UniBone: Linux-to-DEC-UNIBUS-bridge, year #1

[Noel Chiappa]

    > From: Rob Doyle

    > http://www.bitsavers.org/pdf/dec/unibus/RH11-C_Engineering_Drawings.pdf

Oooh, thanks ever so much. Not sure how I missed that when I looked on
BitSavers for RH11 stuff! Very illuminating - eventually! The M7294-YA seems
to be a manual ECO to the M7294; there's a detailed rework list on page 6 of
the PDF.

I'm still trying to work out what the changes do. E66 is a 'component
carrier' header, so it seems like in part the ECO adds a bunch of
option-controlling jumpers there (see pg. 2 for a table of what they do).

The main thing, though, seems to be the addition of E22, a 74191 binary
up/down counter, on page DBCA (pg 10 of the PDF). It seems to modify the
operation of 'Bus Hog' mode - maybe to do 16-cycle bursts? (All the
bit inputs and outputs are unused; only the Max/Min output - pin 12 -
is connected to anything.)

That would make sense; with UNIBUS A and B tied together, the original Bus
Hog (below) would lock out the CPU from the RH11 until the end of the
transfer. Actually, though, even without the cross-connect, having
the RH11 going flat out Bus Hog might lock out the CPU from the _KS10
memory_...

    > However there is plenty of DEC documentation that mentions that the
    > RH11C has a "bus hog" mode for the KS10 disks so that the Unibus can do
    > back-to-back 18-bit transactions.

The RH11-AB has Bus Hog too; see Section 4.12.10, "BUS HOG Mode", pg. 4-22
(59 of the PDF) in "RH11-AB Option Description" for details; it "hold[s] the
Unibus ... until the required number of words have been transferred".

	Noel