ATTO Technology Determining cabling and termination, Cable types, Examples, Connectors

3.1 Determining cabling and termination

Cables and devices must be chosen to maximize performance and minimize the electrical noise from the high-speed data transfers available with the SCSI protocol. Cabling and termination methods become important considerations for proper performance. SCSI cables and devices are subject to specific length and number limitations to deal with electrical problems that arise at increased operating speeds.

Cable types

Single-ended and differential SCSI host adapters use the same cables. Use high-quality cables rated for the type of SCSI transfers required: well- insulated SCSI cables ensure error free communications. Try to keep cable lengths as short as possible to ensure higher signal quality and performance.

Examples

The SCSI specification limits total bus cable length for Single-Ended SCSI in a non-UltraSCSI

environment to 3 meters (combined length of both internal and external cable lengths).

In an UltraSCSI workgroup environment with a 7- drive tower, you are limited to 1.5 meters between the host and the tower, including the cabling for the tower. If the 7-drive tower requires 1 meter of cabling to connect all of its drives, the distance from the tower to the host must be .5 meters.

NOTE UltraSCSI is very sensitive to SCSI bus noise, cable distances and the number of devices connected on the SCSI bus. Carefully connect your devices when working with UltraSCSI.

Exhibit 3.1-1 Various types of SCSI require different bus lengths to support a certain number of devices.

	Bus speed	Bus width	Max. bus lengths, meters			Max. device
STA terms	MB/sec. max.	bits				support
			Single-ended	Differential	LVD
Fast SCSI	10	8	3	25	12	8
Fast/WIDE SCSI	20	16	3	25	12	16
UltraSCSI	20	8	1.5	25	12	8
UltraSCSI	20	8	3	n/a	n/a	4
Ultra/WIDE SCSI	40	16	n/a	25	12	16
Ultra/WIDE SCSI	40	16	1.5	n/a	n/a	8
Ultra/WIDE SCSI	40	16	3	n/a	n/a	4
Ultra2 SCSI	80	16	n/a	n/a	12	8
Ultra2/WIDE SCSI	80	16	n/a	n/a	12	16

Connectors

ATTO ExpressPCI PSC: one industry standard 68- pin “P” (16-bit) and one 50-pin (8-bit) cable connector for internal device connections and one 68-pin “P” (16-bit) cable connector for external device connections.

ATTO ExpressPCI DC: two industry standard 68- pin “P” (16-bit) cable connectors for internal

device connections and two 68-pin “P” (16-bit) VHDCI (Very High Density Cabled Interconnect) cable connectors for external device connections.

NOTE When installing SCSI devices on the ATTO ExpressPCI PSC, use any two SCSI ports on your ATTO ExpressPCI SCSI adapter, but not all three. Use all four channels on the ATTO ExpressPCI DC.

7 ATTO Technology ExpressPCI Ultra/WIDE SCSI Host Adapters Installation and Operation Manual