Minimum Suggested Testing to Check Z84 CPUs

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Flatulentia
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Minimum Suggested Testing to Check Z84 CPUs

Post by Flatulentia »

Hi guys,

I'm taking delivery of 12x used Z84C0006PEC CPUs soon and was wondering what you'd recommend as a minimum test I can carry out to check they're OK?

The only testbed I have is a socketed ZX81, and the reason I'm asking is because I may be selling some of these on as "tested & working" as I don't actually need all 12.

Why did I buy 12 if I don't need them all? It was a lot of 12 and the price was too good to turn down.

Thanks in advance.
ZX81 with ZiLOG Z84 CMOS Z80, 32KB battery-backed Toshiba 55257 SRAM with charge status LED, Wilf's "Why Wait?" mod and switch-mode 5V regulator. 50/60Hz mode switch, composite video output with Zigg's back porch mod and 1080p60 HDMI output.
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1024MAK
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Re: Minimum Suggested Testing to Check Z84 CPUs

Post by 1024MAK »

Well, really, you need to test as many address, control and data pins as possible, plus as many instructions as possible.

So, of the Sinclair computers, a ZX Spectrum 48K or ZX Spectrum+ would be better candidates. Especially when combined with say, a diagnostic expansion card (so that the /M1 control line can be tested).

The Spectrum uses all the addresses lines and the diagnostic/RAM tests are very comprehensive. And most instructions can be tested with available software (originally developed to test that emulators worked correctly).

I’m not sure that such comprehensive testing is available ‘off the shelf’ with the Zeddy. Having said that, if you use expansions with more than 16K of RAM and test a number of different games and applications, you’ll get pretty close.

Oh, one other thing, all Sinclair computers (including those manufactured by/for Amstrad) use NMOS Z80A (4MHz) processors. The ones you are buying are actually Z84C0006PE (assuming they are not remarked NMOS parts - see below) 6MHz CMOS types. Some machines may need some resistors changing due to the slightly different characteristics that affect the busses. However, this also depends on what type of RAM chips are in use. It’s to do with Sinclair using resistors to ‘isolate’ different parts of the busses. It shows up more on ZX Spectrums.

More about the Z84C0006PEC part here.

The reason I mentioned remarked parts, is that various people have received parts marked as Z84C0020PEC (or similar) and found that they are actually 4MHz NMOS parts…

Oh, one more thing, the easiest way to tell a NMOS from a CMOS is to measure the current drawn by it. NMOS typically consume between 100mA and 200mA, while CMOS (at 4MHz) typically consume less than 30mA.

The main web site resource for the Z80 (not including Zilog’s site) is www.z80.info.

Oh, you may also want to try your ‘new’ chips in this circuit “The absolute minimum Z80 CPU test circuit ever seen” before trying them in a computer ;-)

Mark
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Flatulentia
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Re: Minimum Suggested Testing to Check Z84 CPUs

Post by Flatulentia »

I've got away with the same CPU in a ZX81 before, so fingers crossed these run properly too.

I'm planning on measuring current draw as part of the test, so if they're any more power hungry than the genuine part I already have, that'll be a good indicator of any fakery.

No ZX Spectrums here I'm afraid, so these will be tested in my 32KB ZX81, so I'll poke around in the upper RAM and see if it behaves as expected. That should get me close-ish.

Being the strange individual that I am, I really enjoy reading datasheets so will have a good look through those too.

Thanks for the simple tester schematic. I may well have a go at knocking that up later. If memory serves, I have all the necessary components in my parts bin.

Many thanks.
ZX81 with ZiLOG Z84 CMOS Z80, 32KB battery-backed Toshiba 55257 SRAM with charge status LED, Wilf's "Why Wait?" mod and switch-mode 5V regulator. 50/60Hz mode switch, composite video output with Zigg's back porch mod and 1080p60 HDMI output.
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