Viper 60s Theory Of Operation And Maintenance Manual page 139

VIPER SCSI 60, 125 & 150 THEORY OF OPERA TION AND MAINTENANCE
6.3 Data Transfer
To write data to tape, a number of conditions must be satisfied. The drive must be selected at
the SCSI interface, the write enable signal must be active, and the cartridge write protect lever
must be positioned to close the safe switch. Differential write data signals must be available at
the VLSI interface. These signals are sent to the drive from the supplied controller through the
SCSI in terface and VLSI.
Unlike the write data circuits, the read circuits are operational during both write and read
operations. As a tape is recorded the read head operates to read each block of data as it is
written.
The difference between only reading data or reading after writing is controlled by the VLSI,
microprocessor, and the host. The read after write data is qualified by the read and write
thresholds. This ensures that the tape contains high-quality data.
During the read mode, the following read/write threshold percentages are applied allowing
efficient retrieval of data:
VIPER DRIVE FORMAT
60 M byles
125 M byles
150 M bytes
6.3.1 Erase Operation
WRITE THRESHOLD
25%
25%
25%
5%
4%
4%
When track 0 is selected, all tracks are erased. As a signal is applied to track 0 by the full-width
erase head, the entire tape is erased. Erasing the entire tape ensures that new data is not
written over old data. To enable the erase/write circuit, the tape cartridge must not have the
write protect lever in the safe position, and the write enable signal must be true.
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