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27256 adapter for IBM 5150

modem7

modem7

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The ROMs on the IBM 5150 motherboard are MK36xxx types (8 KB in size).

If you desire (or need) to fit a 27256 EPROM (or 27C256) instead of a MK36xxx, you can use the following adapter.
With this adapter, only the first 8 KB in the 27256 will be read by the 5150 motherboard.


Code: 
    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) |
 |          |                 |          |
 +----------+                 +----------+

On the 27256, by tying VPP to VCC, and tying /CE to ground, the 27256 is being placed in READ mode. In READ mode, the /OE pin is used to gate the data out, and that is equivalent to the U33 /CE pin.

The following picture shows the adapter that I recently constructed.

27256_adapter_for_5150.jpg
 
Hi! Nice! Great addition -- bookmarked for later!

Thanks! Have a nice day!

Andrew Lynch
 
Looking at that pinout, it looks like a 68764 or 68766 EPROM would be a direct replacement (as they are for most 24 pin 8K ROMS), no?
 
MikeS said:
Looking at that pinout, it looks like a 68764 or 68766 EPROM would be a direct replacement (as they are for most 24 pin 8K ROMS), no?
Click to expand...
Yes, according to http://bama.edebris.com/manuals/hp/3456a/ there are quite a few different replacements, but obtaining 27xxx is easier/cheaper and modern EPROM programmers are sure to support 27xxx (and unlikely to support the MK36xxx and equivalents).
 
By the way. Yes, the adapter looks horrible, but I produced it for diagnostic use. Using it I was able to fix a faulty 5150 type 1 motherboard. Specifically, by modifying the 5150 POST, I was able to determine that one of the RAM chips in the first (soldered-in) bank was dud, and which chip it was. The modified POST outputted codes to a POST card. I'll post details in a separate thread later when I have some time.
 
modem7 said:
Yes, according to http://bama.edebris.com/manuals/hp/3456a/ there are quite a few different replacements, but obtaining 27xxx is easier/cheaper and modern EPROM programmers are sure to support 27xxx (and unlikely to support the MK36xxx and equivalents).
Click to expand...

There was an outfit that produced a ready-made adapter to fit 2764s in the 24 pin ROM socket.

Why not an adapter to fit 4 of the 5150 ROM into a single 27256?
 
Chuck(G) said:
There was an outfit that produced a ready-made adapter to fit 2764s in the 24 pin ROM socket.
Click to expand...
Yep. In the past, there would have been sufficient demand for such an adapter.
If anyone knows of any entity still selling such adapters, please post details for forum readers.

Chuck(G) said:
Why not an adapter to fit 4 of the 5150 ROM into a single 27256?
Click to expand...
It can be done of course, with the adapter needing to plug in all of the sockets of the ROMs that it is replacing.
Considering that the 5150 has five ROMs (one BIOS and four BASIC), I think it would be better to fit 5 lots of ROM code into a 27512.
Support chips would be required (e.g. to OR the Chip Enable lines from the sockets).
But would there be demand?
 
I think I still have one of those adapters somewhere; AP tools IIRC, or maybe Cambion, but I haven't seen any in a looong while.

They're pretty easy to make though, as long as you have the vertical space: plug a low-profile 28-pin socket into a 24-pin socket and just jumper the 2 or three pins that need swapping; the rest all line up. If the application could use an alternate image, at the same time you can add a pull-up and run a wire to a switch to select either half.

Sorry for the out-of-focus pics, but you get the idea. This is a 27128 replacing a 24-pin 8K monitor ROM in a Cromemco, with a header to connect to a switch selecting two different versions.

Oops - forget about EPROM2, see EPROM3 below...
 
Last edited:
Chuck(G) said:
I doubt it. Why would anyone want to replace the ROM in a 5150, assuming that the original works and is present? And clones, by and large used 28-pin PROMs, so there's no need there.
Click to expand...
I expect no demand for a multi-ROM adapter (replaces multiple ROMs as different from switching between multiple ROM images) but very small demand for a single-ROM adapter:

1. Repair 5150 motherboard where the BIOS ROM (U33) has failed.
2. Diagnose unresponsive 5150 motherboard (such as what I did).
3. Upgrade BIOS ROM (U33) to the third revision - required for hard drive and EGA/VGA support.
 
modem7 said:
I expect no demand for a multi-ROM adapter (replaces multiple ROMs as different from switching between multiple ROM images) but very small demand for a single-ROM adapter:

1. Repair 5150 motherboard where the BIOS ROM (U33) has failed.
2. Diagnose unresponsive 5150 motherboard (such as what I did).
3. Upgrade BIOS ROM (U33) to the third revision - required for hard drive and EGA/VGA support.
Click to expand...

I don't imagine that (1) happens too often, but I do have a diagnostic ROM for (2). I suppose that EGA/VGA might be an issue, but I've run hard drives off of 5150s with early BIOSes. In fact my first hard disk was an 8" Shugart SASI model cabled to a homebrew adapter card in my 64K 5150. You needed to boot from a floppy, but no big deal.
 
modem7 said:
Can you elaborate?
Click to expand...

It's a special-order version of Landmark's 2764 POST ROM.

Yes, there's a variant to that at http://www.selectric.org/old5150/index.html
By "required for hard drive", I meant 'native' support.
Click to expand...

I've also got an Ampex SASI board for the PC, called a "Megastore".

Hard disk support on the 5150 was never "native"; all hard disk adapters had to have the BIOS extension ROM to work; there was no hard disk code in the planar BIOS.
 
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