WORK IN PROGRESS

The Oric Microdisc Controller

By Kenneth Bernholm

The Oric computer has four brands of floppy disk systems. The original Oric Microdisc, the Cumana disk, the Jasmin disk, and the ByteDrive 500 disk. All of them are pretty hard to come by and quite expensive when up for sale. Disk drives are by far the most sought after peripherals for the Oric and I want one for my Oric Tanos.

With some perseverance and a good bit of luck I managed to secure an unpopulated printed circuit board (PCB) for an Oric Microdisc MT O128 controller. Now I just need to source the correct components and make it work. Fortunately, the web abounds with Oric documentation and resources for this type of project:

• The Bestel Oric Floppy Disk Controller by Thierry Bestel in CEO-MAG
• The Oric Projects and Repair page on S. D. Marshall's Oric site
• The INSERT SYSTEM DISC... ORIC MICRODISC NEARLY THERE thread on the Atari Owner Club website
• The Oric floppy images support thread on Gregory Estrade's HxC2001 Support Forum

However, researching for the bill of materials (BOM) is made a bit difficult by the fact that at least three diagrams exists:

• The Microdisc BN O136 diagram used for the Bestel controller
• A Microdisc unnamed diagram incorporating the changes pointed out by Thierry Bestel
• A Microdisc KiCad diagram which appears to be based on the unnamed diagram

Also, at least two revisions of the MT O128 PCB exists. My unpopulated PCB has "PSS 8431" on the soldering side but the other revision has no similar code. This leaves me with no choice but to establish my own:

Based on the facts learned from the three diagrams and the two PCB revisions, I cautiously conclude that:

• The unnamed diagram matches the rev. C PCB.
• The BN O136 diagram represents the MT O128 rev. S PCB.
• The rev. C PCB predates the rev. S PCB when considering the extra components and wires soldered to the rev. C PCB and the fact that C9 is unpopulated on the rev. C PCB and left out entirely on the rev. S PCB.

I do not know why the diagram is labelled BN O136 and the PCB is labelled MT O128.

• MT O128 rev. C component side posted by JihemB at the HxC2001 Support Forum.
• MT O128 rev. C soldering side posted by JihemB at the HxC2001 Support Forum.
• MT O128 rev. S component side
• MT O128 rev. S soldering side

Microdisc MT O128 rev. S Bill of Materials

For the capacitor and the resistor values, some discrepancies exists between the KiCad diagram and the Bestel FDC. I have listed values for them both, and I have chosen the values of the Bestel FDC over those of the KiCad digram.

Pros for chosing the Bestel FDC values:

• The Bestel FDC is a known, working controller but the KiCad diagram is unsigned and for all I know untested.
• The Bestel FDC is based on the BN O136 diagram which should match my rev. S PCB.
• The Bestel FDC implements RP3 but this component is not immediately found in the KiCad diagram.

Cons for chosing the Bestel FDC values:

• The Bestel FDC does not implement R20 but the KiCad diagram and the rev. S PCB does.

Since neither the Bestel FDC nor the KiCad diagram implements C16 and R19 but the rev. S PCB does, this has no effect on the choice of component values for these parts.

A star (*) indicates that there is a discrepancy for this part. A dash (-) means that the component is not represented in this context.

Capacitors

Capacitor types are abbreviated to E for electrolytic, C for ceramic, T for tantalum, and P for polypropylene.

Resistors

For my rev. S PCB, I have used 0.25 watt carbon film resistors.

Integrated circuits

Miscellaneous