The Bestel Oric Floppy Disk Controller

Text and illustrations by Thierry Bestel. Originally published in CEO-MAG issues 147, 149, and 150. Copyright Thierry Bestel and CEO-MAG. Translated, edited, and reproduced without permission by Kenneth Bernholm.

The Bestel Oric Floppy Disk Controller (FDC) is a double sided printed circuit board (PCB) with a component side and a soldering side based on the Oric Microdisc BN O136 diagram and the Oric Microdisc MT O128 rev. C controller. (NOTE: The revision C controller and the BN O136 diagram do not match up which accounts for the discrepancies reported below.)

When building this FDC, you must be aware of the following:

• The Microdisc diagram incorrectly specifies two NOR gates as OR gates. This has been corrected on both the Microdisc and the Bestel FDCs.
• The Microdisc diagram contains handwritten modifications around IC20 (TTL 221). These are implemented on both the Microdisc and the Bestel FDCs.
• The Microdisc diagram specifies a 3k3 resistor between Phi2 and +5V. This resistor is left out on both the Microdisc and the Bestel FDCs.
• The Microdisc diagram calls for IC2 and IC5 to be grounded through capacitor C16 and resistor R19, but both the Microdisc and the Bestel FDCs grounds IC2 and IC5 directly. If you wish to add C16 and R19 to the Bestel FDC, you must cut two traces, add a wire between pin 10 of IC2 and IC5, and solder in C16 and R19. See this grounding illustration.

The Bestel FDC requires a bus amplifier as described in CEO-MAG of June 2002 page 16-18. Without a bus amplifier, the physical size of the Bestel FDC seems to cause the loss of some signals from the Oric even with an amplified Phi2 clock signal.

To alleviate noise problems, keep the ribbon cables under 30 cm for each connection and consider wrapping them in aluminum foil. Also, you should ground the Bestel FDC to the metal housing.

With everything connected but without the ICs mounted, the Oric should boot as usual. With the ICs mounted, the Oric should show one of the following boot messages:

If you cannot reach a suitable adjustment, try monitoring the Phi2 clock signal and the output of the two monostable multivibrators on TTL 221. The output of the first multivibrator (pin 13) is high as soon as Phi2 drops back to low. RV1, associated with R15, is used to adjust the duration of this high state. As soon as this state drops, the output of the second multivibrator switches to a high state, the duration of which is determined by R16. The output of the second multivibrator must never drop after the next clock signal has also dropped. Change the values of R15 and R16 to reach the proper timings.

Bill of materials

See the component layout for reference.

*) See details in the text.

Other components

• One terminal block to connect the PSU
• Two HE-10 2×17 pin male shrouded headers
• Ribbon cable and female connectors for connecting the Bestel FDC to the floppy disk drives and the bus amplifier
• A reset push button