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Question about EPROM...please help!

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Jan 6, 2007
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Question about EPROM...please help! ST M27c256b

Hey guys! Ok, here's the deal...I have several old out of production Eqaulizers that I need to update to the latest firmware. I got ahold of the company and he was able to send me 2 eproms, but that was all he had. He suggested I copy the eproms to make more, which is what I'm trying to do.

The Eprom being used is an STMicro M27c256b chip.

Now, I haven't played with these since college many years ago, so here's what I need to know...

1. Can this eprom be read, and then the program saved and put onto a fresh chip?

2. Is there an inexpensive solution to accomplish this?

I have a BP Microsystems PLD-1128 burner, however, it seems it doesn't support this particular chip so I'll need to get something else...

Any suggestions? Thanks!

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Yes, the 27C256 is completely readable otherwise it couldn't be used!
It is a very standard device, almost all EPROM programmers can handle it. Beware that there are two varieties with the same part number, one uses 12.5V VPP and the other uses 21V VPP, which to use will be marked on the device.

another Brian.

You can buy cheap EPROM programmers on eBay like this one **broken link removed**

Ok...they have stickers over the windows and part numbers, and I don't want to remove them until I'm able to read them! I have peeled one side back enough to see this though...

12F1 L
96001 V5
MYS 8B 426

That is on the newer chip that they sent me.

Hmm...I just peeled back some of the older chip and found this next to the "ST" manufacturer symbol...


Apparently they used different chips in the past...I assume its an older variant of the m27c256b..., here's a few more questions then...

1. Is there a cheap burner that will let me read and write both of these if need be? I assume if it'll do one it should do the other and all the varients?...but I want to be sure before I buy something that doesn't work!

2. Also, you mentioned that some chips want certain voltages, how do I know if the burner is capable of both the voltages that will possibly be needed?

3. Once I retrieve the data from the eprom, can I then erase the old one's and put the new program on them? I know these need light to be erased, but what kind of light? How much? etc.etc.

Thanks so much guys for your help!


The 27256 and 27C256 are essentially the same device, in the early days they used a slightly different technology, PMOS and NMOS. The 'C' version uses CMOS technology which had much lower power consumption but was otherwise almost identical. It was quite normal for non 'C' to be replaced by 'C' to make the circuit more power efficient. Electrically they are interchangeable.

The 27C256B 12F1 uses 12.75V programming voltage and has 120nS access time. Most, if not all, later devices used 12.75V and modern day replacements will almost certainly use that voltage. The access time is important but any device with 120nS or faster access will work OK. The only danger is using slower access time devices which *may* not work but probably will.

To erase the ICs you need a UV light source. Don't worry about removing the labels for a brief period, even in bright sunlight they took days to erase so a few moments in room light is quite safe. There are special erasers which are safe to use, they have a drawer in which the IC is placed which slides inside a light proof box under the UV tube. The tube will normally use mercury vapor and emit light at around 245nM which is extremely dangerous to look at or expose skin to for more than a few seconds, hence the light proof box. Erasure is usually complete in 20 to 25 minutes with the IC around 25mm (1") away from the tube. I advise you to get a purpose made EPROM eraser rather than try to make one yourself.

You can reuse the same IC many times over. Although they do degrade each time they are reprogrammed, you should get about 500 cycles per IC which in most cases equates to centuries of use!

Where in the World are you, it helps to know so we can advise on suppliers.


Hey Brian,
Thanks for that great info...

I was doing some searching and found I can get replacement eproms for under $4 each, at that price I don't think a purpose built eraser is warranted. I really only need to do about 6 of these...

I'm in the USA, Ohio...!

Ok, so, if electrically these are all the same, then the programmer should be able to read/write them all...correct?

I can't believe this BP Microsystems burner doesn't do these!

Anyway, so I need to find an inepensive burner to do these...I looked at that one the other poster provided on Ebay, I did see a similar part number from another manufacturer, but didn't see the actual part number from ST micro. I assume it will work on them all makes of comparable part numbers?

Also, I know with the BP micro burner and software, I had to pick a specific maker/chip number in order for it to connect, I'm a little worried about a generic programmer not having the ACTUAL part I need listed and then not working? Do you have any experience with this? Maybe the newer programmers work differently?

