22 Nov
1999
22 Nov
'99
10:30 a.m.
I wrote:
Actually, DEC used over-the-edge cabling on an early
minicomputer, and
abandoned it because it was LESS reliable than backplane connection,
and also made the machines much harder to service.
Reference: _Computer Engineering: A DEC View of Hardware Systems Design_
Bill Pechter <pechter(a)pechter.dyndns.org> wrote:
A great book... but wrong on this one.
The hp stuff had one over the edge interconnect cable of less than 10
inches.
The DEC 11/40 (45,50,55,70, etc) in the BA11-F cabinet (the worst
offender) had about 1/3 to 3/4 of an inch of ribbon cables (thickness)
running through the cable trough an often popping off the Berg connector
at the top of the Hex, Quad or Dual board (most had no locking ears on
the connector. Yup, the backplane is more reliable as over the top
connectors, but between board cables (RH11, RH70, Cache, etc) were my
biggest pain in the @#$%^ at DEC). The 11/780 was a lot better with the
exception of the lousy SBI cables that were failure prone and a bitch to
troubleshoot.
I'm not talking about I/O cables that are many feet long between a card and
an external device, nor cables under two inches between just a few boards
in the same backplane.
I'm talking about using over-the-edge cabling for bussing signals between
many cards in the same backplane, and that's what I was referring to the
book about. This was done in the PDP-6, IIRC, and they decided that it
was a maintenance nightmare. If HP somehow made this scheme work reliably
and somehow kept it from being a headache to troubleshoot, I'd be interested
in hearing how they did it. Obviously HP must not have thought it was
a particularly great technique, since they generally aren't using it today.