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Parallel SCSI Interface Product Manual, Rev. A ) 35
Figure 8 shows an example of transfers with a negotiated transfer period of 6.25 ns at the receiving SCSI
device’s connector. There is no difference as to when data is latched on paced transfers, however, the relation-
ship between the data and REQ or ACK is required to be adjusted in the SCSI devices receiver to match the
synchronous transfers DT Data shown in Figure
5. For paced transfers, the clocking signal (i.e., REQ or ACK)
may occur when the data bus is changing state.
Figure 5. ST latching data vs. DT latching data
Figure 6. ST synchronous transfer example
DT Data
(paced
transfers)
REQ or ACK
DT Data
(synchronous
transfers)
DT & ST
Clock
DT & ST
Clock
DT
Clock
DT
Clock
ST Data
(
synchronous or
asynchronous
transfers)
R
EQ or ACK
ST
Clock
ST
Clock
ST Data
<----- transfer period = 25 ns ----->
Example: A negotiated transfer period of 25 ns equates to a
transfer rate of 40 megatransfers per second.