Thanks again for your help...I just need a reasonable working solution for this...thanks!


That's correct. Its worth remembering that these devices were made in their millions as the main BIOS memory for computers as well as many other appliances so they are very 'standardized' across manufacturers and programmng systems. There are minor differences from one manufacturer to another but they are mostly to do with how fast they can be programmed, this was important to very high volume users. The basic principle of storing data in them is to set up the address, data and programming voltage then pulse the -CE or -CS pin. The data is then read back and compared to what was written, it if matches the process is repeated a few more times to be certain the memory cell is fully charged, if it doesn't match the writng process is repeated maybe 50 times before giving up. Manufacturers concentrated on how quick the pulses had to be and minimizing the number of tries before the data 'stuck' but the differences were minimal and of no consequece if you are handling small quantities.

Your BP programmer is really for PLD devices rather than EPROMS, the way they are programmed is slightly different. Many programmers can handle both types equally well.

I would suggest you read the master devices and keep a copy of the data inside them on a CD in case you need it in the future. There are several standard formats for saving EPROM data, the most common being "raw binary" which is just a list of the binary numbers from each EPROM address and "Hex" format which is the same information but converted to text with added address information and checksums to ensure integrity.

For 6 ICs it might be worth asking around locally to see if someone has a programmer you can use, it only takes about 30 seconds per IC to program them so it may not be worth buying one.


forget about prefixes on EPROMs the relevant part of the marking is 27(C)256, as betwixt pointed to you, and the 'speed' of the chip, eg -12, -10, ... in suffix. C stands for CMOS technology, early ones were PMOS or NMOS (higher current consumption).

Don't bang your head much (otherwise it will HURT), just use any 27C256 EPROM that you can find in your market (which is BTW huge), take care only on the access time.

^^^, would that $39 deal on ebay be the way to go then?

It's out of the country (which sucks), I wish I could find something in the states...but if not maybe thats the way to go?

It looks like a good buy and it programs microcontrollers as well. I have ordered from China and Hong Kong many times using Ebay and only once had a problem of non-delivery, even then the seller sent out a replacement straight away. I don't know if the same applies in the US but sometimes the Post Office here will intercept international packages and add a customs charge to them. The amount is usually small but the hassle of paperwork and having to travel to the local PO to pay it is a pain, especially when you live out in the wilds like me!


Ok, I'll shoot them an email to ask about the software...thanks!


---------- Post added at 07:08 ---------- Previous post was at 06:48 ----------

Also, is there an EEPROM that I could use instead of this EPROM? Something that's a direct replacement that will accept the program from the M27c256b once it's extracted?

There's a 28256 EEPROM, but unfortunately, the pinning is slightly different from 27256. Two pins have to be exchanged on the target board.

The trouble with EEPROMs is you need a mechanism to write into them as well as read them. This usually means an extra pin to handle the 'write' control signal. They also use a rather complicated method of unlocking the internal write process which involves repeatedly sending commands to them. Unfortunately they are not simply like RAM that you just don't write to again if you want to retain data, if that was the case, any glitch on the 'write' pin could corrupt the contents.

All is not lost though, you don't need to use that specific type of 27C256B, almost any make of 27256 or 27C256 will work as a replacement. They are plentiful and inexpensive.


Writing to FLASH EEPROM's is specific but, reading is SAME as reading an ordinary EPROM, no need for any "mechanism" during read.

I have some, rather old, vending machines, and replaced 2 27x512 with one 29F010 (a small adapter PCB, of course), and the 29F010 is FLASH EEPROM. Machines are still working 24/7.

The trouble with EEPROMs is you need a mechanism to write into them as well as read them.
Not necessarily, if used as a read-only EPROM replacement.

Thanks guys, I think I'll stick with the original chips just to be sure they will work...Thanks again!


Sorry, I didn't mean to imply there was a special way to read them. Yes, they behave just like EPROMs when programmed, it's just getting the program into them in the first place thats more complicated.


it's just getting the program into them in the first place thats more complicated.
Yes, that's true. But you can rely on the capability of most general purpose programers to handle this step.

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