IBM 727, Manual Of Instruction

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12.0 RESUME OF LINES ENTERING AND LEAVING TAPE UNITS
The control lines entering and leaving the tape unit are pictured in Figure 15. A
brief functional description of each, line is given below.
Select Lines (75.01) (10 lines). The select lines are routed from external con­
trol; they determine which tape unit is to be selected.
Write Bus (75.07) (7 lin es). The seven DC lines making up the write bus trans­
mit the seven bits in parallel from the external circuitry.
Read Bus (75.06) (7 lines). The read bus consists of seven DC lines over which
the seven bits are transmitted in parallel to the external control. Reading at a speed
of 75 inches per second with a bit density of 200 characters per inch gives a read
frequency of 67 m icroseconds per character.
Write Echo Bus (75,07) (7 lines). These seven lines send the image of the infor­
mation as it is written back to the external control where it is used for checking
purposes.
Select, Ready, and Read (75.02). This line is an output from the tape unit indi­
cating that the unit has been selected and is ready to begin reading. The term "ready”
here means that a tape is loaded into the machine and the machine is not in rewind
status. This is, in reality, a response to the set-read line coming from the control
system.
Select, Ready and Write (75.02). This line is also an output of the tape unit in­
dicating that the unit has been selected and is ready to begin writing. It is a response
to the set-w rite line coming from the control system.
Set Read (75.02). This line is an input from the control system indicating desire
to read from the tape unit. It is preceded by the proper select line indication.
Set Write (75.02). This line is an output from the control system indicating de­
sire to write in the selected tape unit.
Select and Rewind (75.08). This line is an output of the tape unit indicating it has
been selected and is rewinding. Reading or writing cannot be perform ed if this line
is up.
Select and At Load Point (75, 04). This line indicates to the control system that
the selected tape unit is at load point.
Select and TI On (75. 03|c This output from the tape unit indicates that the selected
tape unitfs tape indicator is on.
Select and TI Off (75.03). This output indicates that the selected unit’ s tape in­
dicator is off.
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