LINC tape

From Computer History Wiki
Jump to: navigation, search

LINC tape (sometimes given as LINCtape) was the predecessor of DECtape. It used 3/4-inch wide tape on small 3-1/2-inch reels, formatted before use into fixed-sized blocks which could be individually read or written. (This differs from conventional magnetic tape, which could only be written at the end of the previously-written section of the tape.)

It was created in 1961 as the primary storage medium for the LINC computer. Instructions in the LINC CPU could seek to a given block, and then read or write multiple blocks without further action on the CPU's part.

LINC tape was random access, so the drive had the ability to move forward and backwards to the desired block. The tape format included a block number at the start and end of each block, to help the drive find the desired block. Tapes could only be read and written while moving in the forward direction, however.

The tape contained timing and mark tracks along with three data tracks; the first two allowed not only the ability to re-write individual blocks, but also meant that it could operate with varying tape speeds - the drives did not have capstans, so tape motion was controlled by the motors which drove the reels, and tape speed did vary considerably.

Since each 'line' on the tape contained only three bits, a 12-bit word took four lines to store. Blocks were potentially of different sizes, but normally were all 256 words; a tape could hold 128K 12-bit words.

The block format included checksums, guard words around the data area to protect the block numbers, and an inter-block zone. The tape format includes end zones on each end of the tape, to prevent the tape running off the reels.

LINC tape also had exceptional reliability; it used a very simple and brute-force mechanism to produce that: it wrote all tracks in duplicate on each half of the tape. It was possible to punch a hole in a LINC tape with a paper punch and leave the tape readable.

The drive had a single head with a symmetrical tape path, and used hydro-dynamic lubrication to reduce wear on the tape; like disk heads, the tape floated on a layer of air while the tape was moving, without touching any metal. (In a test, a tape make 30,000 passes over a head without significant wear.)

Specifications

  • Nominal tape speed - 50 inches/second (+/- 25%)
  • Reading/writing rate - 25K lines/second (40 micro-seconds/line)
  • Density - 420 lines/inch
  • Tape - 3/4 inch x 150 feet
  • Tape capacity - 1,572,864 bits
  • Block length - 2-1/2 inches
  • Block transfer time - 1/25 second
  • Start/stop/reverse time - 1/10 second

Further reading

Considerable detail about the development of LINC tape is given in:

  • Severo M. Ornstein, "Computing in the Middle Ages"

Detailed information is given in:

  • "Small Computer Handbook", 1967 edition