IBM 1 Series - Page 37

2-26 GA34-0033
6. Upon deactivation of both ‘service gate’ and ‘data strobe,’ the device
deactivates ‘address bus bits 0-15,’ ‘condition code in bus,’ ‘cycle byte
indicator,’ and ‘cycle input indicator,’ as appropriate, and ‘data bus’
(on an input transfer). These lines must be deactivated prior to the
drop of ‘service gate return,’ as seen at the device interface.
The device deactivates ‘service gate return.’ The permissible delay, T4,
from the deactivation of ‘service gate’ and ‘data strobe’ to the
deactivation of ‘service gate return,’ is 3 microseconds, maximum, as
seen at the device interface. This delay is for attachment convenience;
however, for performance reasons, the delay should be held to a
minimum. All device action for the transfer must be completed prior to
dropping ‘service gate return.’
7. The total duration of the cycle-steal service sequence is timed out by
the channel for error-detection purposes. The total duration is
measured in the same way as for an interrupt-service sequence. If the
time-out occurs, the channel activates the ‘halt or MCHK’ line. If the
device attachment adheres to the specified times over which it has
control, the total duration of the cycle-steal sequence is within the
channel time-out under normal operation.
The sequence described here applies to cycle-steal servicing in burst mode,
also; however, there is an additional consideration in burst mode, as noted
by CTS in Figure 2-10. ‘Service gate’ for the next cycle-steal transfer may
activate immediately at the device interface after deactivation of ‘service
gate return.’ Therefore, the device does not directly control the demanded
rate of servicing in burst mode. (The device does control this demand in
normal cycle-steal transfers because one request corresponds to one
cycle-steal service sequence.) In burst mode, the device can only exert
minor control over the demand made by the channel by indirectly
controlling delays of ‘service gate return’ activation and deactivation;
however, this is generally not recommended because there are attendant
risks of device underrun and channel time-out. The recommended
mechanism for burst mode is to buffer for a size of data equal to the
length of the burst. No specific timing characteristics of the channel may
be assumed by the device in burst mode operation. The burst mode device
must be able to operate with an indefinite delay in the rise of ‘service gate’
after the deactivation of ‘service gate return’ (CTS time).