On 4/22/24 16:06, Paul Berger via cctalk wrote:
On 2024-04-22 5:21 p.m., Chuck Guzis via cctalk wrote:
On 4/22/24 13:02, Wayne S wrote:
I read somewhere that the cable lengths were expressly
engineered to provide that signals arrived to chips at
nearly the same time so as to reduce chip “wait” times
and provide more speed.
That certainly was true for the 6600. My unit manager,
fresh out of
UofMinn had his first job with CDC, measuring wire loops
on the first
6600 to which Seymour had attached tags that said "tune".
But then, take a gander at a modern notherboard and the
lengths (sic) to
which the designers have routed the traces so that timing
works.
--Chuck
Shortly after I started at IBM I assisted one of the
senior CEs doing engineering changes on a 3031 and the
back of the logic gates was a mass of what IBM called
tri-lead, when I saw it I wonder how it could possibly
work. The tri-lead is basically a 3 wire ribbon cable
that has the two outer wires grounded and is precisely
made to have reliable characteristics. It was explained
to me that sometimes they would change the length of the
tri-lead in a connection to adjust signal timing.
I am not sure when IBM started using tri-lead
IBM 360's used Chabin TLC (transmission line cable) that
were essentially a ribbon cable version of tri-lead. 18
signals wide, terminating in the standard 24-pin connectors
just like the SLT cards had. I think that the Tri-lead and
TLC both had a 91 Ohm impedance. The reason for splitting
the ribbons into individual signals was to reduce
crosstalk. Interesting note, 370's version of ECL used a
+1.25 V and -3V power supply that shifted the logic levels
to +400 mV and -400 mV, and were terminated to ground. If
you wanted to scope a signal, you could unplug a tri-lead
and connect it to a scope with a 91 Ohm terminator.
Jon
