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One of the most exciting things about SCSI-3 is the proposed Serial SCSI, a scheme that may use only a six-conductor cable and that will be able to transfer data at up to 100MBps. The switch to serial instead of parallel is designed to control the delay, noise, and termination problems that have plagued SCSI-2, as well as to simplify the cable connection. Serial SCSI will be capable of transferring more data over six wires than 32-bit Fast Wide SCSI-2 can over 128 wires. The intention is that Serial SCSI be implemented on the motherboard of future systems, giving them incredible expansion and performance capabilities.

Although Serial SCSI may not make the older host adapters and cables obsolete overnight, it does make future cabling possibilities even more of a puzzle. Serial SCSI offers the possibility of longer cable lengths, less electromagnetic interference, and easier connections on laptops, notebooks, and docking stations. Expect SCSI-3 to offer almost pain-free installations with automatic Plug and Play SCSI ID setup and termination schemes.

In any practical sense, SCSI-3 is still some ways away from being approved. Because the standard exists in draft documents before being officially approved, if the portions of the standard become stable, we may very well see products claiming SCSI-3 compatibility well before the standard truly exists. Because SCSI-3 actually incorporates all of what is in SCSI-2, technically anybody can call any SCSI-1 or SCSI-2 device a SCSI-3 device as well. Beware of product hype along these lines. Some of the new SCSI-3 features will likely be incompatible with previous SCSI implementations, and may take a while to appear on the market.

SCSI Cables and Connectors

The SCSI standards are very specific when it comes to cables and connectors. The most common connectors specified in this standard are the 50-position unshielded pin header connector for internal SCSI connections and the 50-position shielded Centronics latch-style connectors for external connections. The shielded Centronics style connector also is called Alternative 2 in the official specification. Passive or Active termination (Active is preferred) is specified for both single-ended and differential buses. The 50-conductor bus configuration is defined in the SCSI-2 standard as the A-cabled.

The SCSI-2 revision added a high-density, 50-position, D-shell connector option for the A-cable connectors. This connector now is called Alternative 1. The Alternative 2 Centronics latch-style connector remains unchanged from SCSI-1. A 68-conductor B-cable specification was added to the SCSI-2 standard to provide for 16- and 32-bit data transfers; the connector, however, had to be used in parallel with an A cable. The industry did not widely accept the B cable option, which has been dropped from the SCSI-3 standard.

To replace the ill-fated B cable, a new 68-conductor P cable was developed as part of the SCSI-3 specification. Shielded and unshielded high-density D-shell connectors are specified for both the A cable and P cable. The shielded high-density connectors use a squeeze-to-release latch rather than the wire latch used on the Centronics-style connectors. Active termination for single-ended buses is specified, providing a high level of signal integrity.

SCSI Cable and Connector Pinouts

There are two electrically different version of SCSI, Single Ended and Differential. These two versions are electrically incompatible, and must not be interconnected or damage will result. Fortunately, there are very few Differential SCSI applications available in the PC industry, so you will rarely (if ever) encounter it. Within each electrical type (Single Ended or Differential), there are basically three SCSI cable types:

  A Cable (Standard SCSI)
  P Cable (16- and 32-bit Wide SCSI)
  Q Cable (32-bit Wide SCSI)

The A cable is used in most SCSI-1 and SCSI-2 installations, and is the most common cable you will encounter. SCSI-2 Wide (16-bit) applications use a P cable instead, which completely replaces the A cable. You can intermix standard and Wide SCSI devices on a single SCSI bus by interconnecting A and P cables with special adapters. 32-bit wide SCSI-3 applications use both the P and Q cables in parallel to each 32-bit device. Today there are virtually no PC applications for 32-bit Wide SCSI-3, and because of the two cable requirement, it is not likely to catch on.

The A cables can have Pin Header (Internal) type connectors or External Shielded connectors, each with a different pinout. The P and Q cables feature the same connector pinout on either Internal or External cable connections.

Single-Ended SCSI Cables and Connectors

The single-ended electrical interface is the most popular type for PC systems. The A cable is available in both internal unshielded as well as external shielded configurations. The pinouts for these are shown in Tables 2.5 and 2.6.

Table 2.5  A Cable (Single-Ended) Internal Unshielded Header Connector.

Signal Name Pin Pin Signal Name

GROUND 1 2 -DB(0)
GROUND 3 4 -DB(1)
GROUND 5 6 -DB(2)
GROUND 7 8 -DB(3)
GROUND 9 10 -DB(4)
GROUND 11 12 -DB(5)
GROUND 13 14 -DB(6)
GROUND 15 16 -DB(7)
GROUND 17 18 -DB(Parity)
GROUND 19 20 GROUND
GROUND 21 22 GROUND
RESERVED 23 24 RESERVED
Open 25 26 TERMPWR
RESERVED 27 28 RESERVED
GROUND 29 30 GROUND
GROUND 31 32 -ATN
GROUND 33 34 GROUND
GROUND 35 36 -BSY
GROUND 37 38 -ACK
GROUND 39 40 -RST
GROUND 41 42 -MSG
GROUND 43 44 -SEL
GROUND 45 46 -C/D
GROUND 47 48 -REQ
GROUND 49 50 -I/O

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