Home

IBM 5150  -  Exchanging the U33 ROM for an EPROM


The BIOS ROM on an IBM 5150 motherboard is located in socket U33.

There are reasons why you may need to exchange it for 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).

The ROM that IBM used for the BIOS has a part number of MK36C35. The MK36C35 is an 8 KB sized device. It is not easy to find a direct EPROM replacement, and if you do have such a replacement, you then have the issue of locating equipment that can program it.

An option is to use a 27xxx series EPROM together with a suitable adapter. 27xxx series EPROMs are readily available and you can still purchase EPROM programmers on eBay (and elsewhere) that can program the 27xxx series EPROMs.


Option 1 - Purchase

An adapter that will work with 2764, 27128, 27256 and 27512 EPROMs (or 27C64/27C128/27C256/27C512) is advertised at http://store.go4retro.com/2364-adapter/

The 2364 is pin compatible with the MK36C35 ROM used in the IBM 5150, and so don't worry that the seller's description omits a reference to the MK36C35. I have one of these adapters, and it works.

Note that the seller offers either the PCB alone, or a complete unit.

This particular adapter ties pins 1, 26 and 27 of the 27xxx EPROM socket to Vcc (via a pullup 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 EPROMs.


Option 2 - Make Your Own

One of the adapters that I've constructed is pictured below. Unlike the above adapter, it has pins 26 and 27 of the 27xxx EPROM socket tied to ground. That results in three things:

1. It is only suitable for 27256 and 27512 EPROMs (or 27C256/27C512).
2. When this adapter is used on the IBM 5150 motherboard, the 5150 motherboard will only see/use the first 8 KB block within a 27256 EPROM.  [because A14=0, A13=0]
3. When this adapter is used on the IBM 5150 motherboard, the 5150 motherboard will only see/use the fifth 8 KB block within a 27512 EPROM.  [because A15=1, A14=0, A13=0]

Of course, in making the adapter, you have the option of tying pins 26 and 27 of the 27xxx EPROM socket to Vcc. That would turn it into the adapter described in Option 1.


 


It is beyond the scope of this web page to describe how to construct the adapter. Following is the schematic diagram.


    U33
   SOCKET                       27256
 +----------+                 +----------+
 |          |                 |          |
 |  1 (A7)  |---------------->|  3 (A7)  |
 |  2 (A6)  |---------------->|  4 (A6)  |
 |  3 (A5)  |---------------->|  5 (A5)  |
 |  4 (A4)  |---------------->|  6 (A4)  |
 |  5 (A3)  |---------------->|  7 (A3)  |
 |  6 (A2)  |---------------->|  8 (A2)  |
 |  7 (A1)  |---------------->|  9 (A1)  |
 |  8 (A0)  |---------------->| 10 (A0)  |
 |          |                 |          |
 |  9 (D0)  |<----------------| 11 (D0)  |
 | 10 (D1)  |<----------------| 12 (D1)  |
 | 11 (D2)  |<----------------| 13 (D2)  |
 |          |                 |          |
 | 12 (GND) |-----------+-----| 14 (GND) |
 |          |           +---->| 20 (/CE) |
 |          |           +---->| 26 (A13) |
 |          |           +---->| 27 (A14) |
 |          |                 |          |
 | 13 (D3)  |<----------------| 15 (D3)  |
 | 14 (D4)  |<----------------| 16 (D4)  |
 | 15 (D5)  |<----------------| 17 (D5)  |
 | 16 (D6)  |<----------------| 18 (D6)  |
 | 17 (D7)  |<----------------| 19 (D7)  |
 |          |                 |          |
 | 18 (A11) |---------------->| 23 (A11) |
 | 19 (A10) |---------------->| 21 (A10) |
 |          |                 |          |
 | 20 (/CE) |---------------->| 22 (/OE) |
 |          |                 |          |
 | 21 (A12) |---------------->|  2 (A12) |
 | 22 (A9)  |---------------->| 24 (A9)  |
 | 23 (A8)  |---------------->| 25 (A8)  |
 |          |                 |          |
 | 24 (VCC) |-----------+-----| 28 (VCC) |
 |          |           |     |          |
 |          |           +-----|  1 (VPP) |  [VPP on a 27256, A15 on a 27512]
 |          |                 |          |
 +----------+                 +----------+