In 1968, three people wrote a book called "How to Build a Digital Computer That Works" to introduce young people to the new science of computers.
With paper clips, cables and lamps, the authors invited to build a small programmable 5-byte 4-bit computer, enough for young minds to understand the basic processes of the new "machines" that are here to stay.
Although I have followed the instructions in the book, I have incorporated some more modern pieces of hardware, such as multipoint switches, diodes, resistors and LEDs.
I have also recycled parts of old computers such as cables, knobs and even screws.
The assembly has been done on small wooden boards, as described in the book, but changing the "look and feel" of the computer.
One of the great advances in computing was made by the German Z4 computer from 1945. According to a book, it was capable of comparing numbers within its programming.
Z4: front view of operating console with relay cabinets in background. Deutsches Museum
Some days ago, I began to develop a unit capable to compare two numbers in order to add this capacity to my Paper Clip Computer.
Yeah, I know, I know.... the numbers that this computer can manage are small how to need a "comparator unit" but the pleasure was not in the "needed" but to build it and add it.
Searching the Internet I did not find a four-bit comparator circuit without chips; my goal was to follow the principles and philosophy of the computer: no chips, just basic electronic components and switches.
As I did not find any circuit, based on the book I created mine which is the one that I expose below.
This circuit is capable of comparing two four-bit numbers with the only limitation that both must be positive or negative; if one of them is negative, this will be the lowest.
I will try to build the circuit that will be integrated into the computer (and the drum) in a blank panel that is on the left of the model and it will look like this:
The comparator will add one more command:
whose parameters will be:
COMP <Word 1> <Word 2>
and Word n should indicate:
two of the five memory locations
accumulator and a memory location
The result of these two parameters will indicate whether:
Word 1 > Word 2
Word 1 = Word 2 Word 1 < Word 2
in the light panel.
The operation will be simple:
Step 1: Indicate is whether the numbers will be positive or negative. Step 2: Select the position of the individual bits. Answer: Read the result and use it according to the instructions of the program.
Today I finished wiring the Control Panel.From the beginning of the project I tried to make it modular since the search for possible errors and maintenance would be much easier.
Making the system modular consumes more time since instead of just "placing" a cable I placed terminals and connectors;debugging or replacing damaged components should be easier this way.
All cables and connectors are either color-coded (I used network cables) or are numbered so they can be easily identified.I also made an Excel with the cable-number-computer word equivalences to print it and stick it on the inside of the back cover and include it in the operations and maintenance manual.
With this modular format I can quickly disconnect all the panels to remove them from the chassis, repair them independently from the rest of the equipment and once tested, reconnect them.