RQDX Mini-BOB for floppy drives

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This simple PCB will enable one or two 3.5″ or 5.25″ floppy drives to be directly connected to a PDP-11 RQDX-series controller.

It is a simplified version of my earlier RQDX Breakout Board. Unlike my earlier board, this board will not connect to an MFM hard drive, and does not have any circuitry to stretch the “Motor On” signal (which is required to use 3.5″ drives in substitution for real RX50 drives).

The most convenient application for this board is to enable one or two surplus PC-era 3.5″ or 5.25″ floppy drives to be connected to an RQDX3, to emulate DEC RX33 floppy drives.

It can also be used to connect real RX50 or RX33 floppy drives to an RQDX controller.


There are some significant limitations to be aware of, resulting from the design of the DEC RQDX controllers:

  • RX33 is supported only by the RQDX3. So if you have an RQDX1 or RQDX2 controller, you will not be able to connect to (or emulate) RX33 floppy drives. You will still be able to connect to real or emulated RX50 drives, but please see the other bullet points below for more information about RX50 emulation.
  • This board won’t reliably support using 3.5″ drives (to emulate RX50s). This is because 3.5″ drives spin down too quickly when the “Motor On” signal goes inactive, causing the next drive access to sometimes fail when “Motor On” goes active. It seems the RQDX firmware relies on the higher rotational inertia of 5.25″ drives to overcome this problem. My earlier design solves this by stretching the Motor On signal, but this simpler board does not address the problem.
  • You can use this board to connect 80-track 5.25″ floppy drives to emulate RX50 drives. Be aware that 40-track drives cannot be used, as RX50 is single-sided double-density 80-track format. If using a high-density 5.25″ drive for this purpose, you will need to slow the drive (from 360RPM to 300RPM). This is usually achieved by pulling pin 2 to GND. This is achieved by inserting a jumper between pins 2 and 3 of jumper JP1. You may also need to set some jumpers on the floppy drive to make this work.

Cabling to the RQDX controller

This is simple: just connect a 50-way cable between connector J1, and the RQDX controller. Note that J2 is provided as an alternative to J1. If you choose to use J1, pin 1 is on the right-hand side. If you chose to use J2 instead, pin 1 of the cable is on the left-hand side.

Note that if you are using an RQDX1 or RQDX2 controller, jumper JP2 should be inserted (to enable the RQDX outputs). If you are using an RQDX3 controller, make sure jumper JP2 is removed.

Drive cabling

Floppy drives would normally be connected to this PCB using a 34-way cable that has pins 10-16 “twisted” between the two drive connectors (this was the standard way in which floppy drives were cabled in x86 PCs). The twisted cable is required because most PC-era 3.5″ drives are pre-configured to each respond only to the DS1 (Drive Select 1) signal. The twisted cable allows the controller to select either of two drives, even though both drives are physically configured to respond to DS1.

If you are using floppy drives that allow you to set the drive-select jumper (5.25″ drives usually allow this), then you have two options:

  1. Use the twisted cable and configure both drives to respond to DS1; or
  2. Use a “straight-through” 34-way cable and configure one drive to respond to DS0 and the other drive to respond to DS1

If you are using the twisted 34-way cable, then only 2 drives can be connected to this board. If you are using the straight-through 34-way cable, then up to 4 floppy drives can be connected.

Note that jumpers JP3 and JP4 on the board need to be set to the “TWST” or “STRGHT” position, to match the type of cable you are using.


Revision history

28 July 2017: Posted initial version