22 Apr
2024
22 Apr
'24
2:06 p.m.
On 2024-04-22 5:21 p.m., Chuck Guzis via cctalk wrote:
On 4/22/24 13:02, Wayne S wrote:
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 but I also recall seeing a
168 that had some third party memory that was in a box hung on the end
of a frame and had a large bundle of tri-lead coming out of it that
disappeared under the covers. The site CE told me that those tri-leads
where all plugged into specific locations on the back of the boards on
one of the card gates, and if they had a problem with memory, they would
call in the techs that looked after the third party memory and have them
disconnect it all. The last system I saw with tri-lead was a 3081 most
of the logic was in TCMs but the memory was on a separate card gate and
connected to teh main CPU board with tri-lead.
Paul.
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