RL01/RL02
Removable-cartridge disk drives for PDP-8 and PDP-11 systems.
The RL01 and RL02 are moving-head magnetic disk drives from Digital Equipment Corporation, built for the PDP-8 and PDP-11 microcomputers. They use removable data cartridges based on the IBM 5440 design, but with servo tracking data pre-encoded onto the cartridge. This servo data prevents on-site low-level formatting and ensures cartridges can be used across multiple drives, though RL01 and RL02 cartridges are not interchangeable despite looking similar. The RL01 stores about 5 MB, the RL02 about 10 MB. Each drive weighs 34 kg and fits in a standard 19-inch rack. Up to four drives—any mix of RL01 and RL02—can connect to a single controller, typically an RL11 for a Unibus PDP-11, an RLV11 or RLV12 for a Q-bus PDP-11, or an RL8A (an M8433 hex-wide Omnibus card) for a PDP-8/a.
The RL01 cartridge has 256 tracks, the RL02 has 512. Each track is split into 40 equal-length sectors. Every sector is divided into six fields. First comes a header preamble of three words (each word is 16 bits) with 47 zeros followed by a single one for synchronization. The header itself is three words: the first identifies the side (upper or lower), track number (1–256 or 1–512), and sector number (1–40); the second is all zeros; the third holds a cyclic redundancy check (CRC) for the header, checked during reads. A header postamble of one zero word separates header and data, allowing for mechanical tolerances. The data preamble repeats the three-word pattern of 47 zeros and a one. The data field contains 128 words (2048 bits) plus a 16-bit CRC. A data postamble of 16 zero bits ends the sector. Before each header and data field are two servo bursts (S1 and S2), prerecorded during manufacture. Data is encoded with Modified Frequency Modulation, where a one bit causes a magnetic flux reversal.
The last track on each cartridge holds a table of bad sectors and the cartridge’s serial number. The drive itself has no logic for handling bad sectors—that task falls to the operating system. Consequently, the bad-sector table could be erased if the OS does not guard against it. The drive communicates serially with its controller via a 40-pin cable.
- Capacity
- RL01: ~5 MB; RL02: ~10 MB
- Form factor
- 19-inch rack mount, 34 kg
- Interface
- Serial via 40-pin cable
- Controller options
- RL11 (Unibus PDP-11), RLV11/RLV12 (Q-bus PDP-11), RL8A (PDP-8/a)
- Cartridge tracks
- RL01: 256 tracks; RL02: 512 tracks
- Sectors per track
- 40
- Data encoding
- Modified Frequency Modulation (MFM)
Lore & Background
The RL01 and RL02 drives were designed to provide reliable, removable storage for Digital Equipment Corporation's PDP-8 and PDP-11 microcomputers. Up to four drives, in any combination of RL01 and RL02, could be connected to a single controller. The controller varied by bus architecture: an RL11 for Unibus PDP-11 systems, an RLV11 or RLV12 for Q-bus PDP-11 systems, and an RL8A (an M8433 Hex wide Omnibus card) for the PDP-8/a.
The data cartridges are based on IBM 5440 cartridges but incorporate servo tracking data pre-encoded during manufacture. This design reduced the need for strict head alignment and allowed cartridges to be used across multiple drives, though RL02 cartridges were not backwards compatible with RL01 drives despite similar appearance. The servo data is protected by drive logic to prevent accidental overwriting, and on-site low-level formatting of cartridges is not possible.
Each track is divided into 40 sectors, with each sector containing a header and data field separated by preambles and postambles. The header includes track and sector identification, a zero word, and a cyclic redundancy check. The data field holds 128 words (2048 bits) followed by a CRC. The last track of each cartridge contains a table of bad sectors and the cartridge's serial number; the drive itself has no bad-sector handling logic, leaving that responsibility to the operating system.
Reader's Guide
The RL01 and RL02 drives represent a transitional point in small-computer storage, offering removable media with moderate capacity for their era. Their use of pre-encoded servo tracking on IBM 5440-style cartridges improved interchangeability between drives, a practical advantage for multi-drive installations. The lack of on-site low-level formatting and the reliance on operating system software for bad-sector management were characteristic of the period's design philosophy, where hardware handled basic read/write operations and higher-level functions were delegated to software. The ability to mix up to four RL01 and RL02 drives on a single controller gave system builders flexibility in configuring storage capacity. The serial interface via a 40-pin cable was typical for the time. The drives' weight of 34 kg and rack-mount form factor reflect the industrial and laboratory environments where PDP-8 and PDP-11 systems were commonly deployed. The RL01 and RL02 are notable for their role in Digital Equipment Corporation's ecosystem, providing a removable storage option that balanced capacity, cost, and reliability for mid-range computing applications.
Did You Know?
- RL01 cartridges have 256 tracks, while RL02 cartridges have 512 tracks.
- The drives weigh 34 kg and are typically mounted in a standard 19-inch rack.
- Up to four RL01 or RL02 drives can be used in any combination from a single controller.
Frequently Asked Questions
What are the RL01 and RL02 drives?
They are removable-cartridge magnetic disk drives built by Digital Equipment Corporation for the PDP-8 and PDP-11 microcomputer families. The RL01 offers roughly 5 MB across 256 tracks, while the RL02 doubles the capacity to about 10 MB over 512 tracks, with 40 sectors per track in both cases.
Why won't an RL01 cartridge work in an RL02 drive (or vice versa)?
Although the cartridges look nearly identical, each model has its own servo tracking data baked onto the media at the factory, so the drives cannot read the other's format. This pre-encoded servo information also rules out on-site low-level formatting as a way to make them interchangeable.
Which controllers and buses do the RL01/RL02 support?
Unibus PDP-11 systems use the RL11 controller, Q-bus PDP-11s take the RLV11 or RLV12, and PDP-8 / PDP-8/a machines connect through the RL8A. All of them link to the drive via a serial 40-pin cable.
What is the physical form factor of the RL01/RL02?
Each drive is a 19-inch rack-mountable unit weighing 34 kg, and a single controller can manage up to four drives in any mix of RL01 and RL02 models.
What cartridge design do the RL01/RL02 cartridges derive from?
They are based on the IBM 5440 removable-cartridge format, but DEC added pre-encoded servo tracking data so a cartridge can be dropped into any compatible drive without a formatting step. This makes day-to-day cartridge swapping straightforward for a given model, even though the two RL models remain incompatible with each other.
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