TAPE

5/23/98

Tape: Half inch reel-to-reel tape has been widely used in the computer industry for four decades. Audio cassettes were used in the early days of PCs. Although both have been and are used for PCs, the primary tapes used with PCs are cartridge tapes in one of many formats. There are three primary formats -- 8mm Digital Logical Tape (DLT), 4mm Digital Audio Tape (DAT), and Quarter Inch Cartridge (QIC/Travan). Two other high end standards are Advanced Intelligent Tape and Exabyte Mammoth.

QIC/Travan offer the least expensive drives, the lowest data transfer rates but the cartridges cost three to five times other systems for the same capacity. QIC was the original standard. Travan uses a longer cartridge in order to get more tape into the cartridge. QIC tapes can usually be read in Travan drives. QIC tapes come in a variety of QIC-xx and DC-xxxx designations. Travan tapes are designated as TR-1, TR-2, TR-3 and TR-4. NS/8 is a common variant of TR4 that includes hardware data compression and read after write data verification. The data used for rapid positioning of the tapes is vendor specific (at best) for QIC, but is part of the standard for DAT.

The slowest tapes can be supported on the floppy bus or via a parallel port. Faster tapes generally require IDE or SCSI. Accelerator cards are available for some floppy bus drives. They often work.

Approximate Costs and capabilities

Technology Drive Cost Media Capacity (Compressed) Media cost/GB Compress Ratio Transfer Rate (Uncomp)
QICcheap.06 to 3.2Gb$8.002<0.3 MB/sec
Travan$3001 to 8Gb$4.002 0.5 MB/Sec*
DAT$10002.6 to 24Gb$1.2521 MB/Sec
DLT-4000$400020 to 40Gb$2.2521.5 MB/Sec
DLT-7000$800070Gb$1.2525 MB/Sec
AIT$350065GbUnk2.63 MB/Sec
Mammoth$400040GbUnk23 MB/sec

*Three or four formats (QIC80, 3010, 3020, (3095)) with different transfer rates may be supported depending on the drive.

Tapes are not random access devices although some drives can position roughly in high speed and find the data at lower speed once roughly positioned. Tapes may require formatting which can be very time consuming and are typically sold pre-formatted.

Tapes are somewhat touchy and can get into trouble if other activities on the buses interrupt data flow. In marginal situations, it may be necessary, to switch to a low resolution video mode or take other action to remove loading -- especially intermittent loading -- from the computer buses.

Other common problems with tape include "shoe shining" and problems sensing end of tape. Shoe shining is a phenomenon featuring short data bursts followed by time consuming repositioning of the tape. It happens when the mechanical motion of the tape is too fast for the data transfers. The cure, is to speed up data transfer or slow down the tape such that the reading and writing is done in continuous "streaming mode". The end of tape is identified mechanically or optically. The sensors may fail or become blocked causing the tape to despool off the end of the reel. QIC-80 is claimed to have a high error rate that virtually guarantees occasional read errors.

Return To Index Copyright 1994-2002 by Donald Kenney.