Basic checks for ZX81

[msknight]

Hi All,

I'm starting to get zeddies (81) and am wondering what the basic checks are for testing an 81 with oscilloscope and logic tester, before trying to hook up to a monitor please.

[1024MAK]

To be honest, because the ZX81 uses a combination of hardware and software to generate the video image, connecting a TV/monitor and seeing what (if anything) the Zeddy is displaying is by far the fastest way to determine if it's well or not.

There are three ULA versions. Only the last version includes circuitry to generate the back porch part of the video signal. If your machine has one of the first two ULA versions, a colour TV/monitor may not correctly display the video image.

For doing a composite video modification, I recommend the ZX8-CCB from Pokemon. His shop is here. This module includes circuitry to generate a back porch. It also cleans up the video signal by removing noise.

In terms of testing signals using a 'scope, logic analyser or logic probe, here are the most useful:

Z80 (or NEC µPD780C-1) processor

Pin Function
6 clock - 3.25MHz (approximately)
16 /INT (should be pulsing)
17 /NMI (should be pulsing)
18 /HALT (should be pulsing)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be pulsing)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Address lines A14 and A15. These give an idea of which area of memory the CPU is accessing and if it is trying to generate a TV display.

You can also test the remaining address lines A0 to A13 and the data lines D0 to D7. A lack of activity is suspicious.

Next, test the /RAMCS signal which on the SRAM documentation is called /S, /CS or/CE on the RAM chips. For the 2114 type, this is pin 8. Check also the /W signal on pin 10. Both 2114 chips should share both these signals. Or for the larger (as in physical size of package) SRAM chip, pins 18 and 20 are the /RAMCS and pin 21 is the /W signal.

For the ROM, pin 20 is the /ROMCS signal.

All the above signals should be pulsing, as the processor needs to access RAM and ROM to generate the video image.

Z80 pin-out here.

Mark

[msknight]

Thanks for that Mark. That's brilliant.

As I've got experience with the RGB to HDMI from the BBC and I have a Pi Zero doing nothing, I'll be using the following for video output...
https://www.sellmyretro.com/offer/details/63615