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View Full Version : 27256 or 27C256 rom chips



k2x4b524[
May 28th, 2012, 03:32 PM
Greetings again, i'm posting this here as i'm trying to source a few for my AT to update the rom for 1.44mb support. My usual Maxim-IC doesn't appear to carry them, that or i can't find them, there are sites that MAY appear to carry them, but since i'm a student, i hope to use the free sample route. Not much luck though, any suggestions on who still carries them?

Chuck(G)
May 28th, 2012, 04:21 PM
Any particular reason that you don't want to use EEPROM? 28C256s are pretty common.

But Futurlec has the 27C256s for less than $3 the each.

k2x4b524[
May 28th, 2012, 06:06 PM
28c256? didn't know about them, but primarily because my burner is good for the glass window chips only *i think*

Chuck(G)
May 28th, 2012, 06:21 PM
Unless it's very old, it should do EEPROMs.

ibmapc
May 28th, 2012, 07:26 PM
Are the 28c256s pin compatable with the 27c256s? If so, I've wasted a lot of time putting 27c256s under the UV light for erasure.


Any particular reason that you don't want to use EEPROM? 28C256s are pretty common.

But Futurlec has the 27C256s for less than $3 the each.

k2x4b524[
May 28th, 2012, 07:53 PM
heh, i scrounged 2 from a dead speedstar 64 and they are under my chip erasure light *yep i spent the cash on one* way back when, so we see how that goes

Chuck(G)
May 28th, 2012, 08:20 PM
Are the 28c256s pin compatable with the 27c256s? If so, I've wasted a lot of time putting 27c256s under the UV light for erasure.

Close enough--pin 1 on the 28C256 is A14, while on the 27C256 it's Vpp.
Pin 27 on the 28C is WE/, while on the 27C it's A14.

Just a simple pin swap--between 1 and 14. Easy enough to work up a simple adapter.

ibmapc
May 28th, 2012, 10:53 PM
Close enough--pin 1 on the 28C256 is A14, while on the 27C256 it's Vpp.
Pin 27 on the 28C is WE/, while on the 27C it's A14.

Just a simple pin swap--between 1 and 14. Easy enough to work up a simple adapter.

I also compared a datasheet for a 29c256 and found the pins to line up better to the 27c256. One difference I found was that on the 29c256, pin 14 was labeled GND but on the 27c256 it's VSS. Is GND the same as VSS in this case? I'm also assuming that we don't care about WE since we won't be programming it in place? I know that the 29c256s are getting kind of rare, but I think I found an EBAY seller that I can work with. He sells them in quantities of 10 for 39.99 but I'm trying to talk him in to selling two for a simalar per piece price.

pearce_jj
May 28th, 2012, 11:39 PM
This is another possible solution (http://vintage-blog.peacon.co.uk/wiki/8bit-rom-board).

Chuck(G)
May 29th, 2012, 12:03 AM
For most chips, Vss = Gnd; Vdd = Vcc.

The issue is mostly one of terminology. A FET is souce-gate-drain, so Vss, Vdd; A junction transistor is emitter-collector-base, so Vee (usually labeled GND) and Vcc.

Regardless, you want a logic high on WE/ so your data isn't inadvertently overwritten.

ibmapc
May 29th, 2012, 09:07 AM
For most chips, Vss = Gnd; Vdd = Vcc.

The issue is mostly one of terminology. A FET is souce-gate-drain, so Vss, Vdd; A junction transistor is emitter-collector-base, so Vee (usually labeled GND) and Vcc.

Regardless, you want a logic high on WE/ so your data isn't inadvertently overwritten.

So... Do ya think the 29c256 will work in place of the 27c256? I don't quite follow about "logic high on WE/"

Chuck(G)
May 29th, 2012, 09:31 AM
The 29C256 should work without any socket mods. Just check pin 1 on the socket with a meter to make sure that it reads somewhere around +5. Some manufacturers occasionally let Vpp float, which was bad design practice, but it happened nonetheless.

ibmapc
May 29th, 2012, 11:24 AM
The 29C256 should work without any socket mods. Just check pin 1 on the socket with a meter to make sure that it reads somewhere around +5. Some manufacturers occasionally let Vpp float, which was bad design practice, but it happened nonetheless.

Socket is U18 and U19 on IBM 5160 64-256K Mother board. Do you think IBM did it right?

Chuck(G)
May 29th, 2012, 01:23 PM
Socket is U18 and U19 on IBM 5160 64-256K Mother board. Do you think IBM did it right?

