Deciphering M792-YB "ROM Diode Matrix" PDP-11 bootstrap

From: Ethan Dicks <ethan_dicks_at_yahoo.com>
Date: Thu Oct 21 11:31:16 1999

In my ongoing excavations, I've located an orphan board that I would like to
return to its nest. It's a diode-strapped boot card, M792-YB. The pattern
looks like this...


- - - - - - - - - - - - - - - -
- o o o o o - - - - - o - o o o 0
o o o - - - - o - - - - - - - -
- o - o o o o o o o o o o o o o
o o o o o o - o o o o o - o o o
o o o - o o - - - o - o - o o o 10
o o o o o o o o - - - - - - - -
- - - o - o o o o o o o o - o o
o o - o o - - - o - - - - - - -
- - - o o o o o o - o o o o o o 20
o o o o o - - - - o - o - o o o
o - o o o o o o o o o - o o o o
o o o - o o - - - - o - o o o o
o - - - - - - - o o o o o o o - 30
o o o o o - - - - - o - o o o o
- - o o - - - - o o o o o o o - W
o o o o - o o o o o - o o - o o O
o o o o o - - - - o - o - o o o R
- o - o o o o o o o o o o o o o D
o o o - o o - - - - o - o o o -
o - - - - - - - o o o o o o o -
o o o - o o - - - - o - o o o o 50
o o - - o - - - - o o o o o o -
o o o - o o o o o - o - o o o -
- - - - - o - o o o o o o o o o
o o o o o o o o o o o o o o o o 60
- o o o o o o o o o o o o o o o
- - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - - 70
- - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - -

0 BIT 15

(- = diode installed o = diode missing)

The question is, What's a one and what's a
zero? Left-to-right? I don't have an -11
frontpanel or handbook in front of me; is
bit 0 the MSB?

Any hints? Worst case, I could translate all permutations of the bits
into octal and figure out what produces the most legible bitstream, but
I'd rather go into this with a little foreknowledge.

The eventual goal is to disassemble the bootstrap, thus revealing what
device it is for.

Thanks,

-ethan


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Received on Thu Oct 21 1999 - 11:31:16 BST

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