There are three versions of the original Radio Shack Multi-Pak Interface:

• 26-3024 Multi-Pak Interface for Color Computer 1 (grey with curved front)
• 26-3124 Multi-Pak Interface for Color Computer 2 (white with sloped front)
• 26-3124A Multi-Pak Interface for Color Computer 3 (white with sloped front)

Note: POKE 65315, 54 : REM &HFF23

Ghost Busting
The reason for the Multi-Pak fix for the CoCo 3 is at least twofold.

First, the older PAL chips used to decode the software slot select port for the Multi-Pak “ghosted” from $FF7F to $FF9F. [Also known as “mirroring” memory ranges]

That is, when a value was written to $FF7F, it appeared at $FF9F also, and vice versa.

This ghosting caused no problems with the CoCo 2, but it plays havoc with the CoCo 3, which occasionally wants to write to $FF9F when talking to its GIME chip.

Similarly, any attempt at slot selection with a ghosting Multi-Pak will send spurious data to the GIME chip.

Thus, the decoding of the software slot selection port had to be made more complete.

Second, Tandy felt it necessary to lock out the $FF80 through $FF9F range ($FFA0 and up are already locked out by the programming of the old Multi-Pak PAL chips).

This was to protect the GIME chip from conflicts with information from other external devices that might be addressed in that range.
~ Excerpt from “A PAL For Your CoCo3” by Marty Goodman, The Rainbow, January 1987 , pages 98-102.

The article also explains in outline form how to upgrade Multi-Paks if you are electronically inclined.

Others in the CoCo community, such as a Mark Marlette (Cloud9) or Richard Lorbieski (BoysonTech) may also be contacted regarding upgrade work if you are not electronically inclined.

http://www.frontiernet.net/~mmarlette/Cloud-9/Images/MPI%20PAL.jpg Upgrade Your Multi-Pak To Work With The CoCo 3

This upgrade allows Radio Shack's 26-3024 Multi-Pak to work properly with the CoCo 3'.

Without this modification, the Multi-Pak and CoCo 3's memory map overlap and can cause unpredictable results, possibly even data corruption under certain conditions.

If you're running an non-upgraded 26-3024 Multi-Pak with a CoCo 3, don't take a chance.

Installation of the upgrade is easy – no soldering is required.

Does My Multi-Pak Need This Upgrade?

Here's a simple test that will determine if your Mulit-Pak has already been upgraded to work with a CoCo 3.

From BASIC's OK prompt, type PRINT PEEK(&HFF90) and press ENTER.

If 126 is returned then the Multi-Pak has been upgraded.

If it returns 255 then it has NOT been upgraded.

Ed “Zippster” Snider has a range of 2 and 4 slot modern MPI replacements with additional features:
https://thezippsterzone.com/2018/05/09/mega-mini-mpi/

http://www.colorcomputerarchive.com/coco/Hardware/ - Cartridge Y Cable Design (have not tested this)

MPI Owners Manual (archive)
MPI Service Manual (colorcomputerarchive)

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