable the LTC...


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