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27256 or 27C256 rom chips

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k2x4b524[

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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?
 
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.

Chuck(G) said:
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.
Click to expand...
 
ibmapc said:
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.
Click to expand...

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.
 
Chuck(G) said:
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.
Click to expand...

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.
 
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.
 
Chuck(G) said:
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.
Click to expand...

So... Do ya think the 29c256 will work in place of the 27c256? I don't quite follow about "logic high on WE/"
 
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.
 
Chuck(G) said:
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.
Click to expand...

Socket is U18 and U19 on IBM 5160 64-256K Mother board. Do you think IBM did it right?
 
ibmapc said:
Socket is U18 and U19 on IBM 5160 64-256K Mother board. Do you think IBM did it right?
Click to expand...

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.
 
k2x4b524[ said:
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
Click to expand...
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.
 
pearce_jj said:
Yes, I booted a 5160 with it with its ROM chips removed.
Click to expand...

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)?
 
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