The "BX720D Project
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The BX720D Project Page

Introduction

Welcome to the BX720D Project Page! The Commodore CBM-II machines have always been my favorite. The top-of-the-line CBM-II would have been the BX-256-80 or BX-700 machine (depending on which side of the world you are in). Commodore announced it in 1983 but fate would intervene and ultimately it was not to be. This project is my attempt to assemble one.

The "X" in "BX" stands for extra processor, in the form of an 8088 board designed to run CP/M-80 and MS-DOS. The board was designed to plug into a standard high-profile B-series machine, although it has been shown that it can also work in the low-profile machines like the B128 provided your power supply can handle it. Other processors were hinted at, but nothing was even prototyped. Only 40 processor boards were ever built.

In europe, when Commodore went under, some 710D and 720D machines were found in the labs. These had built-in disk drives, but not no 8088 processor boards.

I will call my machine the BX720D as a kind of mash-up of the different model numbering schemes, and in keeping with the tradition of Commodore constantly changing their model designations...

The Pieces

I have the 710 computer and it is socketed to accept an extra 128K RAM. This unit has an internal IEEE cable wired into it, as well as extra cables, which I believe connected a Proxa7000 board, which sadly I did not get. The 710 case top already has mounting poles/holes for the drives.

I have the 8088 processor board, however it is currently not working. I have verified that the PLA is the correct revision to support the board. I have been told that my board might be missing a pull-up resistor of some kind.

I have the drives. I have an extra 8296D that can donate the 8250LP drive unit. I salvaged the drive faceplate from another gutted 8296D computer. I also have an SFD-1001 board with 8250MINI.

Current Status - Planning

The challenge will be to install the drives. The 8250LP drive has the proper mounting hardware for the 8296D computer, NOT the CBM-II machines. The 8296D computer has it's power supply mounted on the bottom, leaving room at the top for the drives, and more importantly, the analog board. The analog board is connected via ribbon-cable and is mounted towards the back of the drive mechanisms. Unfortunately, in the 710 computer, the power supply is mounted on the top, where the analog board wants to go. So, to make it fit the analog board will need to be re-located, or a smaller power supply would have to be installed.

The other option is to install the SFD-1001 with 8250MINI boards. This will require building the proper mounting hardware, or using the 8250LP hardware. So far, I have not been able to get the SFD+8250MINI combination to work. It is possible my 8250MINI board is defective or I might have the wrong firmware on one or both boards to make it work.

Contact

Last updated: Oct 27/2009, 3:20pm EST

Send comments or feedback to Steve Gray(sjgray@rogers.com)