Seagate Ultra 160, Ultra 320 manual ST latching data vs. DT latching data

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.

REQ or ACK

ST Data (synchronous or asynchronous transfers)

DT Data (synchronous transfers)

DT Data (paced transfers)

DT & ST	DT	DT & ST	DT
Clock	Clock	Clock	Clock

Figure 5. ST latching data vs. DT latching data

<----- transfer period = 25 ns ----- >

REQ or ACK

ST Data

ST	ST
Clock	Clock

Example: A negotiated transfer period of 25 ns equates to a transfer rate of 40 megatransfers per second.

Figure 6. ST synchronous transfer example

Parallel SCSI Interface Product Manual, Rev. A ) 35