MFM Hard Drives

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This page is a collection of notes regarding MFM hard drives.

Drive-Select Jumper Settings

When using an MFM drive on my RQDX Breakout Board, install link DS1 on the MFM drive.

When using an MFM drive in a DEC BA23 chassis, install link DS3 on the MFM drive (if you install DS1 instead, the MFM Hard Drive and the RX50 Floppy Drive will both respond to the DS1 signal).

Seagate ST-412

The specifications for this drive are:

  • Cylinders: 306
  • Heads: 4
  • Sectors/Track: 17
  • Precompensation: 128
  • Landing Zone: 319
  • Bytes/Sector: 512
  • Capacity (formatted): 10MB

See stason.org’s ST412 page for more information about this drive.

Underneath the ST-412 there are two un-populated IC sockets near the 34-way edge connector.

The 14-way IC socket is for the terminating resistor pack. The Pin 1 end of the package is furthest from the edge connector. When tested with a multimeter, the 12 input pins should each measure 138 ohms to Pin 14 and 147 ohms to Pin 7.

The 16-way IC socket is for jumpers (or possibly a dipswitch). It has 8 Options (Jumpers). Option 1 is closest to the 34-way edge connector. The assignment for each Option is as follows:

  • Option 1: DS1
  • Option 2: DS2
  • Option 3: DS3
  • Option 4: DS4
  • Option 5: Reserved (leave Closed)
  • Option 6: Reserved (leave Closed)
  • Option 7: Reserved (leave Closed)
  • Option 8: Daisy-Chain (Open) or Radial (Closed). Leave Open

Only one of Options 1 through 4 should be Closed.

This drive was used by DEC in the PDP-11 range, designated as an RD51 drive.

Seagate ST-225

The specifications for this drive are:

  • Cylinders: 615
  • Heads: 4
  • Sectors/Track: 17
  • Precompensation: 300
  • Landing Zone: 670
  • Bytes/Sector: 512
  • Capacity (formatted): 21MB

See stason.org’s ST225 page for more information about this drive

The last drive must have a terminating resistor pack fitted. This is a single-in-line package that plugs in underneath the PCB.

The manual for the ST-225 is available here.

The configuration jumpers are on the back of the ST-225. From left to right they are:

  • Pins 15-16: DS1
  • Pins 13-14: DS2
  • Pins 11-12: DS3
  • Pins 9-10: DS4
  • Pins 7-8: Life Test (leave Open)
  • Pins 5-6: Write Fault (leave Open)
  • Pins 3-4: Recovery Mode (leave Open)
  • Pins 1-2: Radial (leave Open)

Only one of DS1 through DS4 should be Closed.

This drive was used by DEC in the PDP-11 range, designated as an RD31 drive.

Seagate ST-4053

The specifications for this drive are:

  • Cylinders: 1024
  • Heads: 5
  • Sectors/Track: 17
  • Precompensation: N/A (1025)
  • Landing Zone: N/A (Auto Park)
  • Bytes/Sector: 512
  • Capacity (formatted): 44MB

More information about this drive is available at:

The configuration jumpers are on the back of the ST-4053. They are physically in the same location as appears on the ST-225, but the pin numbering is different. From left to right they are:

  • Pins 1-2: DS1
  • Pins 3-4: DS2
  • Pins 5-6: DS3
  • Pins 7-8: DS4
  • Pins 9-10: (leave Open)
  • Pins 11-12: Write Fault. Stason.org’s page says this should be Closed, but I leave Open
  • Pins 13-14: Recovery Mode. Stason.org’s page says this should be Closed, but I leave Open
  • Pins 15-16: Daisy-Chain / Radial (leave Open, for Daisy Chain)

Only one of DS1 through DS4 should be Closed.

The ST-4053 was not used on DEC’s PDP-11 MFM controllers (eg RQDX3), but can be used on the PDP-11 with Webster’s SRQD11 controller.