Parallel SCSI Cable and Connector Pinouts
| This section details the pinouts of the various SCSI cables and connectors. Two electrically different versions of SCSI exist: SE and differential. These two versions are electrically incompatible and must not be interconnected; otherwise, damage will result. Fortunately, very few differential SCSI applications are available in the PC industry, so you will rarely (if ever) encounter one. Within each electrical type (SE or differential), there are basically two SCSI cable types:
Although the 50-pin A cable was the most common cable in earlier SCSI-1 and SCSI-2 installations, it does not support today's high-speed SCSI implementations for servers. In servers, you most often find the 68-pin P cable originally developed for SCSI-2 Wide (16-bit) devices. If you need to install a standard (Narrow) SCSI device on the same SCSI bus that you use for Wide devices, you can use adapters that convert P cable connectors to accept A cables. SCSI cables used for external drives are specially shielded, with the most important high-speed signals carried in the center of the cable and the less important, slower ones in two additional layers around the perimeter. A typical SCSI cable is constructed as shown in Figure 7.14. Figure 7.14. A cross-section of a typical SCSI cable. This specialized construction is what makes SCSI cables so expensive, as well as thicker than other types of cables. Note that this specialized construction is necessary only for external SCSI cables. Cables used to connect devices inside a shielded enclosure (such as inside a PC) can use much less expensive ribbon cables. The A cables can have pin-headertype (internal) connectors or external shielded connectors, each with a different pinout. The P cables feature the same connector pinout on both internal and external cable connections. LVD SCSI Cables and ConnectorsVirtually all recent servers with integrated or add-on SCSI host adapters support LVD signaling. Table 7.4 lists the pinouts for a 68-pin LVD P cable. Note that in the P cable, the RESERVED lines are left open in SCSI devices and the bus terminator assemblies. A hyphen preceding a signal name indicates that the signal is Active Low.
SE SCSI Cables and ConnectorsSE SCSI cables and connectors use the pinouts listed in Tables 7.5 and 7.6 (the same pinouts used by HVD SCSI cables and connectors). Although servers don't incorporate SE-only host adapters, both integrated (embedded) and add-on card LVD SCSI host adapters are designed to work with both LVD and SE devices. To avoid slowing down LVD devices, SE devices should be connected to a separate bus from the one hosting LVD devices.
The A cable is available in both internal unshielded and external shielded configurations. A hyphen preceding a signal name indicates that the signal is Active Low. The RESERVED lines have continuity from one end of the SCSI bus to the other. In an A cable bus, the RESERVED lines should be left open in SCSI devices (but may be connected to ground) and are connected to ground in the bus terminator assemblies. HVD SCSI SignalsHVD SCSI is not normally used in a PC environment but is very popular with minicomputer installations because of the very long bus lengths that are allowed. This has changed with the introduction of LVD signaling for SCSI, bringing the benefits of differential signaling to lower-end and more mainstream SCSI products. Differential signaling uses drivers on both the initiator and target ends of the bus and makes each signal work in a push/pull arrangement rather than a signal/ground arrangement, as with standard SE SCSI. This enables much greater cable lengths and eliminates some of the problems with termination. Almost all PC peripherals produced since SCSI was introduced have been SE types. These are incompatible with HVD devices, although HVD devices can be used on an SE bus with appropriate (and expensive) adapters. This is possible because HVD cables use the same pinouts as SE cables (refer to Tables 7.5 and 7.6). The LVD devices, on the other hand, can be used on an SE bus if they are multimode devices, in which case they switch into SE mode. If all devicesincluding the host adaptersupport LVD mode, all the devices switch into that mode, and much longer cable lengths and higher speeds can be used. The normal limit for an SE SCSI bus is 1.53 meters maximum and up to 20MHz. If run in LVD mode, the maximum bus length goes up to 12 meters, and speeds can go up to 80MHz. HVD SCSI supports bus lengths of up to 25 meters. Almost all modern SCSI hard disks are Ultra 2 or Ultra 3 devices, which means that by default they are also LVD or multimode LVD/SE devices. ExpandersSCSI expanders separate a SCSI bus into more than one physical segment, each of which can have the full SCSI cable length for that type of signaling. They provide a complete regeneration of the SCSI bus signals, allowing greater cable lengths and incompatible devices to essentially share the same bus. An expander can also be used to separate incompatible parts of a SCSI busfor example, to keep SE and HVD SCSI devices in separate domains. Expanders are transparent to the software and firmware on the bus, and they don't take up a device ID. They are usually capable of providing termination if located at the end of a bus segment, or they can have termination disabled if they are in the middle of a bus segment. Figure 7.15 illustrates the interior of a typical SCSI expander. Note that a SCSI expander is usually housed in a protective shell. The dime at the lower right provides an indication of the relative size of the expander. Figure 7.15. The interior of a typical SCSI expander. Because of their expense (often more than $100 each), expanders are not normally used except