Seven-Track tapes

The 7-track feature allows a tape unit to read and write 7-track tapes on reels. This special feature consists of a 7-track read/write head (instead of a 9-track head) and control unit changes, including a translator. Data can be read or written in densities of 556 or 800 bpi with either odd or even parity. Nine-track tapes cannot be read on tape units with the 7-track feature installed. The translator takes 8-bit EBCDIC characters from your buffer and writes them as 6-bit binary coded decimal interchange code (BCDIC) tape characters and translates the opposite way during a read operation. Density and parity can be set and the translator can be turned on and off, by mode setting control commands. When the translator is off, only the 6 low-order bits of the characters in your buffer are written on tape; during reading, the 2 high-order bits are set to zeros.

ISO and ANSI standards do not include a specification of 7-track magnetic tape for information interchange. Therefore, the 7-track feature is not applicable for tapes with ISO or ANSI labels.

The data conversion feature can also be installed with the 7-track feature. The data conversion feature makes it possible to write binary data on 7-track tape. It writes 3 characters from your buffer as 4 tape characters, and converts the opposite way during reading. Conversion is turned on and off by mode setting control commands and is mutually exclusive with translation. You must use the data conversion feature to process format-V (variable-length) tape records because the length field of such records contains binary data. You cannot use the data conversion feature with the read backward (RDBACK) processing method.

The operating system supports the various densities of the 7-track feature. You specify the density with the DEN parameter of the DD statement or DCB macro. Table 1 shows the DEN parameter codes. If not specified, the default value is 800 bpi.

The job step is abnormally terminated if the DEN parameter specifies a density incompatible with the tape unit.

Mode information other than density is specified with the tape recording technique (TRTCH) parameter of the DD statement or DCB macro instruction. The codes for the TRTCH parameters are listed in Table 1.

You use the DEN and TRTCH parameters (or their default values) to specify the density and mode of the data to be read or written. If the tape contains standard labels, the DEN parameter also specifies the density of the labels. IBM recommends that all data sets on a tape containing standard labels be written in the same density. IBM standard labels on 7-track tapes are always written in BCD, with the translate bit on, and even parity, regardless of the value of the TRTCH parameter.

Nonstandard labels on 7-track tapes can be read or written in any code with any parity. The density of the labels need not be the same as the density of the data, but the density of associated tape marks should be carefully planned. System recognition of tape marks is ensured only when they are read in the density in which they were written.