IBM 1 Series - Page 40

10.
Burst mode (‘burst return’) (Figure 2-12):
After ‘burst return’ is activated, the next activation of ‘service gate’
begins the burst transfer. All other devices on the I/O channel are
preempted, and the burst device continues to get service until the burst
return is deactivated or an error or time-out condition is detected.
‘Burst return’ must be deactivated within 100 nanoseconds of the
activation of ‘service gate’ for the last transfer, T4. The I/O device
must not present another cycle-steal or interrupt request until ‘service
gate return’ goes inactive for the last transfer, T5. This allows time for
the processor to start a new polling sequence and to service a different
request, if one is present.
In both sequences, (1) with ‘poll return’ and (2) with ‘burst return,’ a
channel time-out may occur. In the poll sequence with ‘poll return,’
the time-out occurs if ‘poll return’ does not go inactive. In the poll
sequence with ‘burst return,’ the time-out occurs if ‘poll’ does not go
inactive. Both of these time-out conditions are indications of a failure
at the I/O device, and do not occur under normal operating conditions
if the timings in the referenced figures are adhered to. If the channel
time-out does occur, it causes a machine check and activates the ‘halt
or MCHK’ line on the channel.
Processor I/O Channel 2-29