RX01/02 floppy drive
The RX01 and RX02 floppy drives were DEC's first floppy drives, and were used on a number of computers, such as the PDP-8 and PDP-11. They used 8" floppies, compatible with the IBM 3740 floppy drive; the media were single-sided, single-density, floppies, which came pre-formatted. (The RX0x drives are not capable of formatting blank floppies.) Each drive cabinet holds a pair of drives.
The drives are mostly identical, except that the RX02 supports optional double-density operation. This double-density is not the same as the 'normal' double-density 8" floppy, but a unique-to-DEC format. Both drives provide 26 sectors per track (1-26), and 77 tracks per floppy (0-76), for a total of 2002 sectors. Single-density sectors hold 128 bytes, and double-density sectors hold 256 bytes, for a maximum total capacity of 256KB for the RX01, and 512KB for the RX02.
Single-density recording uses a double frequency (FM) coding (compatible with IBM 3740 devices), and double-density uses a modified Miller code (MFM). In an especially bizarre design choice, in both single- and double-density media, the sector headers are all recorded using FM coding; on double-density media, the data portions of sectors are recorded with MFM.
Both drives have UNIBUS and QBUS controllers; the RX11 and RX211 for the UNIBUS, and RXV11 and RXV21 for the QBUS, as well as controllers for the PDP-8. The first of each pair are for the RX01, and are programmed I/O; the latter are for the RX02, and provide DMA data transfer. The maximum number of drives supported by a single controller is two.
The RX02 can be strapped to emulate an RX01, in which case the RX01 controllers (RX11 and RXV11) can support them.
In the single-density double frequency (FM) coding, there is a flux reversal every clock time, and an additional flux reversal in the middle of the bit for a '1' bit; for '0' bits, there is no flux reversal. In the double-density Miller coding (MFM), a flux reversal indicates a '1' bit, and no flux reversal indicates a '0'; a clock is recorded only between data 'zeros'.
Since the maximum flux reversal rates are the same for FM, and MFM with a data rate double that of the FM, the same hardware can thus hold twice as many bits with the MFM coding.
|v • d • e PDP-11 Computers and Peripherals|
|UNIBUS PDP-11s - PDP-11/20 • PDP-11/15 • PDP-11/35 • PDP-11/40 • PDP-11/45 • PDP-11/50 • PDP-11/55 • PDP-11/70 • PDP-11/05 • PDP-11/10 • PDP-11/34 • PDP-11/04 • PDP-11/44 • PDP-11/60 • PDP-11/24 • PDP-11/84 • PDP-11/94|