Switch board

Parts:

At the moment red power cable is GND and black power cable is +5V. The quick rewiring was required to make the negative output strong enough to power 20+ gates, like 74LS-compatible stuff is supposed to be.

Maximal output on high is 5 mA, on low it's practically unlimited (directly connected to the power rail, of course the cabling will fry at some point).

As 74LS TTL are supposed to source at least 8 mA on low, in the original wiring the board would emit quite weak signals, being able to drive only 12 gates, instead of at least 20.

This is a problem, as two signals ALU uses are fed to 16 XOR gates each. Actually each of this signals is transfered over 2 wires, as each XOR gate sinks twice as much current as normal gates.

LED board

Parts:

The logic is inverted (it emits light on low voltage). This is more practical solution, due to asymmetric 74LS output current limits. Maximal output current on high (0.4 mA) is 20 times lower than on low (8 mA).

ALU board

Parts:

Because it got way too dense, I decided to split ALU board into main ALU board (21 of 31 chips), and ALU extension board (the other 10).

Main ALU board does all work on first 8 bits. Other 8 bits are processed as follows - negation layer is on the main board, xored inputs, carry-8, and select signals go to the extension board, output and carry-16 return to the main board, where they're checked for being zero and forwarded to output.

It's not all wired yet.

Connector IN:
Top
A 8A 9A 10A 11B 8B 9B 10B 11 A 12A 13A 14A 15B 12B 13B 14B 15 AiBiNCS1
A 0A 1A 2A 3B 0B 1B 2B 3 A 4A 5A 6A 7B 4B 5B 6B 7 AiBiCarry-INS0
Bottom

Connector OUT:
Top
R 8R 9R 10R 11R 12R 13R 14R 15 Carry-OUTZeroNCNC R 7R 6R 5R 4 R 3R 2R 1R 0
NCNCNCNCNCNCNCNC NCNCNCNCNCNCNCNC NCNCNCNC
Bottom

Connector EXT:
Top
R 8R 9R 10R 11R 12R 13R 14R 15 NCNCNCNCNCNCNCNC NCNCCarry-OUTS1
Ax 8Ax 9Ax 10Ax 11Bx 8Bx 9Bx 10Bx 11 Ax 12Ax 13Ax 14Ax 15Bx 12Bx 13Bx 14Bx 15 NCNCCarry-MIDS0
Bottom

ALU extension board (planned)

Parts:

Register file (planned)

At first I thought I would use 74LS273 octal D flip flops and 74LS151 8-input muxes, but after some experience with wiring the ALU I see what a cabling nightmare it would probably be. So I think I'll use some 3->8 decoders and octal 3-state drivers. Unfortunately I only have a few 74LS240 3-state inverters (that's a minor problem) and no 3->8 decoders.