in extreme situations in which no other alternative remains. In most cases, it is better to stick within the recommended cable and bus length requirements and keep incompatible devices, such as HVD devices, off a standard SE or LVD bus. SCSI TerminationBecause a SCSI bus carries high-speed electrical signals, it can be affected by electrical reflections that might occur within any transmission line system. A terminator is designed to minimize the potential for reflections or noise on the bus, as well as to create the proper load for the bus transmitter circuits. Terminators are placed at each end of the bus to minimize these problems. Despite the simple rules that only two terminators must be on the bus, and they must be at each end, termination are still the most common cause of problems in SCSI installations. Several types of SCSI terminators are available, depending on the bus signaling and speed requirements:
The first three of these are used on single-ended SCSI buses only. Passive terminators use a passive network of 220-ohm and 330-ohm resistors to control bus termination. They should be used only in Narrow (8-bit) SCSI buses running at 5MHz. Passive terminators allow signal fluctuations in relation to the terminator power signal on the bus. Usually, passive terminating resistors suffice over short distances, such as 1 meter or less, but for longer distances or higher speeds, active termination is a real advantage. Active termination is required with Fast SCSI. Passive terminators can be plugged in to the unused SCSI port or be built in to many low-speed SCSI devices, such as scanners, optical drives, and removable-media drives. On such devices, you can activate or deactivate built-in passive termination with jumper blocks, a toggle switch, or some other form of selector switch, depending on the device. Figure 7.16 shows the schematic of a typical passive terminator. Figure 7.16. Passive SCSI terminator schematic. Active terminators use built-in voltage regulator ICs combined with 110-ohm resistors. An active terminator actually has one or more voltage regulators to produce the termination voltage, rather than resistor voltage dividers alone. This arrangement helps ensure that the SCSI signals are always terminated to the correct voltage level. Active terminators often have some type of LED indicating the termination activity. The SCSI-2 specification recommends active termination on both ends of the bus and requires active termination whenever Fast or Wide SCSI devices are used. Most high-performance host adapters have an "auto-termination" feature, so if it is the end of a chain, it terminates itself. Figure 7.17 shows the schematic of a typical active terminator. Figure 7.17. Active SCSI terminator schematic. A variation on active termination is available for single-ended buses: FPT. FPT is an even better form of active termination, in which diode clamps are added to eliminate signal overshoot and undershoot. The trick is that instead of clamping to +5 and ground, these terminators clamp to the output of two regulated voltages. This arrangement enables the clamping diodes to eliminate signal overshoot and undershoot, especially at higher signaling speeds and over longer distances. FPT is technically not found in the SCSI specifications but is a superior type of termination for SE applications that experience high levels of electrical noise. FPT terminators are available in several versions. FPT-3 and FPT-18 versions are available for 8-bit standard SCSI (Figure 7.18 shows the schematic for an FTP-18 terminator), whereas the FPT-27 is available for 16-bit (Wide) SCSI. The FPT-3 version forces the three most highly active SCSI signals on the 8-bit SCSI bus to be perfect, whereas the FPT-18 forces all the SCSI signals on the 8-bit bus except grounds to be perfect. FPT-27 also forces all the 16-bit Wide SCSI signals except grounds to be perfect. Figure 7.18. FPT18 SCSI terminator schematic. HVD buses require HVD terminators, constructed using a passive network of 330-ohm/150-ohm/330-ohm resistors. The only choice is that the terminator matches the cable or device connection. The same is true for LVD buses. They require LVD terminators for the bus to function properly. One twist is that special LVD/SE (active) multimode terminators are available. These function as LVD types on an LVD bus and as active types on an SE bus. Note that if any SE devices are on the bus, the bus functions in SE mode and never uses LVD mode, severely limiting bus length and performance. If any SE-only terminators or SE devices are on the bus, the bus defaults into SE mode.
Note Several companies make high-quality terminators for the SCSI bus, including East/West Manufacturing Enterprises, Inc. (formerly Aeronics) and the Data Mate division of Methode. Both of these companies make a variety of terminators. EWME is well noted for some unique FPT versions that are especially suited to problem configurations that require longer cable runs or higher signal integrity. One of the best investments you can make in any SCSI installation is in high-quality cables and terminators. Contact information for both of these companies is in the vendor list in Appendix C, "Vendor List." Special terminators are also required for LVD and HVD SCSI, so if you are using those interfaces, you need to be sure your terminators are compatible. With LVD or HVD buses, you don't have much choice in terminator types, but for SE buses, you have at least three choices. Tip The best rule for terminators, as well as for cables, is to get the best you can. You should never use passive terminators; instead, use active or, if you want the best in reliability and integrity, use only FPT. |
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