SCSI - Computer Busses - page 153

7.2.1 SCSI-II
SCSI-II supports fast SCSI which is basically SCSI-I operating at a rate of 10 MB/s (using
synchronous versus asynchronous) and Wide SCSI which uses a 64-pin connector and a 16-
bit data bus. The SCSI-II controller is also more efficient and processes commands up to
seven times faster than SCSI-I.
The SCSI-II drive latency is also much less than SCSI-I due mainly to tag command
queuing (TCQ) which allows multiple commands to be sent to each device. Each device then
holds its own commands and executes them in whatever sequence that will maximize per-
formance (such as by minimizing the latency associated with disk rotation). Table 7.1 con-
trasts Fast SCSI-II and Fast/Wide SCSI-II. It can be seen that both disks have predictive fail-
ure analysis (PFA) and automatic defect reallocation (ADR).
The normal 50-core cable is typically known as A-cable, while the 68-core cable is
known as B-cable.

Table 7.1 Comparison of SCSI-II disks

Seek
time
(ms)

Latency
(ms)

Rotational
speed (rpm)

Sustained
data read (MB/s)

PFA ADR

1 GB SCSI-II fast 10.5 5.56 5400 4 9 9

4.5 GB SCSI-II fast/ wide 8.2 4.17 7200 12 9 9

7.2.2 Ultra SCSI

Ultra SCSI (or SCSI-III) allows for 20 MB/s burst transfers on an 8-bit data path and 40
MB/s burst transfer on a 16-bit data path. It uses the same cables as SCSI-II and the maxi-
mum cable length is 1.5 m. Ultra SCSI disks are compatible with SCSI-2 controllers; how-
ever the transfer will be at the slower speed of the SCSI controller. SCSI disks are compati-
ble with UltraSCSI controllers; however, the transfer will be at the slower speed of the SCSI
disk.
SCSI-I and Fast SCSI-II use a 50-pin 8-bit connector, whereas fast/wide SCSI-II and Ul-
tra SCSI uses a 68-pin 16-bit connector. The 16-bit connector is physically smaller than the
8-bit connector and the 16-bit connector cannot connect directly to the 8-bit connector. The
cable used is called P-cable and replaces the A/B-cable.
Note that SCSI-II, and Ultra SCSI require an active terminator on the last external device.
Table 7.2 compares the main types of SCSI.

Table 7.2 SCSI types

Data
bus
(bits)

Transfer rate
(MB/s)

Tagged
command
queuing

Parity
checking

Maximum
devices

Pins on
cable and
connector

SCSI-I 8 5 7 50

× × / 9 (optional)

SCSI-II Fast 8 10 (10MHz) 9 9 7 50

SCSI-II fast/
wide

16 20 (10MHz) 9 9 15 68

Ultra SCSI 16 40 (20MHz) 9 9 15 68