One can add slow mode hardware to a ZX80, but I decided to buy a board with this designed in already.

2023-08-19

I bought these boards on eBay.

The logic board has some nice features. The bus signals are available on a holes for a header,  allowing the dreaded RAM-pack wobble to be avoided. The board can take up to 32K RAM, so you don't even need an external RAM pack.

Power is from a USB-B socket. I'll have to check the electric current budget.

On the down side, the clock is a crystal-and-inverter which mean I can't change frequencies by swapping an oscillator can. I think that can be hacked!

The on-board SLOW-mode generator will need hacking to run with a 14.7456 MHz clock. I'll design something that can be easily switched between two frequencies.

The keyboard is on a separate PCB, so no mechanical stress is transmitted to the motherboard. It has the key letters but not the associated keywords. 

The boards are due in about a week, so I shall get hunting for components. 

2023-08-21

Printed out the circuit diagram and parts list. I have:

• Z80 CPU
• ROM
• RAM
• HC165 shift register
• HC74 D-type latches
• HC04 inverters
• HC138 one-of-eight decoders 
• ACT86 xor-gates
• ACT574 eight-bit edge-triggered latch

What I don't have:

• HC541 (but I could use some ACT245 with a bit of hacking)
• HC08
• HC32
• HC125
• HC393
• HC4002

I could use some PAL chips to mop up the random logic. I have plenty and they are easy to modify.

2023-08-23

Boards arrived. I have soldered the key switches to the keyboard.

The motherboard is very packed.

2023-08-25

I fitted 0603 surface mount resistors where there are vertical through-hole resistors.

The ACT574 and HC165 chip are now soldered to the board.

2023-08-31

I have no HC541 chips but plenty of HC245 chips. So I took two HC245 chips and bent pin 1 under the chip and wired it to VCC. I then fitted it to the HC541 sites.

2023-09-27

Fitted SIL sockets for keyboard.