Compaq Portable Plus Rev C BIOS fix for error 401

[glk]

I am restoring a Compaq Portable Plus and replaced the Compaq floppy drive/printer board (000012-001 rev J) that has LPT1 support only at port 0x3BC with a new controller board from http://www.malinov.com/Home/sergeys-projects/isa-fdc-and-uart. The new controller has UART support but no parallel port support. Hence, the my Compaq Portable Plus reports error 401 at boot - no LTP1 at 0x3BC even though a Diamond Flower MFPLUS board with LPT1/LPT2 support is installed. The Diamond Flower board only recognizes 0x378 and 0x278 LPT ports - hence the 401 error at boot.

This required a modification of the the bios to change the 0x3BC to 0x378 at two locations in the bios rom. This Compaq Portable Plus came with bios rev C. Following the procedure outlined in "Adrian's Digital Basement" video at https://www.youtube.com/watch?v=ILFVoZksjNc, I managed to modify the rev C bios to eliminate the 401 error at boot. The Compaq Deskpro 286 bios, as discussed in the video, is very similar to Rev C bios, except that I found the checksum for the rev C bios is actually 16 bit not 8 bit as discussed in the his video. It was necessary to change the last to bytes in the bios rom from 9A 00 to 22 01. Very happy, no more 401 error at boot. I have attached the modified rev C bios for the Compaq Portable Plus.

 

[Andrettigto]

That's quite an accomplishment and you should be happy. He does have some good youtube videos and I'm a subscriber.
I myself am trying to learn what I need to program an 2764 EPROM but for a different reason as my Compaq Portable 1 doesn't support BIOS expansion ROMS's so I can't install an XT-IDE.
How do you like the Floppy Controller? I have a Monotech based on Sergey Kiselev's design installed in an 5150 but, with no Floppy installed, it takes what seems like forever for it to pass the boot to my Hard Drive.

 

[glk]

The floppy controller works great. I have two OEM 5.25 inch floppy drives (Mitsubishi M4851-352U) connected to the controller. The floppy drivers needed a little lubrication on the sliding rails to make them work smoothly and quietly. I also have installed a Lo-tech XT-CF-lite rev.2 to replace the broken hard drive that took one of the floppy mounting ports. Boot time is about 54 seconds from power on and about 16 second from ctrl-alt-del. An image of the boot screen is attached.

 

[retro-pc_user]

Gorgeous. Going to diagnose my newly acquired Portable 1 that has a dual 360KB drive setup and a HardCard (might pull the data off of it if it works).

Waiting for the foam contact pads to arrive along with the CF-IDE assembly (bracket, the adapter itself, and the power connector) and spare parts in case something broke, plus a CT1350B + CMS card.