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System/360; Data Bytes; Status Byte - IBM 2702 Information Manual

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SYSTEM/360 I/O INTERFACE
The I/O interface sequences, data bytes, and signal
lines are described in detail in the Original Equip-
ment Manufacturer's Information manual, Form A22-
6843. The meaning of sense and status bytes for a
2702/03 are described in this chapter.
Data Bytes
The 2702 and 2703 differ in the number of data bytes
transferred during anyone I/O channel data-servicing
sequence. The 2702 transfers one byte at a time,
while the 2703 transfers data in bursts of up to four
bytes.
Status Byte
The status byte signals the I/O interface the status
of the 2702/03 during initial selection of a command
and at the completion of a command.
Figure 2-8 shows the status bits that can be set
by the 2702/03.
Initial Status
All Zeros. This signals the channel that the 2702/03
is able to attempt to perform the command.
Bus-In Bits
Status Condition
0
Attention
Not used.
Status Modifier, Control Unit End, and Busy. This
signals the channel that the command is incapable of
being performed at this time. This status-byte
configuration is due to the 2702/03 control-unit
portion being busy or in a machine-reset condition.
Unit Check. This signals the channel that the com-
mand is incapable of being executed. This status-
byte configuration is dUEl to either a parity check of
the command byte or an invalid command code.
Status Modifier. This status-byte configuration is
sent to the channel upon decoding of a Test I/O instruc-
tion.
If
the control-unit portion of the 2702/03 is
busy, the status for a Test I/O instruction will be as
noted under "Unit Check" preceding.
Channel End and Device End. This status-byte con-
figuration is sent to the channel upon decoding of an
I/O No Op command.
Ending Status
The conditions turning on the ending-status bits for
different commands and terminal controls are shown
in Figures 2-9 and 2-10. The meaning of combina-
tions of status bits follows.
Use
1
Status Modifier
Initial Selection. Positive Poll response received. (Included for BSC
are timeout, and polling address received during Address Prepare.)
2
Control Unit End
Initial Selection
.1.
3
Busy
Initial Selection.l.
4
Channel End
Command Termination.
I.
5
Devi ce End
6
Unit Check
Error (Caused by any sense bit ON).
7
Unit Exception
Unusual Condition (See figures entitled
"Ending Status . . . ").
Figure 2-8. Status Bits
2-26

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