BBC Model B 32K

Description



BBC Micro Model B, 32K RAM. Acquired in the UK in April/May 2013, condition unknown. I decided not to power it up while I was over there, saving it instead as a project for the MARCH 2013 Repair Event.

This is a 240V unit with PAL output.

These machines contain an OS ROM that selects one of a number of programs in paged ROM to boot into. Most machines destined for general purpose use would boot into BBC BASIC. As the machine found many uses beyond BASIC programming (word processing, terminal emulation for example), the default boot environment could be changed.

Repair Day Work

My goals for the repair day were to replace the line capacitors which are known to fail in older machines, and to get the Disc Filing System operational. The caps arrived from Digikey in time for the event. I removed the PSU in its entirety and replaced C1 and C2 with new X2-rated versions as described elsewhere on the web.

This unit was equipped with an Intel 8271 floppy disk controller device. These did not ship with the base model by default, so the disk option must have been installed at one time. The 8271 is said to have been obsolete before Acorn even used them in the design of the BBC Model B. Later units have a 1770 controller. A couple of days before the event I burned a copy of Acorn DFS 0.98 which is compatible with the 8271.

Incidentally, the Utility ROM slot of the BBC uses the 27128 device pinout. 27256s *may* work but I decided not to take the chance. A standard 34-pin floppy flatform cable connects to the floppy disk connector on the underside of the board and a DSDD drive was configured as Drive 1.

When you bought the DFS option, you would have received the controller chip, ROM, and a utility disk containing programs for formatting diskettes. In the absence of the utility disk, I used a short BASIC program, keyed in, to format a diskette. The BBC treats each side of the diskette as a separate disk unit, unit 0 being side 1 of the first drive, unit 2 being side 2 of the first drive. If you try this at home do nor forget to format both sides of your disk if you are using a double-sided drive. I had no problems formatting DSDD diskettes on the 8271 with Acorn DFS 0.98.

The screen shot below shows the output from the *CAT command on the drive with a newly formatted diskette in place.

Micro-PROLOG

While the cover was off I installed a copy of Micro-PROLOG in remaining empty ROM slot. The *PROLOG command switches you from BASIC to the Micro-PROLOG environment.

Links

Herb Johnson - page on the MARCH 2013 Repair Event

Bill Degnan - page on the MARCH 2013 Repair Event

Manufacturer

Acorn

Approximate Year

1981

Category

MICROCOMPUTER

Status

ACTIVE