• ### Four-bit comparator circuit

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.

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:

COMP

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 Encoder

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

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.

• ### Cabling system

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.