OT the telephone network
On Mon, 25 Sep 2017 15:40:47 +0100, Andy Burns
In large cables they were all coloured (using resistor colour codes)
and twisted together in pairs, so every pair could be uniquely
Not quite (for telecoms cables at least) the usual domestic extension
commonly seen in 4 pair cables, continues with
GPO Primary colours: Blue, Orange, Green, Brown, Slate. (Acronym
Secondary colours: White, Red, and I can't remember the rest but there
are five of them.
At first, solid primaries are paired with each solid secondary in
turn, giving 25 combinations. Then each colour can be used with a
"tracer" or stripe of any of the other colours. I can't remember the
order in which they're used, but this gives effectively another 20
primary colours and another 20 secondary colours, so a total of up to
another 400 combinations in addition to the 25 using solid colours.
This is why the Ishihara test was included in BBC job interviews.