This is prompted by memories I have of a project that I and other pupils contributed to at Wellsway School around 1973/4. It was led by our Maths teacher Mr. Grace. Before I started to investigate, as far as I could recall:
the machine had a binary counter with lights, could possibly perform simple binary arithmetic and could be used to test logical propositions
it was based on one described in a magazine article from a few years before, referring to it by the acronym BEATLE (well done to whoever was responsible for that!)
it made extensive use of flip-flops for memory. This was the first time I had come across the term and I remember putting together one or two of these
the counter could be driven by a high frequency signal and hence act as a clock or timer, the first light in the chain seemingly permanently on, with the next one flickering rapidly and the ones after that flashing ever more slowly, at half the rate each time.
After some research, I found mention of the BEATLE machine in a book, Teaching School Mathematics, edited by W. Servais and T. Varga, published in 1971 by Penguin Books & Unesco, available here. In turn, the book refers to two articles by A. Wilkinson, a teacher, in the journal Mathematics Teaching, published by the Association of Teachers of Mathematics:
MT 31, Summer 1965: A Small Computer Demonstration Model (i.e. the BEATLE)
MT 32, Autumn 1965: A Computer Logic Demonstration Board
In addition, in the first article the author refers to another article in the previous journal:
- MT 30, Spring 1965: A System of Logic to Aid in Teaching Mathematics, by G.J. Flanagan & L.Molyneux of the University of Newcastle-upon-Tyne.
These journals are available online, but are behind a paywall.
At the core of the BEATLE is a toggle flip-flop, that is used as a divide-by-two circuit and is referred to as a scale-of-two. It is built using NOR gates.
The machine comprises the following main units:
a Binary Arithmetic Unit: a binary counter with 9 scale-of-two units to accumulate and display sums (up to 511) of the numbers input
a store: a binary counter with 5 scale-of-two units which stores the twos complement of the number to be added
a pulse clock to increment the contents of the BAU and store
a control panel to allow decimal numbers in the range 1 to 9 to be input via switches or a rotary dial, and with a button to start the addition of the input number selected
a routing network: twos complement wiring between the switches/rotary dial to the elements of the store
The mechanism to add the contents of the store to the BAU is interesting, if limited:
a decimal number, n, 1 to 9, is entered by the user
32 - n is put into the store
the Add button starts the addition by means of an electronic switch, an AND gate, with input from the Add button and pulse clock
for each pulse of the clock, the contents of the BAU and store are incremented by 1
after n pulses of the clock, the content of the BAU has increased by n and that of the store has become 0
when the store contains 0, the electronic switch is set to OFF and the pulses from the clock no longer get through to the store and BAU.
The computer logic demonstration board is a set of NOR gates that can be wired together to form logical circuits such as half or full adders. It can also be used to model logical puzzles and, when connected to the BEATLE machine, to solve them. When a solution is found, the appropriate lights are lit and the machine stops - reminiscent then of the code-solving machines at Bletchley Park during the war.
Comments:
although referred to as a computer demonstration model, I don't think it merits that description: while the functions can be enhanced through wiring of new logical elements, it cannot be programmed
using this method of addition, the input number range could be extended up to 16 without increasing the size of the store
although not mentioned in the article, the machine could be modified to subtract numbers as well: a subtract button could allow the input number n to be stored directly, using a different routing network, but some extra logic would be needed to handle the unwanted digit arising from using this method.
the decimal number input method cannot be scaled up easily to large numbers, so is clearly limited in practice. Perhaps it should be replaced by binary input only and the twos complement obtained by extra logic to invert the number (easily done with NOR gates) and then to add one (but how?)?
I have also found the following relevant publications:
Electronic Engineering magazine, July 1964: A Low Speed Single Supply Logic System, also by G.J. Flanagan & L.Molyneux. Useful background to the NOR gate implementation. Can be downloaded for free from here.
Computer Models, also by Wilkinson, published in 1968 by Edward Arnold (Publishers) Ltd. Describes a later machine which performs addition and subtraction using half and full adders and shift registers. The author also shows how it can be enhanced to multiply two numbers, or to compare them. Can be downloaded for free from here.
I may at some point decide to re-create either the BEATLE machine or the one described in the book. In the meantime, I've started a page with some simple circuits based on the literature.