Avago Technologies LSI53C320 - Manual (Page 27)

Ultra320 SCSI Functional Description 2-9
Version 2.2 Copyright © 2003 by LSI Logic Corporation. All rights reserved.
expanders. For Ultra320 SCSI environments, the information in this
section takes precedence over the information in
Section 1.2,
“Applications.”
2.2.3.1 Maximum Electrical Cable Length
The SCSI Parallel Interface-4 (SPI-4) standard states that the electrical
length between the hosts arbitrating on different ends of a SCSI bus
must not exceed 800 nanoseconds (ns), when operating at Ultra320
SCSI data transfer rates. Due to this constraint, LSI Logic specifies that
a maximum of four expanders can be cascaded on a SCSI bus.
There are additional electrical length constraints imposed by the SPI-4
standard. When ending a paced transfer from DT
OUT
to any other phase,
the target must wait 800 ns before issuing a REQ. The SPI-4 standards
allows 200 ns for the host to recognize the phase change and stop the
free running ACK. This requirement reduces the electrical length of the
bus to 600 ns.
Because the expander resides in the middle of the bus and retimes data
to the active free running clock, the expander must switch its internal
logic from ACK to REQ. The expander performs this switch 600 ns after
detecting the phase change. Activity on the ACK or REQ line during this
period adversely affects the expander. This reduces the electrical length
of the bus to 400 ns.
Figure 2.4 illustrates the bus timing. Assuming a 400 ns electrical length
and that the host uses the full 200 ns time allotment, the last free running
ACK from the host returns to the expander at the 600 ns mark.