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.

pearce_jj
May 29th, 2012, 01:34 PM
...or use an add-in ROM card.... :)

Chuck(G)
May 29th, 2012, 01:54 PM
Does that ROM card work when it's mapped into the default BIOS space (F6000-FFFFF)? I wouldn't think that it does.

k2x4b524[
May 29th, 2012, 03:53 PM
The bios works, buy the at still refuses to employ 1.44 Mb floppies. But now I can feed it my Everex mfm card so I can get around it. I think that stock controller hates 1.44 floppies it 3.5inch in general

pearce_jj
May 29th, 2012, 10:05 PM
Does that ROM card work when it's mapped into the default BIOS space (F6000-FFFFF)? I wouldn't think that it does.

Yes, I booted a 5160 with it with its ROM chips removed.

modem7
May 30th, 2012, 01:56 AM
The bios works, buy the at still refuses to employ 1.44 Mb floppies. But now I can feed it my Everex mfm card so I can get around it. I think that stock controller hates 1.44 floppies it 3.5inch in general
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.

Chuck(G)
May 30th, 2012, 07:30 AM
Yes, I booted a 5160 with it with its ROM chips removed.

That is interesting. My initial impulse would be that the decoding for the planar ROMs would happen separately from the ISA bus. Time to go back and stare at the schematics--that would seem to imply that the F0000-F5FFF area is fair game for such stuff as adapter RAM and memory-mapped I/O.

Does it work similarly on a 5170 (such as what the OP is using)?

brain
May 30th, 2012, 10:54 AM
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

Chuck(G)
May 30th, 2012, 12:54 PM
I don't think you can do much better than Futurlec (http://futurlec.com/ICEPROM.shtml) for these things at $1.80 the each, new.

MikeS
May 30th, 2012, 02:18 PM
I don't think you can do much better than Futurlec (http://futurlec.com/ICEPROM.shtml) for these things at $1.80 the each, new.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).

k2x4b524[
May 30th, 2012, 03:12 PM
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.


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.

k2x4b524[
May 30th, 2012, 03:13 PM
Time to start scavenging rom chips from some of them thar 1 and 2mb pci video cards for the windowed chips....


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

Chuck(G)
May 30th, 2012, 03:23 PM
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).

Calling those an EPROM is just plain false advertising. There's nothing erasable about them.

MikeS
May 30th, 2012, 05:11 PM
Calling those an EPROM is just plain false advertising. There's nothing erasable about them.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.

brain
June 1st, 2012, 03:28 PM
I don't think you can do much better than Futurlec (http://futurlec.com/ICEPROM.shtml) for these things at $1.80 the each, new.

They have a good price. If someone wants to talk quantity, I can probably do $1.50/piece.

Mine are new as well, and I can get other sizes if desired.

Jim

pearce_jj
June 2nd, 2012, 12:56 PM
Does that ROM card work when it's mapped into the default BIOS space (F6000-FFFFF)? I wouldn't think that it does.

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.

k2x4b524[
June 2nd, 2012, 02:09 PM
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.

Chuck(G)
June 2nd, 2012, 02:18 PM
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).

Chuck(G)
June 2nd, 2012, 02:20 PM
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.

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.

ibmapc
June 9th, 2012, 01:56 PM
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.

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.

ibmapc
June 10th, 2012, 12:50 PM
It works great. Now I can make changes to the bios without getting out the UV eraser. I used two of these as adapters.
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.

Chuck(G)
June 10th, 2012, 01:43 PM
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... :)

ibmapc
June 10th, 2012, 02:00 PM
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... :)

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.

Chuck(G)
June 10th, 2012, 02:08 PM
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.

ibmapc
June 10th, 2012, 02:21 PM
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.
Actually, it's a 5155, but as you know it uses the early 64-256K 5160 motherboard. Thanks for looking into the matter!

Chuck(G)
June 10th, 2012, 03:07 PM
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.

MikeS
June 10th, 2012, 04:27 PM
It works great. Now I can make changes to the bios without getting out the UV eraser. I used two of these as adapters.
9303
Something wrong with that link?

ibmapc
June 10th, 2012, 04:51 PM
Something wrong with that link?

Sorry Mike, It's fixed now.

ibmapc
June 10th, 2012, 05:01 PM
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.

That would be pretty cool, but I would need to be able to select between U18 or U19. The code for this is above my abillities right now, but maybe in a year or two or less if I can find the time to get really serious about learning Assembly.

MikeS
June 10th, 2012, 07:22 PM
Sorry Mike, It's fixed now.Ah, thanks; for some reason I thought you were using the PLCC version. Could I ask you where you bought the DIP 29C256s, what speed and for how much (PM if you prefer)?

