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27256 or 27C256 rom chips

Chuck(G) said:
I don't think you can do much better than Futurlec for these things at $1.80 the each, new.
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Assuming he has the particular type you need I think I'd pay the extra .20 and buy from Jim; he's contributed quite a few useful and reasonably priced items for the vintage computer community, especially Commodore, and deserves our support.

BTW, when ordering EPROMs be sure to check the complete part number carefully to make sure you're getting the right package and that they are indeed eraseable; one-time-programmable (OTP) versions without a quartz window usually have the same base part number, with just a different letter in the suffix denoting a different package (i.e. one without a window).
 
I'm trying to use one of my dualie drives as i'm short on slide rails, so it's either the controller or the drive, it may be the drive in reality, tonight is the night i try my everex controller and see what happens.

modem7 said:
I have a first generation 5170 (with all-IBM parts). I got it to boot from a 1.44M sized MS-DOS 3.3 boot diskette by:

1. Upgraded BIOS ROMs to third revision (U27=62X0820, U47=62X0821); then
2. Connected a 1.44M drive to the hard/floppy controller (it is a first generation IBM one); then
3. Ran GSETUP, and set drive A: to 1.44M.
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Time to start scavenging rom chips from some of them thar 1 and 2mb pci video cards for the windowed chips....

brain said:
I think using EEPROM is a great idea, but I'd be remiss if I didn't mention that I've got CMOS EPROMs in the store for exactly this reason: http://store.go4retro.com/categories/Electronic-Components/Memory/

They are getting harder to find at reasonable prices, and windowed ones are nearly impossible to find at sane prices.

Jim
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MikeS said:
BTW, when ordering EPROMs be sure to check the complete part number carefully to make sure you're getting the right package and that they are indeed eraseable; one-time-programmable (OTP) versions without a quartz window usually have the same base part number, with just a different letter in the suffix denoting a different package (i.e. one without a window).
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Calling those an EPROM is just plain false advertising. There's nothing erasable about them.
 
Chuck(G) said:
Calling those an EPROM is just plain false advertising. There's nothing erasable about them.
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Well, I suppose technically they're the same chip inside but just in a package that doesn't allow erasing, but yes, it's certainly confusing and misleading and I often read about folks who thought they'd ordered a UV eraseable and got an OTP instead; that's why I mentioned it.

For example:
AM27C256-55D - 28-pin DIP (ceramic), UV eraseable/reprogrammable
AM27C256-55P - 28-pin DIP (plastic), OTP, NOT eraseable/reprogrammable.
 
Chuck(G) said:
Does that ROM card work when it's mapped into the default BIOS space (F6000-FFFFF)? I wouldn't think that it does.
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Hi Chuck, my ROM board works in the 64-256K mobo with system BIOS image as 64KB at F000h, but not in the 256-640KB board it seems. I don't know why this is.
 
That doesn't surprise me. There's a bipolar PROM involved in the address decoding that allows you to do some interesting things (see the 1MB mod to the 5160 640K board postings).
 
k2x4b524[ said:
hey chuck, i got a question, could the AMI AT bios not natively do 1.44? i got a feeling it is the controller, i haven't had a chance to do anything to it yet as i'm trying to leave it as stock as i can.
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As far as I know, yes it can. I'll check and see if I can't give a better answer. I think that the Phoenix 2.07 BIOS may also be 1.44M capable as well.
 
Chuck(G) said:
IBM never intended that EPROMs would be used. According to the schematic that I have, pin 1 is connected to pin 27 (check this with an ohmmeter) and labeled A14. This isn't good. You'll have to lift pin 1 of the EEPROM from the socket and tie it to pin 28. Again, not a difficult mod.
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Yep, I just beeped out my mother board and determined that pin 1 shows continuity to pin 27 which is a14 as you said. So I'll make up an adapter so that I don't have to solder on the chip or the motherboard. The 29C256 eeproms arrived today, so I'll try it out and give results, for those interested.
 
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It works great. Now I can make changes to the bios without getting out the UV eraser. I used two of these as adapters.
View attachment 9304

I clipped off pin 1 on the bottom side then removed the top and soldered a small wire between pin 1 and pin 28 to supply +5v to WE to protect the contents of the eeprom. Then, with some minor modification to the plastic to make room for the jumper wire and solder, I put the tops back on and plugged them into the Mother board, inserted the programmed 29c256's and booted it up. The Zif sockets with the eeproms plugged in stand pretty tall, but they make removing and installing the chips so much easier.
 
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It'd be even more interesting to decode WE/ and farm it to this motherboard socket. Then, you wouldn't even have to remove the chip from the socket...

You could then brick your BIOS just like the big boys... :)
 
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Chuck(G) said:
It'd be even more interesting to decode WE/ and farm it to these chips. Then, you wouldn't even have to remove the chips from the sockets...

You could then brick your BIOS just like the big boys... :)
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You mean "Flash" 'em in place? I've thougt the same thing, but don't know where to start. Maybe I could run some wires from my programmer (TOP853)to the sockets. Naw, that would be hokey. There must be something more ellegant that wouldn't need any external hardware.
 
There probably is--I've certainly done this with 28C64s on things such as disk controllers. Even more interesting is to substitute a 6264 SRAM, write the contents and then do a warm boot.

That's a 5160 you have? I'll take a look at the schematics and see if anything looks good. You shouldn't need your programmer.
 
Chuck(G) said:
There probably is--I've certainly done this with 28C64s on things such as disk controllers. Even more interesting is to substitute a 6264 SRAM, write the contents and then do a warm boot.

That's a 5160 you have? I'll take a look at the schematics and see if anything looks good. You shouldn't need your programmer.
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Actually, it's a 5155, but as you know it uses the early 64-256K 5160 motherboard. Thanks for looking into the matter!
 
It looks as if you can connect pin 1 of your 29C256 to pin 5 of U13 (XMEMWR/) and write to the EEPROM memory space.

Programming is a little tricky--you do it in blocks of 64 bytes and then wait for the programming to be complete by reading the address of the 64th byte written. While the EEPROM is busy writing, it will return the complement of the 64th byte; when the result comes back as the actual value, you've programmed in that byte is returned and can move on to the next 64 bytes.

A little code involved, I know--but then you've got a on-board flash BIOS, which I think is pretty cool.
 
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