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EPROM flash and change


Newbie level 6
May 1, 2024
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I hope this the correct place to post this, feel free to move it if not.

Please excuse my ignorance but i require some knowledge please. I have an old 8 bit computer which i believe needs the BIOS Rom replacing, my intention is to replace it with an Eprom and flash it myself. I have purchased a cheap programmer form China and downloaded some software which i believe to be compatible. However, nothing i look at seems to list the chip i am trying to flash.

The original chip is a Toshiba TMM23256P which i would like to replace.

The replacement is an AM27C256-120DC which i think is an AMD IC (??)
( This is UV Erasable ? Is it usable or do i need something else?)

Pics attached.

The programmer is a SP200S-V2.00 which connects via USB using a CH340 Driver. And i have ASprogrammer. NEOprogrammer and WLPro-V220 that seem to lauch ok but not detect or list the specific chip(s) i have.
Do i need a different chip, a different program(s) or is this possible with what i have? All i have in way of a UV light is a curing light for solder mask, is this useable or not ?

Any advise is greatfully received.



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They are pin compatible and the EPROM has a slightly faster access time so it should work as a replacement.

Yes, it is UV erasable, the quartz window is to let light through to the silicon, after programming it is advisable to place a label over it to keep light out. I'm not sure what wavelength the curing light is or whether it will be powerful enough. I use 8W tubes at a distance of about 20mm and they take about 15 minutes to fully erase. Make sure the window is clean, old label glue or similar will cause a shadow on the silicon and it will not fully erase. I'm not familiar with your programming hardware but you should find an "erased check" facility somewhere to check it is really empty. Note that an empty device reads back as having all bits set, in other words 0xFF in all locations.

Almost all EPROMs of the same size will program in identical fashion, the major difference is the voltage you have to apply to put them in programming mode. For the AM27C256 series it has to be 12.75V +/- 0.25V on the VPP pin and 5V on VDD. Other than that you can probably program it with any '256' EPROM type selected.

Hi and thanks for the reply

This is likely to be a one-off job and i dont want to invest in UV lighting if there is an alternative. Could you recommend a replacement that could be flashed with this Programmer i have and, is a direct replacement for the old one please?

your help is appreciated.
UV is the only way to erase them I'm afraid. Your picture shows one that has had a sticky label on it at some time so it probably has a program in it already so needs erasing before it can be used again.

The devices are still available from most suppliers and even from Ebay but if you buy a new one make sure it has sufficient speed rating. Usually the part number ends with "-xxx" (yours is -120) where the xxx is the number of nanoseconds it takes to select and read each byte of data. The TMM23256 has a rated speed of 150nS and any lower number, meaning faster, should be OK. If you try to use a slower device, it might work but the chances are it will read incorrectly at some point and crash your machine.

If you are willing to wait you can use direct sunlight.

I have had decent luck, though not with your specific
memory P/N, using the Willem programmer on UV
@betwixt - i am not too far from you i beleive, and agree the sun is in short supply at the moment. But when it comes who would be anywhere else.

I think it will be best to get a "non UV"replacement, they dont break the bank so the Wife shouldnt object :)

@dick_freebird thanks for the suggestion, i think this programmer will work ok once i get the right components, but i'll keep your suggestion to hand in case.

i'll try a few reads of the current eproms and see if they are blank or not, i should imagine they have already been done and just need reprogramming and covering up. Once i get the software to recognise one, i'll give it a go.


If the xxROM is ony written in the programmer (not in the application) then one may use pin compatible EEPROMs, FLASH, or battery_backupped SRAMs

The whole thing sounds strange. Never have seen an universal EPROM/Flash programmer not supporting AM27256 and AM27C256. Seems that the listed programmers are only supporting uC and serial flash. If so, there's no suitable parallel memory device.
Unfortunately programmers on the market will tend to move
with the times and maybe we are losing coverage of "classic"
parallel EPROMs. And then you lose 5V operation, going modern,
which then obviates using 74HC/AC logic for playing around.
A cascade of rip-up "for want of a nail".

You might be needing to pursue older programmers on the
aftermarket, although present vendors might have some
old stock that isn't advertised anymore for "shelf space".

Might begin for Googling your part type and "programmer"
with an eye to interfacing with your computer (like, mine is
so old it uses DB25 parallel port, when's the last time you saw
one of those?). I keep WinXP machines around just for this
kind of thing.
You get new designed universal programmers with USB interface that still support all industry standard EPROMs, but probably not for $10.
Presently I'm happy with a 15 or 20 years old Galep-4 as long as I have a PC with legacy LPT port.
Using a TL866 here with USB interface and "minipro" software under Linux. Programs just about everything, including EPROMs.
I've checked my stock and have lots of '256 EPROMs but none rated fast enough to replace the original ROM, if I had you could have one. I do have some stock elsewhere and will check as soon as I can.

Question for Deviousdexter: do you have the binary or hex file to program into the new IC?

Hi, sorry was onto other things for a while. Got CO2 laser not in good health. :)

Yes i downloaded an image for the bios, which came from someone who used the image succesfully for his own machine.

I would of thought, because all the labels are removed, the existing ROMs have been cleared and should just need programming. I will run up some of this software and see if i can get it open a'compatible' chup and see if i can read one. I bought 5 originally at a low price so not very hopeful ( of the programmer or IC)



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A good while back I was building a digital modulator
engine (AWG as EPROM+DAC, and a mixer) and I
found some "one time programmable EPROM" parts
which had no window, plastic packaged, so no erase
but they were faster (like 75ns vs 150ns?) and dirt
cheap (basically 256K PROM).

If your success rate is high and reprogramming (after
this) is not really much of the plan, check it out.
I had'nt looked at it but it a Microsoft product. I can see the line near the end i think you are referring to, but this is the file the original user has uploaded to their site ( with several others) ROMs/ROM's RepairBas/ - Please use caution as with all links, i cannot vouch for the site.
i have ordered another programer as i cant get the first one to recognise the IC, perhaps ill have better luck with the new one and can progress further. It should arrive shortly and i'll give it a go. Then just need to find a reasonably priced replacement.
I checked my stocks and unfortunately I only have 150nS 27C256 devices. I can program one for you if you want me to. It might work but not knowing the expected speed it might be too slow to work. 120nS means it can be read about 8.3 million times per second but 150nS means only 6.7 million times, without knowing the design of the product it isn't possible to say whether that would be fast enough or not.


The IC is for an 8 Bit MSX computer, i suspect this may have gotten corrupted over time and the machine is not displaying to a screen although preliminary testing shows out puut on a cheap oscilloscope. It may not be this and may very well be the 'LVA 510' chip which i understand does some video conversion ???

Is a socketed IC so when i get the replace programmer i will try to read it and see if i get anything off it or it is indead corrupted.

I can see a few IC's are available if this programmer works, so fingers crossed.
Well, just my luck to find this programmer is, it looks , a parallel programmer taking external power from a barrel jack or usb. I dont think i have a parallel port machine or a expansion card (Not sure if a parallel to USB convertor would work ?). I saw the USB port and wrongly assumed it could flash through it too. So my quest continues!

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