GE 235 System Manual page 67

Special Binary Format
The GE-235 system also operates in a special binary format. This records 3 positions on tape
instead of 4 without altering the bit structure. This feature effectively expands compatibility to
cover both the decimal and binary modes of other tape systems. The feature i s also useful for
GE-235 applications using mixed binary and decimal data.
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It i s advantageous to write numeric information in the binary format because the equivalent of
5-1/2 decimal digits in memory may be recorded in only 4 tape characters. In BCD format,
however, 3 BCD characters require 3 tape positions. If memory data consists of mixed formats
of both BCD and binary in memory, the binary writing format is employed.
CHECKING FEATURES
Each GE-235 tape handler has two s e t s of magnetic heads; read heads, and write heads. During
reading operations, only the read heads a r e used. During write operations, both s e t s of heads
operate, a s follows:
Write Heads
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Record the characters on tape a s the tape passes;
Read Heads
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Read each character immediately after it is written and checks its validity.
Thus, if e r r o r s occur, they a r e detected during the write operation and can be corrected
under program control.
Write Permit Ring
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Fits into a groove on the back side of the tape r e e l to permit writing
on the tape. Removing the ring from the r e e l prevents writing o r accidental erasure of data
already on a tape.
The GE-235 magnetic tape system incorporates a wide variety of e r r o r detection circuits to ensure
accurate data transfers between memory and tape. These circuits are:
CONTROLLER INPUT-OUTPUT REGISTER EXHAUST OR OVERFLOW. Checks against an
excessive wait for memory access on the part of the Tape Controller during multiple input/
output operations.
LATERAL PARITY.
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This parity check associated with each tape character ensures the
accuracy of each character when read from, o r written onto, tape.
LONGITUDINAL PARITY. This i s a parity check on each of the seven record tracks, o r
channels, recorded a s a parity character a t the end of each tape record.
MODULO THREE OR FOUR. When information i s read from, o r written onto, tape, a check
is made to determine that the proper number of characters (three in BCD and special binary
formats, and four in binary formats) have been read o r recorded.
All information words a r e parity checked when transferred between memory and the Tape
Controller.