Disk Controllers


When it comes to LSI-11 Q-bus systems, there were only a few storage subsystems to choose from. These were the RX01/RX02 8-inch floppy systems, the RL01/RL02 and RK05 cartridge disk systems, the RC series MSCP-based fixed disk system and the RD series MSCP-based hard and floppy disk systems.

The table below contains a comparison of these controllers.

Disk Controllers and Their Capabilities
Module Number M7946 M8029 M8639-YA M8639-YB M7555 M8013/M8014 M8061 M72695
Model Number RXV11 RXV21 RQDX1 RQDX2 RQDX3 RLV11 RLV12 RKV11-D
Introduced 1976 1978 1982 1985 1978 1981
Module Size Dual Dual Quad Quad Dual Quad x 2 Quad Dual
Q-bus Addressing 18-bits 18-bits 22-bits 22-bits 22-bits 18-bits 22-bits 18-bits
Drive Series RX01/RX021 RX02 RD/RX RD/RX RD/RX RL RL RK05
Drives Supported 2 2 4 4 4 4 4 8
Floppy Drives 22 22 23 43 43,4 0 0 0
Hard Drives 0 0 2 4 4 4 4 8
Max. Heads 1 1 8 8 16 2 2 2
Sector Size 128 256 512 512 512 512 512 512
Interface RX01/RX02 RX01/RX02 ST412 ST412 ST412 RL01/RL02 RL01/RL02 RK05
DMA Capable no yes yes yes yes yes yes yes
Block Mode N/A no yes yes yes no no no
Interface Speed 2.5 Mb/s 2.5 Mb/s 5 Mb/s 5 Mb/s 5 Mb/s 4.1 Mb/s 4.1 Mb/s

1 The RX02 is only supported in single desity mode.
2 Floppy drives are 8" IBM 3740 type.
3 Supports RX50 5.25" floppy drives.
4 Supports RX33 5.25" floppy drives.
5 Bus interface only.


Mass storage is always a difficult item for ancient hardware builders, as it all relied on mechanical media, which has a very short life span. Trying to obtain drives (and media) for any of these controllers is problematic. Certainly, new drives are not available, except for 5.25" floppy drives, which can be found as new old stock.

RL01, RL02, RK05, RX01 and RX02 drives can occasionally be found, as can their media. These drives are very large and the media densities are comparatively low, limiting their usefulness.

By far, the RQDX3 controller is the most practical option. Used Winchester drives are not uncommon and the controller works with TEAC FD-55G series floppy drives, which can be jumpered to look like an RX33 drive. The 5.25" floppy disks can be written to from a PC, with a similar drive and special software, to aid in loading software.

Third-party manufactuers, such as Emulex, also made SCSI controllers. High voltage SCSI drives are becoming less common, but they can provide more storage and the controller could support tape devices, as well.

Another option is to use emulated drives. The following folks have put a lot of work into these projects and they are pretty intriguing.

Emulating an RX01/RX02 is not terribly difficult, since these drives have an onboard controller that communicates in a coherent manner with the interface. The other drives are extremely difficult to emulate, since they receive and transmit a continuous stream of raw MFM-coded transitions that are recorded and played back from the media. The hardware requirement to emulate these drives can be quite substantial. David Gesswein resorted to using a BeagleBone Black running Linux to accomplish this feat.

The PortaOne DREM appears to be the first production hard disk emulator. It not only emulates ST412 drives that would normally interface with the RQDX series controllers, but it can also emulate RL01/RL02 drives that interface with the RLV series controllers, providing many more options to PDP-11 owners.

One last option would be to use a bridge controller, between the drive and controller. These bridges allow SATA drives and solid state media to be attached to a SCSI bus. One example is Michael McMaster's popular SCSI2SD SCSI Hard Drive Emulator. This controller connects to a SCSI bus and can emulate up to four targets, with one SD card.


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