Scor­pio News


January–March 1987 – Volume 1. Issue 1.

Page 40 of 63

A Beginners’ Guide to 1/2″ Magnetic Tapes

by Timeclaim

[Ed. – As well as using their Gemini Multi-Format (M-F-B) Systems for transferring files between many different disk sizes and formats, a growing number of users are also using them for transferring data to and from 1/2″ magnetic tapes. This article is an extract from the manual for Timeclaim’s 1/2″ magnetic tape sub-system for the M-F-B and describes some of the some of the basics of 1/2″ tapes. Our thanks to Timeclaim for allowing us to reproduce this documentation, which is copyright (c) Timeclaim.]


1/2″ Computer tape comes on plastic reels which are usually kept in a plastic box or with a strip of plastic round the edge for protection. The diameters of the reel are 7″ (holding either 300′ or 600′ of tape), 10.5″ (holding 1200′ of tape) or 12″ (holding 2400′ of tape). The centres of these reels are the same so that they all fit the same tape drives. (History will reveal some exceptions but these are almost never seen today).


Tapes are said to have 9 tracks because the recording head consists of 9 sections one above the other. All tracks are recorded at the same time unlike tape cartridges which usually are recorded one track at a time. There are some old 7 track tape machines about but these are rarely encountered.


Progress has meant that the density of data recorded on tape has been increased over the years. The densities seen nowadays are as follows:

800bpiNRZI(Non Return to Zero Inverted)
1600bpiPE(Phase Encoded)
3200bpiPE        "           "
6250bpiGCR(Group Coded Recording)

The density refers to the number of bits per inch on each track. As one complete byte is written at a time (with the 9 head sections) the number of bytes per inch is equal to the number of bits per inch. In the above table it will be seen that there are some further letters. These describe the method by which data is recorded on the tape. This information really needs to be known only by the tape deck engineer.

800 bpi tape are almost obsolete in the data processing world where there is a need to store a great deal of data on a tape. However, many CAD/​Engineering/​Scientific users still use 800 dpi as they do not usually need to store so much data. Often they will only use the first few feet of tape on a reel

1600 bpi has become the data exchange standard. Most tape decks can read and write at 1600 dpi so this is how data is usually transported from one systen to another.

3200 bpi. This a double density version of 1600 bpi. This density is usually quite a cheap add on for 1600 bpi tape decks but is almost never used for data interchange. Systems using this are fairly unusual.

6250 bpi. This is the Latest development in 1/2″ open reel tape and is coming into use at many mainframe computer sites. Most 6250 bpi drives will also read and write at 1600 dpi. 6250 bpi drives are much more expensive than 1600 bpi and are not available in some countries,

Data Layout on 1/2″ Tape

Data is written onto tape in one complete 8 bit byte. The 9th track is used for a parity bit which is dealt with automatically and is not seen by the programmer. A number of bytes on a tape are grouped together into a block. Blocks are separated by gaps in which no data is recorded. These gaps are from 0.6″ for Phase Encoded tapes to 0.75″ for NRZI tapes. Blocks can be any length but blocks shorter than 11 to 14 bytes are not allowed. The actual minimum

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