[prev in list] [next in list] [prev in thread] [next in thread] 

List:       classiccmp
Subject:    Re: Aluminum rectifiers (was Dummy Loads)
From:       ard () p850ug1 ! demon ! co ! uk (Tony Duell)
Date:       2011-04-29 19:16:01
Message-ID: m1QFtAh-000J43C () p850ug1
[Download RAW message or body]

> I like to read old magazine copy (particularly the advertisements) 
> because it reinforces the idea that there's really nothing new under 
> the sun.

Yes... Old books on technical subjects are similarly interesting. You 
find all sorts of things in them that them pop up again later.

For example, I have a set of books called 'Modern Electrical Engineering' 
which appear to date from just after the First World War. I bought them 
becuase when I flipped through them in a second-hand bookshop, I saw a 
good description of the Baudot telegraph.

But after readingg them mroe fully I found somethign that was even more
interesting. A description of the signaling system used on the London
Underground ('Tube', call it the Subway if you must :-)). Apparently even
then there were too many trains for the signalmen to remember the order. 
They had an electormechanical FIFO buffer. A drum with 4 rows of pins
round it. Each axial positon of 4 pins on the drum corresponded to a word
of the store and held the description of one of the trains on the line. A
'write device' moved the 4 pin in the current postion in or out, encoding
the description of a particular train. A read device sensed the psitions
of the pins. The read and write devices moved independantly relative to
the drum, which makes the whole thing very similar to the stnadard
implementation of a FIFO buffer using read and write pointers. 

When a new train came into that block of track, its description was 
stored as a 4 bit code in the next position on the drum and the write device 
moved on to the next (unused) position. The read device effectiviely told 
the signalman what the next train to come out of that block would be, 
when it emerged, the read device was moved on one position so as to look 
at the next train's word, and so on.

Oh yes, the 4 bits read from the store were decoded to 1-of-16 using a 
relay tree and then used to operate indicator lights for the signalman. 

There is one of these devices as a static exhibit in the London Transport 
Museum, but when I visited it (some years ago, I admit), the description 
wasn't too clear as to how it worked. Having read the book, I realised 
what it was.

There are, of course many other examples

-tony
[prev in list] [next in list] [prev in thread] [next in thread]