DEC RX33 Floppy Drive

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The RX33 is an 80-track double-sided 5.25″ floppy drive that was used by DEC in some PDP-11 and MicroVAX computers. It was also used in some other late-model DEC equipment as well, such as HSC cluster controllers.

The only QBUS controller that supports the RX33 is the RQDX3. There was no UNIBUS controller for the RX33.

Total capacity is 1.2MB (in RX33 mode). The RQDX3 also supports use of the RX33 drive to read and write RX50 (400KB) disks.

The RX33 is essentially the same drive that was used in many PC/AT-compatible computers between about 1984 and 1995.

DEC was not the manufacturer of the RX33. The RX33 is simply a re-badged (or co-branded) Teac FD-55GFR drive. Note there were many variants of the FD-55GFR, as evidenced by the suffix that follows the FD-55GFR part number.

The actual formatted capacity of the RX33/RX50 disks are:

  • In RX33 mode: 1,228,800 bytes (80 cylinders x 2 sides x 15 sectors/track x 512 bytes/sector)
  • In RX50 mode: 409,600 bytes (80 cylinders x 1 side x 10 sectors/track x 512 bytes/sector)

In RX33 mode, the bit rate is 500 kbit/sec, and the drive spins at 360RPM. These two specifications are identical to the standard 8″ floppy drive. The drive has to slow to 300RPM (which is achieved by the controller pulling Pin 2 low) for RX50 mode (where the bit rate will be 250 kbit/sec). Getting the drive configured properly to support these two rotational speeds is sometimes a challenge. You need to know the correct jumper settings for the drive, to make this work.

Note that a “High Density” disk must be used in RX33 mode, and a “Double Density” disk must be used in RX50 mode. The disks have different magnetic coercivity and are not electronically interchangeable.

Almost all PC/AT series “1.2MB” 5.25-inch floppy drives can be used as a substitute for a real RX33. The key requirement is that the drive can operate at both 360RPM (in HD mode, when Pin 2 floats high) and at 300RPM (in DD mode, when Pin 2 is pulled low by the controller). The exception is that there may be some drives out there that ONLY operate at 360RPM. It was common for some PCs to always operate at 360RPM in both modes (HD and DD). The PC would simply increase the bitrate for DD mode by 20% (from 250 kbit/sec to 300 kbit/sec) to compensate for the non-standard rotational speed in DD mode. Unfortunately the RQDX3 does not support 300 kbit/sec bitrate, so the drive must be able to reduce the rotational speed to 300RPM for DD mode, if you want to have dual RX33 and RX50 capability.

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Jumper Settings

I have managed to get a number of different Teac drives to work as RX33 substitutes. Although you should work through the full set of jumper settings for your drive, the key ones (for Teac drives) are as follows:

  • Look for a Jumper marked “I”. Sometimes it is by itself. Otherwise it might be in a cluster marked “I/II/IS” or similar. You want the “I” jumper to be IN, and the II and IS jumpers (if any) to be OUT. This tells the drive to operate in 2-speed mode (HD=360RPM, DD=300RPM)
  • Look for a Jumper marked “LG”. Sometimes it is by itself. Otherwise it might be in a cluster marked “LG/HG” or similar. You want the “LG” jumper to be OUT, and the HG jumper (if any) to be IN. This tells the drive to go into DD mode when Pin 2 goes low

Getting original Teac information about jumper settings is not easy. There are a number of documents floating around on the Internet, but they often don’t exactly match the FD-55GFR drive that you are working with.

Perhaps the most useful generic document is this one -> Teac FD-55GFR Specifications. I am not sure if this was a genuine/original Teac document, or whether it is a compilation of various pages that have been thrown together subsequently. Either way, it contains a lot of relevant information.

Below are the jumper settings required for a number of Teach drives.

FD-55GFV-57-U (a genuine DEC RX33)

Note: “GFV” is NOT a typo.

