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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 you fix some termination issues. Other processors were hinted at, but nothing was even prototyped.
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...
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 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.
The challenge will be to install the drives. The 8296D drive assembly 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.
| CBM-II HP | The 8088 board did not work with the 710 motherboard so I swapped in another HP motherboard. This board has 256K ram with BASIC128. |
| Internal Drive Selection | I have decided to use the 8296D drive mechanism. I believe this will be more authentic. The SFD+8250MINI will involve more work modifying the mounting hardware. Also, the 8250MINI board is not a Commodore product. The 8296D mech will also involve modification to the mounting hardware. |
| Drive modification | I have removed the metal mounting plate that held the analog board. This was located in the same place the power supply wanted to be, so one had to go... the plate was welded on and it took a lot of prying to remove it, resulting in bent areas on the main mount. |
| Analog board location | After much contemplation I found the perfect spot for the analog board. I tried moving the board all around to see where to mount it. Every location I thought of would require some type of cable extension. The most logical place seemed to be to the right side (opposite the rod to hold up the case top) but that would involve extending the drive control and head cables for the drive opposite the board. As it turns out, my thinking was not 3-dimensional enough, and I was dismissing the perfect location for the board... I had forgotten that the power supply was mounted on posts, and amazingly the board will fit between the top case and the power supply, beside the fan. This is very close to where the board originally was mounted except that before it was mounted on an angle and now it's flat to the top of the case. One small problem... one of the mounting posts was in the way. The analog board can't be trimmed easily, so one post must go. I chose the post closest the drive. It came off easily. So, no messing with cables is needed at all. I have to make a small clip or something to hold the board in place. |
| Power Supply connectors | The cable powering the monitor goes directly from the power supply to the monitor pcb.
This makes removing the monitor very inconvenient. So, I have spliced in a connector so that
it can be disconected easily without removing the back monitor casing. There is one additional unused power connector that supplies +5V and +12V on a 4-pin connector (push-on type). Removed the connector and replaced with a new solder type. Extended cable with second connector. First connector plugs into Digital board, and second plugs to analog board. |
| Internal IEEE connector | With the new motherboard the Proxa connectors are gone. Also gone is the IEEE to centronics cable. Since I am using the 8296D drives i will need to create a cable that goes from the IEEE pin header to the HP motherboard. I may use a ribbon cable with connector to plug into the pin headers and then solder the other end directly onto the motherboard IEEE ferrit beads. I also am considering building a little board with pin header and wire that directly to the motherboard. That way I can remove the cable if needed. - Decided to start with a directly soldered ribon cable, but have parts on order for a header. |
| Misc | This machine has no speaker. Moved speaker from CBM128-80 machine and discovered that the SID chip is not producing sound. I don't know if the SID is defective or it's some other problem. The 8088 board is working but it requires an 8050 disk drive. The internal 8250LP unit will not work unless put into "8050 mode". There is a jumper on the drive to change that mode, so I plan to install a switch. Will need to mount it hopefully without having to modify the case or drill holes, etc. It might also be nice to be able to turn the drives off when not needed, or to set the internal drive's device#. Will require additional switches. |
Here are the construction photos, in approximate timeline order:
Page Created: Oct 27/2009. Last updated: Dec 10/2014, 8:00pm EST
Send comments or feedback to Steve Gray(sjgray@rogers.com)