RQDX3 MFM Disk & Floppy QBUS Controller

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RQDX3 MFM Disk & Floppy QBUS Controller

The RQDX3 MFM Disk & Floppy QBUS Controller connect the Series of MicroPDP-11 computers and the older models of the QBUS-based MicroVAX / VAXstation systems (MicroVAX I, VAXstation I, MicroVAX II, VAXstation II, VAXstation 3200) to the RX50 and RX33 floppy drives, and the RD31, RD32, RD52, RD53, and RD54 hard disks.

Description taken from the RQDX3 User's Guide

The RQDX3 is the enhanced version of the RQDX family of MSCP RX/RD device controllers. Disks previously formatted on RQDX1/2 controllers will not be compatible with the RQDX3 controller. The RQDX1/2 stores the disk geometry information in ROM while the RQDX3 stores the data on the disk itself and reads in the track that contains this information during the power up sequence. Further the RQDX3 uses a 17 sector per track format while the RQDX1/2 uses an 18 sector per track format. There are also a few performance enhancements to the RQDX3 that add to its preference as a disk controller over the older versions, those being a 1:1 disk interleaving versus 3:1 interleaving for the earlier controllers, an elevator seek reordering algorithm is used as well as an algorithm to handle overlapped seeks when multiple RD5ns are resident in a system. Further the problem that existed with the RQDX2 hold-off timer has been fixed on the RQDX3 by increasing the length of the timer so as not to steal DMA grants from a second RX/RD controller or other DMA devices in the system.

Board layout

The RQDX3 MFM Disk & Floppy QBUS Controller consists of the M7555 Dual QBUS Module.

RQDX3 MFM controller

Version details

All RQDX3 versions support the range of DEC MFM disks RD51, RD52, and RD53 as well as the RX50 dual floppy drive. There is conflicting information about the firmware version from which the RD54, RD31, and RD32 hard disks and the RX33 floppy disk drive are supported.

Firmware versions:

  • Version 1.10 ROMs: 23-216E5 and 23-217E5
  • Version 2.?? ROMs: 23-243E5 and 23-244E5
  • Version 3.?? ROMs: 23-285E5 and 23-286E5
  • Version 4.?? ROMs: 23-339E5 and 23-340E5

The RQDX3 employs the ususal value of 17 sectors per track for MFM disks (as PCs do), the precedessors RQDX1 and RQDX2 controllers used 18 sectors per track. This results in a modest loss of available space when a hard drive is reformatted on an RQDX3.

For RD52 and RD53 hard disks, the RQDX3 controller allows a 1:1 sector interleave on disk surfaces, reducing the average transfer time from 3.5 to 1.5 revolutions per track. For RD51 hard disks, the RQDX3 controller allows a 2:1 sector interleave.

Upgrading from an RQDX1 or RQDX2


Whenever a computer is upgraded by adding an RQDX3 controller in place of an RQDX1/2, the disks (RD51/52/53) will have to be reformatted using the newest XXDP or MDM formatter. The disks are NOT forward compatible as they were when upgrading with the earlier RQDX1/2 controllers.

Disk drive configuration

The disk drives connected to the RQDX3 controller must be configured so each drive responds to a different DRVSELn output signal from the controller.

The RQDX3 assigns LUNs to the physical drives based upon the drive select jumper in the disk drive and the number of drives in the system. The controller looks for disk drives in the following order of drive select settings:

  1. DRVSEL3
  2. DRVSEL4
  3. DRVSEL1
  4. DRVSEL2

Adding external drives

If you want to connect external drives to an RQDX3 controller, you need the RQDXE Extender Module.

External drives are housed in so-called "Leprechaun" boxes, which were available in tabletop and rackmount versions.


To format MFM drives on the RQDX controller in a PDP-11, use XXDP programs:

  • For the RQDX3, use ZRQCF0.

To format MFM drives on the RQDX controller in a MicroVAX, use:

  • MicroVAX I: The RQDX3 is not officially supported in MicroVAX I systems, although it works flawless.
    The only reason for this is that there is no diagnostic software for the RQDX3.
  • All other MicroVAX models: MicroVAX Diagnostic Monitor

Number of devices supported

  • The RQDX3 controls a maximum of four fixed disk drives, or two fixed disk drives and an RX50 (or up to two RX33).


Only one fixed disk drive can be installed in early BA23-A enclosures. (Power & Cooling restrictions)

See also

External links