Sense Bytes; Examples Of Sense Bytes - IBM 2671 Manual

bit 1, intervention required, turns on) if one of the follow-
ing conditions is satisfied:
1. An end-of-tape is detected (EOF pushbutton not oper-
ated).
2. An overstep occurs on a normal tape stop, except on
channel contention and supply stacker stops.
3. The stop pushbutton is operated during the tape read
operation.
Channel End and Device End and Unit Check
The channel-end and the device-end and unit-check status
byte, when issued, indicates that an unusual condition has
occurred in the 2671 to prevent further data characters
being read properly. Operator intervention may be
required, dependent upon the sense bit that set unit check.
By issuing a sense command, the System/360 program
determines the exact condition that caused the channel-end
and device-end and unit-check status byte issued by the
2671, as follows:
Sense Bit 1 (Intervention Required): Indicates that the 2671
became not-ready before the normal completion of a read operation
(for example, overstep on a tape stop because of the channel con-
tention, a supply stacker stop, or the load switch pressed).
Sense Bit 3 (Equipment Check): Indicates that a timeout has
occurred since the last data character was read. A timeout condition
is caused by a 2671 (unit) failure or by detection of a length of
blank tape. The 2671 remains in a ready status. The System/360
program can initiate a new read command or request operator inter-
vention.
Sense Bit 4 (Data Check): Indicates that incorrect parity has been
detected on a data character. However, this data character is trans-
mitted to the system I/O channel, and the 2671 remains in a ready
status. The System/360 program can either initiate a new read com-
mand or request operator intervention.
Channel-end and device-end and unit-check is also
returned to a test I/O command issued after the status byte
was generated and before the status byte is accepted by the
channel.
SENSE BYTES
When a sense command is received by the 2671, one byte
of sense information is transmitted to the system I/O chan-
nel. The following bit assignment is used in the sense byte:
Bit Condition
0
1
2
3
4
5
6
7
Command Reject
Intervention Required
Bus Out Check
Equipment Check
Data Check
Not used
Not used
Not used
These conditions are reset at the beginning of the next
read command with the exception of 'intervention
required', which is reset by pressing the start pushbutton
after all requirements for a ready condition are satisfied.
Examples of Sense Bytes
Command Reject
The command reject condition is detected when a read
backward or a write command is received at the 2671, or
when valid commands have invalid modifier bits. A parity
error on the command prevents the command reject con-
dition from being detected, and instead a bus-out check
condition occurs.
Intervention Required
The intervention-required condition means that the 2671 is
not ready. (This is best described by stating the conditions
for ready.) The 2671 is ready when:
1. Power is on.
2. Tape is properly loaded.
3. Overstep lamp is off.
4. Start pushbutton is pressed. V
The 2671 becomes not-ready immediately when:
1. The stop pushbutton is pressed while no read operation
is in progress.
2. The load switch is pressed.
3. The power-on-and-reset pushbutton is pressed.
The 2671 becomes not-ready at the end of a read oper-
ation when:
1. There is an overstep condition on a normal or abnorma
tape stop. The overstep condition is reset by pressing the
load switch or the power-on-and-reset pushbutton.
2. The end of tape is detected at the read head (EOF lamp
being off).
3. The stop pushbutton has been operated during the read
operation, and a normal or abnormal completion has
occurred.
As soon as the 2671 is restored from any one of the not-
ready conditions to the ready condition, a device-end
interrupt is generated to inform the program of this
transition.
Bus-Out Check
Bus-out check is set when a parity error has been detected
for a command issued by the System/360.
Equipment Check
The equipment-check sense bit indicates that a timeout has
occurred in the 2671 since the last data character was read.
This timeout could be caused by:
1. A failure of the photocells or the light projection sys-
tem.
2. A tape drive motor failure.
3. An unusual tape creeping condition.
4. A length of blank tape.
5. Tape jamming in the stackers.
Operational Characteristics 13