DIVMMC - where to get 128K ROM

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
tomxp411
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Re: DIVMMC - where to get 128K ROM

Post by tomxp411 » Thu Dec 13, 2018 1:30 am

Ast A. Moore wrote:
Wed Dec 12, 2018 10:44 pm
tomxp411 wrote:
Wed Dec 12, 2018 9:02 pm
You can still transfer programs the old-fashioned way.
That would be loading a TAP file on the PC and hooking my sound card up to the tape input, right? The purpose of the DivMMC is so I don't have to do that.
Sure, I understand that. But what’s the point of having a Spectrum, and why would you shelve it if it didn’t play ball with the DivMMC? I mean, I have a couple of Spectrums and no DivMMC or any other SD-card interface for them.

I may like the look of the hardware, but that doesn't mean I want to fiddle with cassettes and audio cables. That was NOT a fun part of owning an 80s 8-bit.
tomxp411 wrote:
Wed Dec 12, 2018 9:11 pm
Are these all pin compatible with the Z80 the Spectrum 128K +2 ships with? Is there any reason to choose a specific part over another? I'm inclined to grab the 20MHz version, just to have the confidence I've gotten the best quality part I can get.
These are likely CMOS Z80s. Most (all?) original Spectrums came with an NMOS Z80 cpu. If I were you, I’d get the 6 MHz version, or even look online for an old-sock NMOS 4 MHz one. There’s no advantage of getting a higher-clock CPU here.
Thanks for the tip on the test program. I'll give that a shot when I get home.

The thing is, the 6,8, and 10MHz chips are all the same price, and the 20MHz chip is only $3 more. So unless there's a good reason the better rated chip *won't* work, I intend to buy that. Typcically, there's no architectural difference between the slower and faster rated chips; the slower ones are just chips that didn't pass when tested at higher speeds.


What worries me is that there might be some electrical difference... can I just drop a CMOS CPU into a socket formerly occupied by an NMOS version?
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tomxp411
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Re: DIVMMC - where to get 128K ROM

Post by tomxp411 » Thu Dec 13, 2018 4:47 am

For reference, the CPU is a Z0840004PSC.
That is an NMOS CPU, according the documentation I've found so far.

Data sheet here:
https://www.datasheets360.com/part/deta ... 876896220/

I ordered a Z84C0010PEG‎ from DigiKey:
https://www.digikey.com/products/en?key ... G%E2%80%8E

We'll see how this goes.
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1024MAK
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Re: DIVMMC - where to get 128K ROM

Post by 1024MAK » Thu Dec 13, 2018 4:57 am

tomxp411 wrote:
Thu Dec 13, 2018 1:30 am
What worries me is that there might be some electrical difference... can I just drop a CMOS CPU into a socket formerly occupied by an NMOS version?
Most ZX Spectrums will run okay with a 6 or 8MHz CMOS Z80. I don’t know if anyone has tried a 10MHz or a 20MHz rated CMOS Z80.

The reason for the caveat is that Sinclair uses resistors to separate the data bus into two sections (does not apply to the black +2A, +2B, +3 or +3B Amstrad machines). Occasionally the different impedance of different chips means that these resistors need changing for a different value.

With some chip designs, some faster rated parts have a quicker output translation time - their edges have quicker rise/fall times. There is a possibility that this may cause problems.

See also http://www.cpu-world.com/CPUs/Z80/index.html and http://www.cpu-world.com/CPUs/Z80/MANUF-Zilog.html

Mark
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1024MAK
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Re: DIVMMC - where to get 128K ROM

Post by 1024MAK » Thu Dec 13, 2018 5:05 am

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tomxp411
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Re: DIVMMC - where to get 128K ROM

Post by tomxp411 » Thu Dec 13, 2018 7:23 pm

1024MAK wrote:
Thu Dec 13, 2018 4:57 am
tomxp411 wrote:
Thu Dec 13, 2018 1:30 am
What worries me is that there might be some electrical difference... can I just drop a CMOS CPU into a socket formerly occupied by an NMOS version?
Most ZX Spectrums will run okay with a 6 or 8MHz CMOS Z80. I don’t know if anyone has tried a 10MHz or a 20MHz rated CMOS Z80.

The reason for the caveat is that Sinclair uses resistors to separate the data bus into two sections (does not apply to the black +2A, +2B, +3 or +3B Amstrad machines). Occasionally the different impedance of different chips means that these resistors need changing for a different value.
Thanks. That's the kind of information I was looking for. I could see a quicker rise/fall time being an issue, but since I've already ordered the chip, I'll have to just try it and see what happens. I should know in a week or so if that's the problem.
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rdhcook
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Re: DIVMMC - where to get 128K ROM

Post by rdhcook » Thu Mar 26, 2020 11:26 pm

Did the Z80 CPU you bought work? Did it solve the problem with your DivMMC?
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