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Compaq Portable  -  RAM Decoder PROM in Socket U35


INFORMATION SUPPLIED BY JACOB  (of Monotech)

July 2019


General information about U35

QUOTE:

Compaq supplied three versions of the U35 PROM:
Most have a Compaq part number written on them.

	• 256K	(64+64+64+64)	(Compaq part number 100340-001)
	• 448K	(64+64+64+256)	(Compaq part number 101256-001)
	• 640K	(64+64+256+256)	(Compaq part number 101257-001)

Some machines already have the 640K version installed, and 256K in each of the two socketed banks.

Only "Board Version 1" needs the different U35 PROM for the different amounts of motherboard RAM.  "Board Version 2" just needs jumper changes.

Refer to pages 115 to 119 of manual for clarification of configuring both board versions: Compaq Portable and Portable Plus - Maintenance and Service Guide

Some Version 1 boards have the RAM Quantity DIP switch not populated.  These will also have BIOS Rev.C or later, and count the RAM automatically.

Some Version 1 boards will have the RAM Quantity DIP switch, and may or may not have BIOS Rev.C
If it has BIOS Rev.C, the switch isn't used. If it has an earlier BIOS, then like the [first two BIOS' of the] IBM 5150, it supports a maximum of 544K RAM.



The 256K version of U35

U35 is an 82S123 PROM.
An image for the 82S123 is at here.



The 640K version of U35

QUOTE:

Regarding the previous files for Compaq Portable, I did succeed in reverse engineering the RAM Decoder PROM, and have recreated the 640K version on an SPLD.
This allows 640K on the system board instead of 256K.

Here is the SPLD source, and an adapter PCB that doesn't get in the way of ISA cards: https://github.com/monotech/Compaq-Portable-U35-640K

I have some left for sale here: https://monotech.fwscart.com/Compaq_Portable_640K_U35_RAM_Decoder_PROM/p6083514_19805777.aspx