Cables and Termination



			Cables and Termination

			Cable Lengths	SCSI cable lengths vary based on the particular SCSI interface used and the number of devices per channel. If there are more than four devices on a channel, the overall bus cable length must be about half of the specified max for optimum signal strength on the bus. This includes devices that are on all channels of the SCSI controller.

				The requirement of a shorter cable for having more devices can be somewhat cumbersome if you are working with Wide SCSI, which allows for 15 devices. If you are planning on having more than 7 devices on a single Wide SCSI channel, consider getting special cables that have the appropriate impedance level to keep the bus signal strength at an optimum level.

				There should be about 12" of space between each connector on a SCSI cable. Unfortunately, this limits a single-ended SCSI bus to about five devices if any are Ultra SCSI. If you need to have more than five devices, consider Wide SCSI, HVD SCSI, or LVD SCSI.

			Termination	Make sure only the devices at the end of the SCSI chain are terminated. For devices on only one connector, the terminated devices will be the SCSI controller and the device at the other end. For devices on two connectors, this would apply to the device at the end of the connector.

				Most SCSI controllers manufactured today have automatic termination. The controller simply determines which connectors have SCSI devices attached and adjusts appropriately. Although this is handy, it's highly recommended you manually set the termination (some adapters don't provide this option).

				You should not allow for more than 3" of cable beyond a terminated device. SCSI signals are not smart and can be lost if they are sent in the wrong direction. It would be a good practice to simply not allow for any cable beyond the last terminated device.

				When terminating devices, use active termination. This type of termination guarantees the SCSI chain is terminated properly. Some older SCSI devices use passive termination, and this type applies a different impedance level. The only devices that can utilize passive termination are SCSI-1 and SCSI-2 devices. Fast SCSI-2, Wide SCSI-2, Ultra SCSI, and newer devices must use active termination. With Wide SCSI devices, you must also make sure both the low- and high-bits are terminated.

				At least one device somewhere on the SCSI chain must provide termination power. This activates the termination system. Usually the SCSI controller does this, unless it's not a terminated device. Most SCSI hard drives provide a TERMPWR jumper option, and they can have several options. The one to pick is "TERMPWR to SCSI bus."