IBM 5150 - Substituting the U33 BIOS ROM
The BIOS ROM on an IBM 5150 motherboard is located in socket U33. The ROM is an 8 KB sized device, with a specific pinout.
Reasons for substitution
There are reasons why you may need to substitute it with something else:
• You need to swap it with something else in order to determine if the ROM is faulty;
• The ROM is faulty;
• You need to upgrade the ROM to a later version (e.g. the 10/27/82 version in order to support an EGA card);
• You want to run a diagnostic ROM (such as SuperSoft/Landmark or Ruud's Diagnostic).
ROM make-models used by IBM
In the supply lifetime of the IBM 5150, IBM used varying make-model of ROM for the BIOS ROM. Some are shown at here.
- AMD AM9264
- AMI S688364
- Mostek MK36000 series. Mostek custom-programmed these for companies. The numbers after the '36' indicate which code is in the ROM.
As shown in the AM9264 cross-reference at here, there were possibly other chips used as well.
Note that if you find these ROM's on eBay (and elsewhere), they may contain something other than 5150 BIOS code.
Use of an EPROM/EEPROM to substitute
OPTION #1: Pin equivalent - MCM68766 EPROM
From a reading perspective, the Motorola MCM68766 is an EPROM equivalent (pin comparison).
if you acquire these chips yourself, you then have the issue of locating equipment that can program/burn them. In 2023, there are some modern programmers available - do an online search using: MCM68766 programmer
There are some people/companies shown at here who can supply MCM68766 EPROM's, programmed/burned to your requirement.
I suggest that you use ROM's rated at 250 ns or faster (because that is what IBM used). But the text at the bottom of here suggests that you can go as high as 350 ns.
OPTION #2: Pin equivalent - "Drop-In Replacement for the MCM68766 EPROM"
Option 1 is a MCM68766 EPROM, but this option is a modern device that emulates a MCM68766. See the YouTube video at here.
OPTION #3: Pin equivalent - 2564 EPROM
The 2564 EPROM is equivalent for reading, if pins 1, 2, 27, and 28, are not used (after the 2564 has been programmed). See the photo at here, a photo showing me successfully using a 2564 in socket U33.
There are some people/companies shown at here who can supply 2564 EPROM's, programmed/burned to your requirement.
I suggest that you use ROM's rated at 250 ns or faster (because that is what IBM used). But the text at the bottom of here suggests that you can go as high as 350 ns.
OPTION #4: 27xxx series EPROM, with adapter
An option is to use a 27xxx series EPROM together with a suitable adapter. 27xxx series EPROM's are readily available and you can still purchase EPROM programmers on eBay (and elsewhere) that can program the 27xxx series EPROM's.
An adapter that will work with 2764, 27128, 27256 and 27512 EPROM's (or 27C64/27C128/27C256/27C512) is described at here.
That adapter is described as "2364 Adapter". From a reading perspective, the 2364 ROM is pin compatible with the BIOS ROM used on the IBM 5150 motherboard. I have one of these adapters, and it works.
Note that the seller offers either the PCB alone, or a complete unit.
That particular adapter ties pins 1, 26 and 27 of its 27xxx EPROM socket to Vcc (via a pull-up resistor). Therefore, when this adapter (as supplied) is used on the IBM 5150 motherboard, the 5150 motherboard will only see/use the last 8 KB block within 27128/27256/27512 EPROM's.
I suggest that you use ROM's rated at 250 ns or faster (because that is what IBM used). But the text at the bottom of here suggests that you can go as high as 350 ns.
A photo of that adapter in use on an IBM 5150 motherboard is at here.
OPTION #5: Winbond W27E257 EEPROM, with adapter
The W27E257 is a 32 KB sized device, and if used with the same adapter as pointed to above for 27xxx series EPROM's, the 5150 motherboard will see/use the last 8 KB block within the W27E257 EEPROM.
Yes, the W27E257 is an EEPROM, however, the IBM 5150 motherboard does not support writing to EEPROM's in any of it's ROM sockets. You will need to use an EPROM programmer, or other.
There are some people/companies shown at here who can supply W27E257 EEPROM's, programmed/burned to your requirement.
ROM content images
ROM content images for programming/burning into an EPROM are at here.
Click here to see some people/companies that can supply and program/burn EPROM's for you.