Sinclair ZX80 / ZX81 / Z88 Forums

Well done 👍 :D The 74LS157 are multiplexer chips to switch between the 14 bit address from the Z80 to the two lots of 7 bit addresses used by the DRAM chips. I don’t know why there are extra resistors fitted to the Memotech 16K RAM pack in that photo. They were not present on the unit that I took ap...

I THEN decided to check at IC6 and got 8v at ground ! Measured again and got 0v, measured again and got 7.6 and rising, then 0v again Now I just need to figure out WHY ! I suspect a faulty ram module leaking volts to ground ? Hmm, what point are you using for the negative lead (black probe) for you...

wietze wrote: ↑Fri Jul 10, 2020 4:39 pm Would you be able to point me to some more in depth resources regarding this?

Read Wilf Rigter’s Video Tutorial first

Note: by default the pages at the linked to web site may not display correctly, so switch your character set to "western" for correct display.

Mark

It maybe worthwhile mentioning that a user program (BASIC or machine code) will only run at the maximum MPU speed if the machine is not generating a display, this is known as FAST mode. This is the same as the system used by the ZX80. The normal ZX81 mode is SLOW mode, also known as compute and disp...

Oh, since I forgot to welcome you, let me do that now...

Welcome on board Rob!



Welcome to Sinclair ZX World

Mark

Oh, my bad, I forgot to welcome you

Time for me to correct that, so...

Welcome on board JetSetWally!



Welcome to Sinclair ZX World

Mark

I don’t recommend testing ULA pin 39. If the ULA produces a video picture, then the 14MHz oscillator is running. If you want to see that the ULA is working, test the Z80 clock signal on pin 6 on the Z80. But note that it may vary slightly as the ULA contention to the Z80 is applied via the CPU clock...

Do you mean A15 on the edge-connector / expansion port? A15 comes from directly from the Z80. The BASIC POKE command and the PEEK function can access the whole 64k memory map regardless of anything else (if the upper RAM is disabled it obviously will ignore any accesses). Hence A15 will go high when...

When you say black square, do you mean that the border is white and the the whole of the text and graphics area is black? The ROM code does not do a RAM functional test as such. The objective of the ROM test is to work out how much RAM is fitted. During the test, you should see thin vertical red lin...

At some point, I’ll split out the relevant posts into a new topic.

Mark