EPROM burning : how to program the bottom half of a chip?

ifkz

Well-known member
Joined
Oct 3, 2006
Messages
1,544
Reaction score
45
Location
Texas
EPROM burning : how to program the bottom half of a chip?

I have Contra PCB that had the mask ROMs stolen sometime during its hard life. I bought four 27C400 to use as replacements and I was told I need to "program the bottom half" to get them to work (along with tying pin 1 on each chip to ground). How do I program them correctly?
 
Umm, are your four 27C400 being used to replace one mask rom (unlikely)?

I assume it's one for each mask rom. The 27C400 might be a larger capacity chip than the originals so you would have to write the same data over and over to it until the entire chip is full.

It also might have a different pinout than the original mask roms too so you would need to look at a schematic and run jumper wires, cut traces, etc.

Hope that makes sense.
 
Here is a very general explanation:

So... lets say you need a 64K chip but you don't have any. You have a 128K chip with the same basic pinout. The 128K chip is broken up (internally) into two 64K memory blocks (64K high +64K Low = 128K). The chip toggles between the high and low block by a trigger signal on one of the legs. Ground that leg and it reads the only the low memory, let the leg float (or tie it to +5... I don't remember as it has never been necessary) and it reads the high memory.

So, there are several ways to do what you want. The easiest is to take your file, open it in an editor, copy the contents, paste the contents at the bottom (doubling the file) and save it. Now instead of a 64K file you should have a 128K file. Burn it onto your 128K EPROM. Now you have the exact same information in both high and low memory. Now tie that trigger pin to ground so the chip will ONLY access the low memory and you have what is essentially a 64K chip.

Using the same formula, if you knew how to ONLY program the bottom half of the memory you could just leave the top blank but that is a google problem. Double the program and you don't have to learn anything new....

Some years ago I posted the specifics of this procedure on this board somewhere when I had to replace a bad masked ROM on my Paperboy.
 
Last edited:
Great, that makes sense given the instructions. Is there a "favorite" editor to use to double the file? I assume something more than notepad, like the EPROM burner software?
 
I'll do that! Ah, DOS, my first love...
 
First, a bit of an academic distinction: not all ROMs without a quartz window are "mask" ROMs; many are in fact OTP EPROMs. They come into existance quite differently: mask ROMs are "build" with their data "in" them, OTP EPROMs are built empty and programmed just like a regular EPROM but lack the window for UV erasure. However, the net effect is mostly the same to you... they can't be erased and reprogrammed.

Secondly, you said you have 27C400 EPROMs. According to the datasheet I find, those are 40-pin devices (can be configured for 8-bit or 16-bit wide storage). Are you Contra EPROMs 40-pin? Or are they 32-pin?? (Or are your new EPROMs actually 27C040? or 27C4000?)
 
Back
Top Bottom