DIVMMC - where to get 128K ROM

[1024MAK]

The keyboard is actually the same as the ZX Spectrum+, which is an evolution of the rubber key using “technology” developed for the Sinclair QL.

The layout was set before the PC keyboard became common in this country. If you look at the various keyboards on home machines available between 1979 and 1984, they are all different...

Mark

[tomxp411]

So after everything, I finally got my DivMMC and my video cable last night. I got everything up and running, and the DivMMC worked for about 20 minutes. Now the screen just shows garbage, and I can't get the computer to successfully start up with a card in the slot.

I've run through all the likely troubleshooting steps, even attempted to re-flash the DiVMMC's ROM (it says "DIVIDE Failure" when I run the flasher.) So I've contacted TFW8B to see what they can do for me.

[stupidget]

I have a board only DivMMC and its prone to not working if i knock it slightly. I have to prop mine up with an old tape and sometimes i have to remove the sd card and reinsert it.

[tomxp411]

Tim at TFW8B suggested that the Z80's M1 line may be faulty, which can prevent it from working with the DIVMMC, even though the system works fine otherwise, without a cartridge.

I'm kind of stuck, since I don't have another Spectrum to test with, and I can't find another compatible computer for sale in the US.

So I'm probably going to try replacing the CPU (I need to see if it's socketed.) If that doesn't work, then I'm just going to call this computer a paperweight and put it on a shelf.

[Ast A. Moore]

I'm just going to call this computer a paperweight and put it on a shelf.

What were you going to use it for, to begin with? You can still transfer programs the old-fashioned way.

Re the M1 line: It might be the problem, but why would the machine work fine for twenty minutes?

[tomxp411]

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.

Re the M1 line: It might be the problem, but why would the machine work fine for twenty minutes?

I don't know, either. What Tim told me certainly lines up with other information out there, but without the hardware to test the CPU and no replacement for either the computer or the cartridge to swap out, I'm basically stuck. I can try another $10 CPU to see if that's the problem, or I can ship the DivMMC back to England. I don't have a third option, unless I find another, closer ZX to test this thing with.

[tomxp411]

This brings me to the next question. Looking online, I found a Z80 CPU at DigiKey:

https://www.digikey.com/products/en/int ... ageSize=25


These all look like different speeds versions of the same part. 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.

[1024MAK]

The CPU that is fitted, if an original part, will be a NMOS type rated at4MHz. NOTE there are versions by manufacturers other than Zilog. Zilog’s orginal part number for the 4MHz version was simply Z80A CPU.

Any CMOS version should work. However, there is no advantage in buying one with a higher speed rating, as the clock speed is set by the ULA.

The most common CMOS versions that are available new are:

• Z84C0006PEG 6MHz
• Z84C0008PEG 8MHz
• Z84C0010PEG 10MHz

I normally use Mouser as I live in the U.K.
They currently have any of the above at $5.38 / £4.09 excluding tax and postage/shipping.

There is a lot of folklore surrounding the M1 line/pin. It’s purpose is for the CPU to signal that it is executing the first part of an instruction op code. In most Z80 systems, this line/pin is not used. Hence it is not used internally on any ZX Spectrum. On the expansion edge connector, a power supply line is right next to it. So my thoughts are that it is much more likely that the Z80 CPUs M1 line/pin suffers damage from mis-use of the expansion port rather than Zilog suppling partly defective parts.

Mark

[tomxp411]

The CPU that is fitted, if an original part, will be a NMOS type rated at4MHz. NOTE there are versions by manufacturers other than Zilog. Zilog’s orginal part number for the 4MHz version was simply Z80A CPU.

Any CMOS version should work. However, there is no advantage in buying one with a higher speed rating, as the clock speed is set by the ULA.

Thanks. That's about what I expected to hear. It seems like, for the <$10 (including shipping), it's a low-risk test.

[Ast A. Moore]

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.

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.

A 4 MHz NMOS Z80 from Zilog is usually marked Z8400APS, sometimes Z8400A PS (with a space).

You can test if your computer has an NMOS or CMOS Z80 by running this little program:

Code: Select all

10 LET a=4e4
20 READ b: POKE a,b: LET a=a+1
30 GO TO 20
40 DATA 243,1,254,0,237,113,237,120,24,247

After it ends with an “E Out of DATA” message, type usr4e4 and press Enter.

If the border turns black, your Spectrum has an NMOS Z80; if it remains white, then it’s a CMOS one. My bet it’s the former.

[tomxp411]

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.

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?

[tomxp411]

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.

[1024MAK]

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

[1024MAK]

Z80 CPU area of the Zilog web site

Mark

[tomxp411]

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.

[rdhcook]

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