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.) Data was protected with a CRC, compatible with that used by the IBM 3740.
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.
The RX02 included the M7744 Controller and M7745 Read-Write card. Each drive cabinet holds a pair of drives.
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.
There are also as RK8E (RX01 only) and RK28 controllers for the PDP-8. 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.
DEC normally left track 0 unused (to hold bootstraps). They also used an idiosyncratic layout of the sectors on a floppy, intended to maximize the performance, etc.:he first 'logical' sector of each data track was offset by six 'physical' sectors from the 'first' sector of the preceding track.
- Seek error rate - 1 in 16 seeks
- Soft data error rate - 1 in 109 bits read or written
- Hard data error rate - 1 in 1012 bits read or written
- Data transfer rate
- Diskette to controller buffer - 4 usec/data bit (FM); 2 usec/data bit (MFM)
- Buffer to CPU interface - 1.2 usec/bit
- Track-to-track move - 6 msec/track maximum
- Head settle time - 25 msec maximum
- Rotational speed - 360 rpm ± 2.5%; 166 ms/rev nominal
- Bit density maximum (on inner track) - 3200 bpi (FM) or modified (MFM)
- Track density - 48 tracks/inch
- Average access - 262 ms, computed as follows:
- Seek - 77 tks/3 X 6 msec = 154 msec
- Settle - 25 msec
- Rotate - 166 msec/2 = 83 msec
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