Selectric Typewriter conversions

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Tue Feb 4 20:08:00 2003

> 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
Received on Tue Feb 04 2003 - 20:08:00 GMT