Subject: Re: Me and my PDP... (long)

From: [email-masked] (Charles Lasner)
Date: Wed, 2 Dec 1992 14:09:17 GMT

In article <[email-masked]> [email-masked] (Lars Persson) writes:
>
>Here the two major bus types. UNIBUS and QBUS. The UNIBUS is a
>replacement of the OMNIBUS (!) from the forerunner the PDP8. The UNIBUS

Wrong:

PDP-8 Positive buss: 1968.  Cables between wire-wrapped assemblies.  The cables
are coax or flat ribbon only if run short.  Buss is always synchronous.

UNIBUS for 11/20 et al. 1969 (in prototypes).  Wide flat ribbon cables between
wire-wrapped assemblies.  Buss is asynchronous to allow longer lengths.

OMNIBUS for PDP-8/e et al. 1970.  Buss is made from the same module blocks as
the other busses but has a printed circuit board to interconnect all fingers
in all slots; there are *no* exceptions, thus no dedicated slots (save the
fact that the CPU stuff has to be at one end and the terminator at the other
end more or less; newer configurations could even redefine which end was the
"front").  Total buss length is 20 slots of module blocks.  Similar to UNIBUS
M920 buss jumpers (for short UNIBUS runs) are the M935 OMNIBUS jumpers which
of course jumper all fingers (M920 do not; typical for UNIBUS it has exception
cases and jumpered fingers, etc.).  Two M935's are used to extend the buss
when it is housed in the PDP-8/e box to a total of 40 slots (38 due to the
overhead of the M935's themselves).  (The smaller PDP-8/f and /m cannot be
extended as such.)  Several years later, the first attempt to expand this
to a second box using BC08H cables.  (BC08H is electrically an M935 only
it looks like a UNIBUS cable, because all of these are made from the same
wide flat stiff ribbon cable; again, BC08H is one-for-one on connection
fingers whereas UNIBUS is not; had DEC rethought the implication of the
few inconsistencies of the UNIBUS here, there would not have been two similar
families of cables.)  Many ECO's later, the attempt is actually viable thus
allowing *reliable* operation of two boxes connected by cables.  OMNIBUS is
synchronous, which is part of the reason for all the problems with extending
it.  Later versions appear (PDP-8/a) with various similar cabling systems
between boxes.  The boxes get smaller (10,12,20 maximum) as the peripherals
get smaller (but likely packaged on hex boards to give a theoretical boost
of "real estate" of 50% if applicable, which often is *not* the case).  The
BC80C cable is created to connect between a hex box and a quad box.  (BTW,
this is the scheme of my own system.)

Still later, the Q-bus appears in various formats.  It is clearly a mechanical
cousin of the OMNIBUS using a printed circuit board on the same blocks.  The
Q-BUS can be defined in two slots, but is often quad (while OMNIBUS is always
quad).

cjl