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BASIC CIRCUITS

A project log for 8 bit transistor computer

An 8 bit computer made of discrete components built on wood and nails.

yagoYago 05/14/2022 at 13:470 Comments

I've already shown the NAND gate:

Note that the pull-up output resistor can either be 680Ω or 340Ω (two parallel 680Ω resistors) depending on the current requirement of the input it's connected to.

Most other computers I've seen use D-latches or something similar for registers. 4 NAND gates per latch is quite a lot considering that the addresing space is 15 bit. The solution I've found is to use SR latches and design the control logic so that it resets registers before writting to them. This is the circuit for a SR latch with an enable input:

We also have the multiplexer circuit. Muxes made out of nand gates require 3 gates, but using a combination of AND/OR/NANDgates we can reduce the component count:

Note that if no input is enabled, the output will default to 1.

Finally, the inverted buffer. This one allows to drive large amounts of OR gates, as in registers' enable signal. It should output up to 100mA in theory:

More circuits will probably appear as I build the computer, I'll post them in the corresponding logs.

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