What is the maximum allowable number of devices that can be attached to a parallel SCSI bus?

Problem

SCSI, pronounced "scuzzy," stands for Small Computer System Interface. SCSI-1 standards are defined by ANSI X3.131.1986.

Resolving The Problem

SCSI, pronounced "scuzzy," stands for Small Computer System Interface. SCSI-1 standards are defined by ANSI X3.131.1986. The newer SCSI-2 specifications are defined by ANSI X3T9.2/375R revision 10K, 1993.

The SCSI standards define common command sets that are programmed into ROM (Read-Only Memory) on the adapters and is accessed via system BIOS as normal hard disk commands. Besides hard disk and CD ROM drives, there are many other SCSI devices available, including tape and re-writable optical drives, printers, scanners, and other devices.

The SCSI architecture allows 8 devices per SCSI bus. The controller itself is a device so seven additional physical devices can be attached. Each physical device can have 7 logical devices attached to it as well. In theory, 64 addressable devices can be attached to a single SCSI controller. In practice, the total is usually closer to 7. But the SCSI specifications allow multiple SCSI adapters to co-reside in a system. IBM Micro Channel systems support up to 4 SCSI controllers (and they can be a mixture of different SCSI-1 and SCSI-2 controllers). IBM's ISA (AT bus) PCs support 2 SCSI controllers.

This sounds complex, so let us simplify. One intelligent SCSI adapter is installed into a system. To this adapter we can attach a SCSI device such as a hard disk, printer or other device. It occupies a physical address as defined by switches or jumpers on the device. It can be addressed as device "0", having the lowest priority, or as device "6", having the highest priority or anywhere in between. Slow devices usually are set for a low priority and faster devices are set for higher priorities. The controller is fixed at address "7" and cannot be altered. If we were to add an intelligent device, one that functions as a "sub" controller, this intelligent device, or more correctly, this controller can have additional devices attached to it. These devices would have logical addresses rather than physical addresses. These addresses are referred to as PUA's (Physical Unit Addresses) or LUA's, (Logical unit addresses).

SCSI devices have a data transfer rate measured in Kilobytes per Second (KBps) for slow devices such as CD-ROM and most tape drives, or Megabytes per second (MBps) for hard disk drives, between the system and the controller, and a different data transfer rate between the controller and the device. For example, the IBM SCSI Adapter with Cache (and its motherboard equivalent) that is standard in many PS/2 models has a data transfer rate of 16.6 MBps between the system and the controller; the transfer rate between the controller and the device is up to 5MBps (for fast devices like hard drives). 5MBps is a limitation of the 8-bit SCSI-1 architecture. SCSI-2 allows for 10MBps (8-bit Fast, or 16-bit Wide SCSI-2 devices), 20MBps (16-bit Fast/Wide, or 32-bit Double-Wide SCSI-2 devices), or 40MBps (32-bit Fast/Double-Wide SCSI-2 devices) data transfers between the controller and SCSI-2 devices designed to operate with those adapters. (Different SCSI-2 adapters support different aspects of SCSI-2; for example FAST SCSI-2 adapters do not support Wide SCSI-2 devices--you would need a Fast/Wide SCSI-2 adapter to support Fast, Wide, and Fast/Wide devices.) SCSI-2 will also support SCSI-1 devices, although at the slower SCSI-1 speeds, on the same controller with SCSI-2 devices. Most SCSI adapters support both internally and externally-attached SCSI devices concurrently.

Various attachment cables and expansion units are also available. An external "option to option" cable is available for device to device attachment. It uses the standard 50 pin SCSI connector. A "card to option" cable is also available to convert the 60-pin connector used on IBM Micro Channel adapters to the 50 pin SCSI standard connector on the first external device. The large 50 pin connector is too big to mount on a Micro Channel adapter so a more compact 60-pin connector was used. The AT-bus adapters have the standard 50-pin connector.

If there isn't sufficient expansion capacity within the computer, external SCSI expansion cabinets may be added, using the Card to Option cable. For example, an IBM 3510 cabinet provides one half-height bay and a 32-watt power supply. The 3511 (which is essentially a PS/2 Model 95 without the motherboard) offers 8 drive bays, both 3.5" and 5.25" and a 400-watt power supply. IBM 3514 disk array subsystem provides additional function, using RAID 5 and other fault-tolerance features, appearing to the operating system as one large disk drive.

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