Jumper settings required (working from left to right):

  • FG: IN (To connect GND to the chassis)
  • LG/HG 3-pin jumper: In the HG position (so Pin 2 low means Low Density)
  • II/I 3-pin jumper: In the I position (to select 2-speed mode)
  • U1 and U2: Both IN (these control the front panel LED. They are not critical)
  • HL: OUT (Tells the floppy drive not to use Pin 4 as a Head-Load input signal)
  • IU: OUT (Tells the floppy drive not to use Pin 4 as an In-Use input signal)
  • DS0, DS1, DS2 & DS3: Jumper IN on DS0 or DS1 (in a single-drive system, it won’t matter which of these two that you choose)
  • ML: Out (Tells the floppy drive to only turn on the motor when the Motor-On input signal goes low)
  • RE: Out (No auto re-calibration of the head read amplifier)
  • DC/RY 3-pin jumper: In the DC position (so Pin 34 goes low when a Disk Change occurs)
  • 16-pin terminating resistor pack: IN (so the open-drain signals are floated high)

Teac FD-55GFR-570-U

  • FG: IN (To connect GND to the chassis)
  • T-shaped 3-way I/II/IS 4-pin jumper: In the I position (to select 2-speed mode)
  • U1 and U0: Both OUT (so front panel LED is controlled solely by the DRIVE SELECT input signal)
  • DS0, DS1, DS2 & DS3: Jumper IN on DS0 or DS1 (in a single-drive system, it won’t matter which of these two that you choose)
  • LG jumper: OUT (so Pin 2 low means Low Density)
  • E2 jumper: OUT (so the INDEX and READ-DATA outputs are masked until seek operation has been completed)
  • IU: OUT (Tells the floppy drive not to use Pin 4 as an In-Use input signal)
  • ML: Out (Tells the floppy drive to only turn on the motor when the Motor-On input signal goes low)
  • RY: Out (So Pin 34 is not used as an active-low READY output)
  • DC2: IN (So Pin 34 is used as an active-low DISK-CHANGE output)
  • 16-pin terminating resistor pack: IN (so the open-drain signals are floated high)

Teac FD-55GFR-149-U5 and FD-55GFR-193-U

These two drives have identical chassis, PCBs and jumper settings.

  • FG: IN (To connect GND to the chassis)
  • E2 jumper (located just above “C2”): OUT (so the INDEX and READ-DATA outputs are masked until seek operation has been completed)
  • U1 and U0: Both OUT (so front panel LED is controlled solely by the DRIVE SELECT input signal)
  • DS0, DS1, DS2 & DS3: Jumper IN on DS0 or DS1 (in a single-drive system, it won’t matter which of these two that you choose)
  • LG jumper: OUT (so Pin 2 low means Low Density)
  • IS jumper: OUT (this is an alternative dual-speed setting – see the description at PDF Page 33 of the Specification document)
  • I jumper (hard to find – its right above the LG & IS jumpers): IN (to select 2-speed mode)
  • DC/RY 3-pin L-shaped jumper: In the DC position (so Pin 34 goes low when a Disk Change occurs)
  • ML jumper (hard to find – its right above the L-shaped CD/RY jumpers): OUT (Tells the floppy drive to only turn on the motor when the Motor-On input signal goes low)
  • IU jumper (hard to find – its right above the ML jumper): OUT (Tells the floppy drive not to use Pin 4 as an In-Use input signal)
  • HL & HS jumpers (right above the IU jumper): On my drive, there are no pins in these locations, and they are both OPEN
  • Terminating resistor pack: This is a SIL module, permanently soldered in

Teac FD-55GFR-7193-U

This is the easiest FD-55GFR drive to work with. The Jumpers are well marked on the silkscreen and there are fewer of them.

  • E2 jumper: OUT (so the INDEX and READ-DATA outputs are masked until seek operation has been completed)
  • U0 and U1: Both OUT (so front panel LED is controlled solely by the DRIVE SELECT input signal)
  • DC/RY 3-pin L-shaped jumper: In the DC position (so Pin 34 goes low when a Disk Change occurs)
  • IU jumper: OUT (Tells the floppy drive not to use Pin 4 as an In-Use input signal)
  • LG jumper: OUT (so Pin 2 low means Low Density)
  • I jumper: IN (to select 2-speed mode)
  • 4-way D0, D1, D2 & D3 jumper block (arranged as a “+” symbol): Jumper IN on D0 or D1 (in a single-drive system, it won’t matter which of these two that you choose)
  • Terminating resistor pack: This is a SIL module, permanently soldered in

Panasonic JU-475 Drives

I have also managed to get a number of JU-475 series Panasonic 5.25″ 1.2MB drives to work as RX33 replacements.

Fortunately, the jumpers for most of the common variants of the JU-475 family are set out in one document -> Extract from “Peripherals – Pocket Service Guide” (see from pages 3-5 onwards).

Links

1994 post by Terry Kennedy regarding jumper settings

2009 post by Jerome Fine regarding RX33 alternatives

Mailing list discussion: Teac Floppy as RX 33