The maximum supported cable length depends on what type of devices are attached (SCSI-1, SCSI-2, and so on). Device types are classified as follows:
SCSI-1: Maximum transfer rate of 5 MB/sec (one byte transfers)
SCSI-2 Fast: Maximum transfer rate of 10 MB/sec (one byte transfers)
SCSI-2 Fast Wide: Maximum transfer rate of 20 MB/sec (two byte transfers)
Ultra SCSI: Maximum transfer rate of 20 MB/sec (one byte transfers)
Ultra SCSI Wide: Maximum transfer rate of 40 MB/sec (two byte transfers) The maximum supported cable length for configurations without any SCSI-2 fast or UltraSCSI devices is six meters (approximately 20 feet).
The maximum supported cable length for configurations that include SCSI-2 fast, but not ultra-SCSI, is three meters (approximately 10 feet).
The maximum supported cable length for configurations of four or less Ultra SCSI devices is three meters (approximately 10 feet); for configurations larger than four Ultra SCSI devices, it is 1.5 meters (approximately five feet).
High-voltage differential (HVD)
The maximum supported cable length for high-voltage differential is 25 meters (approximately 80 feet).
Low-voltage differential/single-ended (LVD/SE)
If a single-ended device is attached to a low-voltage differential adapter the entire bus will operate as a single-ended bus, with the same cable length limitations. To operate in low-voltage differential mode, all devices on the bus must be low-voltage differential, including the terminator. High-voltage differential devices are not supported on a low-voltage differential bus and will prevent any devices on the bus from working and may damage the devices or adapter.
The maximum allowed cable length for point-to-point low-voltage differential SCSI bus configurations is 25 meters (approximately 80 feet). The maximum cable length for multi-drop low-voltage differential connections is 12 meters (approximately 40 feet).
Most IBM devices, cards, and cables will be marked with the kind of SCSI they can connect to. If there are no markings ensure that you are using compatible items. See Figure 5-1 on page 145 for labelling examples
Figure 5-1 Example of SCSI labelling
SCSI problems can be caused by either a device, cables, or a terminator. You can sometimes end up diagnosing a SCSI problem when the original call was, for example, a disk drive. There are a number of ways to diagnose the various faults, some of which are listed in the following sections.
5.7.1 Basic SCSI checks
Check the cabling thoroughly for the security of the cables connected to the devices; also, cross reference the cables used with the diagrams in the
publications RS/6000 Adapters, Devices, and Cable Information for Multiple Bus Systems, SA38-0516, and IBM RS/6000 Adapters, Device, and Cables
Inforamtion for Micro Channel Bus Systems, SA38-0533. In each of these publications, you will find wiring diagrams for many different device scenarios.
Each diagram will give you the feature code and part number of all components in the diagram; cross check with your setup, paying particular attention to the termination of the bus. In addition, make sure that all devices are compatible and that you do not have a mixture of differential and single-ended cables or
terminators. Also, check the documentation for your particular devices and adapters.
Next, run the following command:
# lsdev -Cs scsi
You should then get similar output to what is shown below:
rmt0 Available 11-08-00-0,0 SCSI 8mm Tape Drive
cd0 Available 11-08-00-1,0 16 Bit SCSI Multimedia CD-ROM Drive hdisk0 Available 11-08-00-2,0 16 Bit LVD SCSI Disk Drive
hdisk1 Available 11-08-00-4,0 16 Bit LVD SCSI Disk Drive hdisk2 Available 27-08-00-8,0 16 Bit LVD SCSI Disk Drive hdisk3 Available 27-08-00-9,0 16 Bit LVD SCSI Disk Drive hdisk4 Available 27-09-00-8,0 16 Bit LVD SCSI Disk Drive hdisk5 Available 27-09-00-9,0 16 Bit LVD SCSI Disk Drive ses0 Available 27-08-00-15,0 SCSI Enclosure Services Device ses1 Available 27-09-00-15,0 SCSI Enclosure Services Device
The output you get should show all of the SCSI devices listed in the as ODM both available and defined. Are all of the devices you would expect to see shown as available? If not, check that the device has the power turned on, check the cabling for bent pins, improper termination, such as additional terminators on the bus, or missing terminators. SCSI requires one terminator at each end of the bus. There is usually one on the adapter and one on the last device. Some SCSI devices have jumpers or switches on the device to provide termination with that device. Another jumper can provide Term Power. For IBM systems, Term Power is usually provided by the adapter and should be set to off at the device. If you find that you are having difficulty finding the problem, then, if possible, start with one device on the bus and keep adding devices until the fault occurs. However, neverhot plug onto a live SCSI bus; it has been known to seriously corrupt rootvg or customer data. The exception to this, of course, are hot plug SCSI disks available in machine types 7017, 7025, 7026, and later. The problem may also be with one of the devices added previously, but the cumulative effect of adding the last device has caused the symptom to show. Poor connections or bad/additional terminations often cause this difficult to diagnose condition, which may only give problems intermittently, such as during heavy workloads or only when data backups are running.
If you cannot get the SCSI bus to work even with just one device on the bus, then before replacing the adapter, refer to RS/6000 and pSeries Diagnostics
Informationfor Multiple Bus Systems, SA38-0509, or RS/6000 Diagnostics Information for Micro Channel Bus Systems, SA38-0532. These two documents contain procedures for testing the adapters PTC thermal cutout and a test for correct bus termination on some adapters.