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With 'read modified' commands (bit 5 = 1), the read
operation can be ended if:
1. EOR character detected.
2. Count in the channel control word reaches zero.
The sense command gates back the sense bytes of the
2671/2822 to the System/360.
The test I/O instruction is given by the System/360 pro-
gram to check the availability of the 2671/2822, if it has
not been accessed for some time.
STATUS BYTES
The status byte informs the System/360 of the current
status of the 2671/2822:
Bit
Condition
0
1
2
3
4
5
6
7
Not used
Not used
Not used
Busy
Channel End
Device End
Unit Check
Unit Exception
All bits of the status byte are reset when the byte is
accepted by System/360, with the exception of bits 3 and 6
if the 2671/2822 is not ready (ready lamp off). Bit 6 is set
on if any of bits 0 through 2 of the sense byte are on.
Examples of Status Bytes
Busy
A busy status byte is returned to a new command when the
2671 is still executing the previous command and before
any end status (including channel end) is generated in the
2822.
Channel End
A channel end status byte indicates that transmission of
data characters over the interface for a given command has
been completed.
Device End
The device end indicates that the 2671 has been restored
from any one of the not-ready conditions to a ready con-
dition.
Unit Check
The unit-check status byte is sent to the system to reject a
command. A sense command can be issued by the system
to interrogate the 2671 as to why the command was rejec-
ted. Note that a sense command is accepted with a blank
status even when the unit generates unit check because of a
not-ready situation.
Sense Bit 0 (Command Reject): Invalid command.
Sense Bit 1 (Intervention Required): The 2671/2822 is unable to
perform a read operation.
Sense Bit 2 (Bus-Out Check): Incorrect parity of a valid or invalid
command.
Unit Exception
The unit-exception status byte is issued by the 2671 to
reject the first command following the end of the last tape
of a file (end-of-file lamp on). The status of the 2671
becomes not-ready after issuing this status byte.
Busy and Device End
If the device-end interrupt, generated by the
not-ready-to-ready transition, is stacked, any subsequent
valid command (except test I/O) is rejected. Busy and
device-end is returned to the channel.
Busy and Channel End and Device End
The busy and channel-end and device-end status byte
rejects any command, except test I/O, when such a com-
mand is issued after channel-end and device-end was
generated in the 2822 and before the status byte is accep-
ted by the channel.
Busy and Channel End and Device End and Unit Check
The busy and channel-end and device-end and unit-check
status byte rejects any command, except test I/O, when
such a command is issued after channel-end and device-end
and unit-check was generated in the 2822 and before the
status byte is accepted by the channel.
Channel End and Device End
The channel-end and device-end status byte is transmitted
to the System/360 each time a tape read operation has been
properly performed. If the modifier bit is not present in the
read command byte, the read operation is completed upon
receiving a stop signal from the system (character count
equals zero). If the read command byte contains the modi-
fier bit, the read operation is normally stopped when an
EOR character is detected by the 2671. (The EOR charac-
ter is not transmitted to the system.) The channel-end and
device-end status byte can also be generated by an end-of-
tape at the read head.
Channel-end and device-end is returned:
1. To a halt I/O when halt I/O occurs after initial selection
and before channel-end is presented.
2. To a control no-operation (NOP) when no status infor-
mation is pending.
3. To a test I/O issued after a channel-end and device-end
status was generated and before it is accepted by the
channel.
As soon as the channel-end and device-end status byte is
accepted by the system, the 2671 becomes not-ready (sense
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