RX01/02 floppy drive

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The RX01 and RX02 floppy drives use 8" floppies, compatible with the IBM 3740 floppy drive. The media are single-sided, single-density, floppies, which come 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.

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. 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.

Coding systems

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.