4 Feb
2003
4 Feb
'03
12:08 p.m.
I've seen a 3-bit-binary to one-of-eight decoder
built out of
huge brass and bakelite relays. It's in the London Transport Museum,
and was used to drive the train indicators on the platforms of
There wasa similar thing used to decode the 4-bit code as to what train
was expected next, used in signal boxes, etc on the London Underground.
That was certainly (large) mechanical relays.
The 4-bit train descriptor codes were stored in an electromechanical FIFO,
consisting of a drum with a number of rows of 4 pegs around it. Each peg
could be in one of 2 states (towards the spindle or shifted away from the
spindle. There was a fixed solenoid mechanism to set the pegs, the whole
drum then moved round one position (equivalent to incremementing the
write pointer in a software FIFO). There was a separately revolving
contact assembly to sense the position of each row of pegs and feed it to
the decoder. This was stepped on as each train came through -- equivalent
to incrememnting the read pointer.
The whole thing is _exactly_ like the classic software FIFO we've all
implelmented many times. I don't know when it was built, but around 1920?
It's described in detail in an old book I have called 'Modern Electrical
Engineering' which alas has no dates on it.
-tony