How to upgrade to a 649 rom?

Discussions about Sinclair ZX80 and ZX81 Hardware
mikeh_nz
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How to upgrade to a 649 rom?

Post by mikeh_nz »

I've purchased an issue 1. A quick check on the oscilloscope, shows the "K" happening.
I plan to do a
  • ZX8-CCB,
  • TSR1-2450
  • internal 16KB, &
  • a WESPI.
I understand that the WESPI doesnt work with the 550 ROM - so I want to upgrade it to a 649.

I tried searching the forums, but couldnt find anything useful and struggled getting search to use my search terms.

I have a TL866-3G programmer and some spare M27C256B eproms.
The pin out of the NEC D2364C rom doesnt really match the M27C256B - but could I see that it could be "made" to work.
(ie tie G&E to CS, tie A13-A14 high (or low), shift around the top half of the right hand side)

But Im guessing that people use something like a CY7C264-45WC, MCM68766C or maybe muck around with the pins.

Questions:
  1. How to upgrade to a 649 ROM?
  2. If its burn an eprom, what eproms are recommended?
  3. Any idea of the speed needed? -30, -35, -45?
  4. The socket on the board is a 28 pin socket - what do the other pins do?
The issue 1 board
The issue 1 board
NEC D2364C pin out
NEC D2364C pin out
M27C256B pin out (Q's are data pins)
M27C256B pin out (Q's are data pins)
Last edited by mikeh_nz on Fri Mar 31, 2023 6:10 am, edited 1 time in total.
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1024MAK
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Re: How to upgrade to a 649 rom?

Post by 1024MAK »

Typically most use a 2764 or a 27128 EPROM.

Have a look at this post.

I'm sure you can work it out for a 27C256. Although where you "burn" the image in the EPROMs address range depends on how the "extra" address pins are wired up. The address space in a ZX81 for the ROM as seen by the Z80 is from 0x0000. It's 8k bytes in size, but the chip is decoded to a 16k byte range. See here. It's okay for a EPROM to respond to the whole 0x0000 to 0x3FFF range.

The image of the 649 is, I'm sure, on here somewhere. Have a look at this topic. However, there is a better version available, see here. Although I don't know if it works with WESPI (due to other distractions, I've not go around to trying a WESPI yet). A topic on the WESPI is here.

I think you may struggle to find a reasonably modern EPROM that is too slow for a Zeddy. So just give it a go.

I hope that helps.

Mark
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mikeh_nz
Posts: 67
Joined: Sat Feb 04, 2023 8:03 am
Location: Seattle

Re: How to upgrade to a 649 rom?

Post by mikeh_nz »

Thanks Mark - that post does the trick. I’ll probably go this “muck around with the pins” route - though it looks there’s less “mucking required” since A12 is on pin 2 of the socket.

In my searching since then - a CY7C264 does seem to be an exact replacement. At $13 for 2 - I might give that a try too

Ha!!! I just noticed that the rom mapping you pointed me to is at the end of the same thread I linked to! :)
mikeh_nz
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Re: How to upgrade to a 649 rom?

Post by mikeh_nz »

The CY7C264 looks like it’s going to be interesting to program.

I’ll come back on thread on whether I end up going this route or the 27xxx route.
Moggy
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Re: How to upgrade to a 649 rom?

Post by Moggy »

Just to add my two bobs worth, I have the 2764 EPROM on all my zeddies using the big bang ROM image Mark has linked to, as there are only 2 pins to lift and the increase in speed of the BASIC is well worth the effort.

If you are UK based and get no joy I will happily post a 649 to you if needed, sadly the pension doesn't run to overseas posting. :(
mikeh_nz
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Location: Seattle

Re: How to upgrade to a 649 rom?

Post by mikeh_nz »

Your two bobs is very welcome Moggy!

And thank you for the very kind offer. :)

I’m going to give programming a CY7C264 a go, as it will look “prettier” - even though it will be much simpler & quicker to go program the 27C256 and do to the two pin lifts and wires.

If it goes horribly wrong, I’ll backtrack this decision and do the 27C256….

To program the CY7C264, I plan to build a simple eprom programmer on the breadboard with an arduino to control the programming. It could be fun! And I might end up with a nice little 24 pin 8KB eprom that fits perfectly.

I’ll post back with the tall tale of how it goes….
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Paul
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Re: How to upgrade to a 649 rom?

Post by Paul »

Yes please, and if it works, please also give Infos about your programmer.
Did you consider to ask for help on programming with the TL866?
Many Retro users have that - me included - it might help many Retro users.
In theory, there is no difference between theory and practice. But, in practice, there is.
mikeh_nz
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Location: Seattle

Re: How to upgrade to a 649 rom?

Post by mikeh_nz »

Ive posted a message on the TL866 forums and sent XGecu an email - I'll let you know if anything happens.

I had gone through the TL866 list of supported chips (of type DIP24) and looked at the datasheets to see if there was anything there that looked similar to the CY7C264 - but wasnt successful. (Though, I wasnt comprehensive in my search)

Pages 5 & 6 of the CY7C264-45WC datasheet shows how its programmed.

I'll be assembling something similar to Ben Eaters EEPROM programmer. He has a great video explaining each component and building up to how it works. The current thinking is a couple of shift registers to handle the address and data pins, and then direct driving from the ardunio to the remaining pins. Plus using a separate 12.5V power supply with a tied ground to handle getting the required voltage on the programming pin (pin 19).

Moggy wrote: Mon Jan 18, 2021 11:37 pm Right after much faffing around I managed to dump the 2 ROMs which I have attached.
Moggy - do you recall how you read the ROMs? Was it with something like a TL866 or some other way?
Moggy
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Re: How to upgrade to a 649 rom?

Post by Moggy »

mikeh_nz wrote: Thu Mar 30, 2023 6:35 pm
Moggy - do you recall how you read the ROMs? Was it with something like a TL866 or some other way?
When it comes to things like this Mike, I'm an ancient dinosaur who does things the hard way but in a way that suites me as it is something I was brought up with and it helps to pass the time.

To read the original ROMs.

I use a device called the ZXpand, which you may have heard of, I load a pre saved 8k REM into the Zeddie, then with a simple loop dump the Zeddie's ROM code into the REM statement.

I then save this as a P file.

For burning purposes I'm again old fashioned and use an EPROM programmer connected to my ZX Spectrum, used mainly because it can handle the 21/25 volt programming voltages that the older EPROMs I have require.

The saved P file containing the ROM code I turn into a WAV using a program called winTZX then load this into the Spectrum and burn it to an EPROM. (2764 in my case)

I should add that this only needed doing once as I now have the different ROM contents stored on various EPROMs and only need to plug one into the programmer, dump its contents into the Spectrums memory then burn another EPROM as required.

I used this same method to dump the contents of the Memotech Z80 assembler modules EPROM code, which when loaded back into the ZXpand at the address the original code sits at, allows it to be used as said assembler module for anyone who doesn't possess this module and would like to try it or for those of us who have it but would rather preserve it for posterity.
mikeh_nz
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Location: Seattle

Re: How to upgrade to a 649 rom?

Post by mikeh_nz »

An update...

Chips have arrived. Played around tonight with getting it to read the rom. In this setup, I used the shift registers to handle the A0..A12 lines. And then separate lines for D0..D8 back to the Ardunio.
Its basically a clone of Ben Eator's EEPROM Programmer at this point.

In the weekend I'll have a go at programming the CY7C264...
Reading the ROM
Reading the ROM
Attachments
romReader.txt
Arduino program to read and print the rom contents.
(2.79 KiB) Downloaded 50 times
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