Thanks again.

Chuck(G)
June 10th, 2012, 07:27 PM
U18 and U19 have different address ranges. So, you'd hook pin 1 (WE/) of both 29C256 chips to the XMEMW/ (pin 5 of U13) and then hook pin 22 of both chips to XMEMR/ (pin 3 of U13). So each 29C256 would occupy a different I/O range.

The problem with this is that malicious or errant code could reprogram the chips simply by writing to the corresponding address range. Fortunately, it's possible for software to enable or disable protection so that this can't happen. It's very cool.

There's a lot of good stuff in the 29C256 datasheet.

ibmapc
June 10th, 2012, 07:45 PM
Ah, thanks; for some reason I thought you were using the PLCC version. Could I ask you where you bought the DIP 29C256s, what speed and for how much (PM if you prefer)?

Thanks again.
PM sent

MikeS
June 11th, 2012, 06:09 PM
PM sentPM received; thanks!

k2x4b524[
June 11th, 2012, 06:22 PM
Thread Re-Hijacking

One of the originals of this thread, the at problem. The damned thing still refuses to do 1.44mb controllers. With ANY rom i feed it, and with ANY drive i feed it attached to any controller. Though replacing the stock ibm one gave me 720kb functionality, i would like 1.44mb functionality here aswell. HELP!!

MikeS
June 11th, 2012, 06:46 PM
...The damned thing still refuses to do 1.44mb controllers. With ANY rom i feed it, and with ANY drive i feed it attached to any controller. Well, "ANY" covers a lot of ground; assuming you're using the ROMs Modem7 mentions in post #19, your controller is 1.44MB capable and you've GSETUP the BIOS properly, that leaves the drive and the diskettes. What does "refuses to do" actually mean?

Chuck(G)
June 11th, 2012, 07:59 PM
Have you tried the AMI BIOS in it?

Another thought: Let's make sure your drive works okay. Install the program attached to this post (http://www.vintage-computer.com/vcforum/showthread.php?30962-IDE-Transfer-DMA-Code&p=229184#post229184). Be sure to configure your 3" drive as a 1.44M and try it (it's a formatting popup). it doesn't care about your BIOS--it's strictly direct controller access. If it doesn't like your drive, then the problem's hardware, not software.

k2x4b524[
June 11th, 2012, 08:07 PM
By any, i mean my everex EV346M, the stock western digital ibm board, a 1003-WAH paired to a KW-503D High Density controller, and am using the AMI Bios, which natively supports 1.44, i'm going to try and download the phoenix bios to see if there is a difference, but the AMI bios is pretty spiffy, or i may just cave and re-assemble my backpack floppy, that or use laplink..

modem7
June 13th, 2012, 03:20 AM
By any, i mean my everex EV346M, the stock western digital ibm board, a 1003-WAH paired to a KW-503D High Density controller, and am using the AMI Bios, which natively supports 1.44, i'm going to try and download the phoenix bios to see if there is a difference, but the AMI bios is pretty spiffy, or i may just cave and re-assemble my backpack floppy, that or use laplink..
I see from an earlier post that you are using a 'dual' drive. Have you tried a standard 1.44M drive?
If no, you may find that a standard 1.44M drive works, thus indicating that your 'dual drive is the problem cause.

If yes, have you tried the known 1.44M-good hardware/BIOS solution I posted at post #19 (standard 1.44M drive / 3rd IBM BIOS / IBM 5170 controller / GSETUP) ?

ibmapc
June 22nd, 2012, 05:21 PM
U18 and U19 have different address ranges. So, you'd hook pin 1 (WE/) of both 29C256 chips to the XMEMW/ (pin 5 of U13) and then hook pin 22 of both chips to XMEMR/ (pin 3 of U13). So each 29C256 would occupy a different I/O range.

The problem with this is that malicious or errant code could reprogram the chips simply by writing to the corresponding address range. Fortunately, it's possible for software to enable or disable protection so that this can't happen. It's very cool.

There's a lot of good stuff in the 29C256 datasheet.

Hey Chuck,

Do pin 22 of u18 and u19 need to remain connected to their original mother board conections, or should they be isolated before connecting them to pin 3 of u13? It looks like they are connected to pin 20 CE. I haven't determined if they are connected to anything else. Both pin 1's are already isolated, so that won't be a problem.

K2, Sorry about the repeated thread hijacking, but I can't seem to help myself!

Chuck(G)
June 22nd, 2012, 06:27 PM
You need to isolate pin 22--otherwise it's always enabled when the EEPROM is addressed--and if you're going to be writing to the EEPROM, you don't want that!