On Fri, 26 Jun 2020 14:30:33 +0100
David Higton  wrote:

> As for PCB traces: I discovered in the early 1990s, when we were
> using 74ABT logic devices, which have nanosecond rise and fall times,

 Around 1980 two of us assembled wire-wrap prototype memory boards
(stuffed with 2kx8 SRAM and 74LS logic) the other guy was an experienced
wireman who made a lovely job of it, the looms of address and data bus were
really clear. I was a student and not so inclined to making things pretty,
every wire made the correct connection but they went all over the place in
a rat's nest. On first sight the boss commented that his was the way it
should be and I needed to do better - fair enough I thought - his was
pretty.

 Both boards passed a DC buzz test (every wire going where it should
be and no extras) and a power on smoke test and so were duly populated with
components and tested. Mine worked first time - his radiated at ~30MHz
strongly enough to be picked up by an open scope probe across the room
purely because of the way the wires were laid out on the board (it didn't
work of course) - he wound up hanging terminating resistors at the end of
the bus loops to shut it up, mine didn't need them.

 Wire wrap looms can behave like transmission lines too.

 We all learned something that day - including the boss.

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