SCSI TERMINATION

On SCSI drives, the Host Adapter also has resistors. These are needed to terminate both ends of the bus. Since the SCSI bus can have up to 7 devices attached to it, only the Host Adapter and the device farthest from it will retain the resistors. All devices in-between should have theirs removed. With the advent of increased use of SCSI for peripherals comes the chance that one day you'll need an SCSI terminating resistor. Prepare for a shock, because you might be very surprised at the prices charged, for what you get. Many Manufacturers still have SCSI peripheral hardware priced for the Workstation market, not the PC market. We may see these prices erode as more PCs adopt SCSI as their disk interface of choice, but for now be prepared to pay a premium for anything to do with SCSI. So here you are, with a disk drive mounted internally, and a CD-ROM hanging off the back of the PC. Everything looks great, but it just doesn't work... Maybe it doesn't even recognise the CD-ROM. You've checked the connectors, and everything looks good... So what's the problem? Well, did you check the terminators? (Say Whaaat??) Improper termination of an SCSI bus can raise havoc with the Host Adapter's interface circuit, and result in missing peripherals, or intermittent operation and possible loss of data. Well, here's a way to build an inexpensive terminator that will connect to the second SCSI connector on many SCSI peripherals.

SCSI termination is very important, there must be two terminator on each SCSI chain, one at the beginning and one at the end device chain. The terminator must be within 10 centimeters (4 inches) of each end of the chain on the bus. Take a look at figure 1 to see what is acceptable and what's not when terminating SCSI devices.
The following ARE acceptable:
+------------+----------+-----------+-----------+---------+
| | | | | |
DEVICE A Unconnected Unconnected Unconnected DEVICE B DEVICE C
Terminated (adapter-Terminated)
+------------+----------+-----------+-----------+---------+
| | | | | |
DEVICE A Unconnected DEVICE B Unconnected Unconnected DEVICE C
Terminated (adapter) Terminated
+------------+----------+-----------+-----------+---------+
| | | | | |
Terminated DEVICE A DEVICE B Unconnected Unconnected DEVICE C
(adapter) Terminated
The following ARE NOT allowed:
+------------+----------+-----------+-----------+---------+
| | | | | |
DEVICE A DEVICE B DEVICE C Unconnected Unconnected Unconnected
Terminated (adapter) Terminated
+------------+----------+-----------+-----------+---------+
| | | | | |
Unconnected DEVICE A DEVICE B DEVICE C Unconnected Unconnected
Terminated (adapter) Terminated
Figure 1: Correct and wrong way of connecting SCSI devices
Note: When putting the host adapter in the middle of the chain,
you must disable the host terminators. If this is impossible you MUST put the host adapter at either end of the cable.
SCSI CONNECTORS

Names
External Connector Types (not to scale)
Maximum
Speed
(MB/s) Number of
Devices (A,B)
Max Total
Length of Cable Uses Comments
SCSI-1
50-pin low-density (SCSI-1, Centronix, A-Cable)

>Apple DB-25C

SCSI-1 5 8 6 mD
Scanners, Zip drives, hard drives, etc. The DB-25 version is standard on Macintosh® computers.C
Many peripherals are available.
SCSI-2
SCSI-2 (Fast SCSI, Fast Narrow)
50-pin low-density (SCSI-1, Centronix, A-Cable)

50-pin high-density (SCSI-2)

10 8 ²4 devices: 3 m
5+ devices: 1.5 m Desktop hard drives, scanners, CD-ROM drives, CD-Recordable drives, and removables Backward-compatible with SCSI-1. Many products available.
"Wide" products use 16-bit instead of 8-bit data paths.
Fast Wide SCSI (Wide SCSI) 68-pin

20 16 ²4 devices: 3 m
5+ devices: 1.5 m Faster hard drives, workstations, servers and RAID systems Backward-compatible with SCSI-1. Many products available.
"Wide" products use 16-bit instead of 8-bit data paths.
SCSI-3
50-pin low-density (SCSI-1, Centronix, A-Cable)

50-pin high-density (SCSI-2)

68-pin

Ultra SCSI
(SCSI-3, Fast-20, Ultra Narrow) 20 8 1.5 mD,LVD
Newer and faster hard drives, performance desktop, workstations, servers, RAID systems Backward-compatible with previous SCSI versions.
SCSI-3 specification still under consideration, but products are available that meet current specifications. "SCAM" specification allows for devices to set own ID automatically.
Some variations of SCSI-3 are hot-pluggable.
Ultra2 devices available now.
Ultra3 devices under development.
Wide Ultra SCSI
(Fast Wide 20) 40 16 1.5 mD,LVD

Ultra2 SCSI 40 8 12 m
Wide Ultra2 SCSI 80 16 12 m
Ultra3 SCSI 80 8 12 m
Wide Ultra3 SCSI 160 16 12 m

(A): Including Host Adapter
(B): Per channel
(C): The Apple DB-25 is not an official standard. It uses SCSI-2 protocols, but at SCSI-1 speeds.
(D): Differential allows for cable lengths of up to 25 meters.
(E): The DB-9 Fibre Channel connector is the industry standard, but only the four outer pins are live.