Fd50to34

fd50to34: Adapt 34 Pin Floppy Drives to 50 Pin Controllers

by Mark J. Blair nf6x@nf6x.net

This board is a cable adapter to allow 3.5" or 5.25" floppy disk drives to be connected to computer systems with floppy disk controllers that are designed for use with 8" floppy disk drives. It is inspired by instructions by Frank Durda IV for connecting 5.25" floppy disk drives to Radio Shack/Tandy model II, 12, 16 and 6000 computers, as found at this URL:

http://nemesis.lonestar.org/computers/tandy/hardware/model16_6000/floppyfix.html

Features

• Jumper-selectable double-sided disk support.
• Jumper selection allows drive motors to be strapped on all the time, or controlled by either the head load or in-use/door-lock signal.
• Bank of shorted resistor footprints allows easy drive select number remapping.
• Drive signals are labeled on bottom of board for debugging purposes.

Schematic Diagram

JP1: Double-Sided Media Detect

Install a jumper at JP1 to enable double-sided disk support. If any single-sided disks will be used, remove this jumper while accessing them, since installing this jumper makes the controller think that all drives are reporting double-sided media.

If desired, connect a remote SPST switch to this jumper, and close it to enable double-sided media.

JP2 and JP3: Motor Control

Frank recommends strapping the motor control line to ground to force the drive motors on, since 8" drive motor control works differently than 3.5" and 5.25" drive interfaces expect. To get this behavior, install a jumper between pins 2 and 3 of JP3 (labeled MTRON and GND on the PCB). JP2 is ignored in this configuration.

Some drives can be configured to qualify their motor controls with the drive select signals. With these drives, you may wish to experiment with different motor control schemes. To enable these schemes, first move the jumper on JP3 to pins 1 and 2. Then, install a jumper on JP2 to connect the motor control signal to either the head load signal or the door lock signal.

When connecting a Lotharek floppy drive emulator to a TRS-80 Model II/12/16/6000, the door lock signal seems to work well for drive motor control. For this configuration, install a jumper between pins 1 and 2 of JP2, and another jumper between pins 1 and 2 of JP3.

JP4: Density Select

Some floppy drives use J2 pin 2 to select different density modes. On compatible 3.5" drives, pulling this pin down will select a 360 RPM mode which may work well on systems expecting 8" floppy drives. To do this, install a jumper between pins 2 and 3 of JP4. Install no jumper on JP4 for default behavior.

Installing a jumper between pins 1 and 2 connects the reduced write current signal on the 8" interface to J2 pin 2, which is also used to select reduced write current on some 5.25" drives. This option is experimental.

R1-R4: Drive Select Remapping

If you need to remap the drive select signals, first cut the exposed traces on the top side of the board which short out resistors R1 through R4 (which are not normally installed). Then, solder wires between the pads of R1-R4 to map the drive select signals as needed.

Suggested Parts

About the Design

This board was designed in Kicad 2014-12-16 BZR 5324 on a Mac. Design rules are 6 mil line, 6 mil trace, 13 mil via hole. It is suitable for fabrication at OSH Park.