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42 PCM-3420 User's Manual
ASPI
Advanced SCSI programming Interface. A standard SCSI soft-
ware interface that acts as a liaison between host adapters and
SCSI device drivers. ASPI enables host adapters and device
drivers to share a single SCSI hardware interface.
ASW
An acronym for Adaptec Software.
ASYNCHRONOUS DATA TRANSFER
A method of SCSI data transfer. This is the type of transfer rate
originally introduced with SCSI 1. With this type of transfer
method, transfer rates of 2 MBytes/sec are common. See also
SYNCHRONOUS TRANSFER.
Is SYNCHRONOUS faster than ASYNCHRONOUS?
Asynchronous is faster on short cables, while synchronous is
faster on long cables. The reason has to do with the propagation
delay of the cable; the turn around time of the silicon; and the
interlocked nature of the asynchronous handshake.
1) We have measured propagation delays from various cables
and found an average of 1.7 nanoseconds per foot, which is
roughly 5.25 ns per meter.
2) The turn-around time is the amount of time the SCSI chip takes
to change an output in response to an input. If REQ is an input
then ACK is an output. Or if ACK is an input then REQ is an
output.
3) The asynchronous transfer uses an interlocked handshake
where a device cannot do the next thing until it receives positive
acknowledgment that the other device received the last thing.