SCSI service hints

Use one or more of the following procedures when servicing SCSI adapter or devices.

General SCSI configuration checks

With any type of SCSI problem, begin with the following steps:

Verify that all SCSI devices on the SCSI bus have a unique address.
Verify that all cables are connected securely and that there is proper termination at both ends of the SCSI bus.
Verify that the cabling configuration does not exceed the maximum cable length for the adapter in use.
Verify that the adapters and devices that you are working with are at the appropriate microcode levels for the customer situation. If you need assistance with microcode issues, contact your service support structure.
If there are multiple SCSI adapters on the SCSI bus, verify that the customer is using the appropriate software to support such an arrangement. If the correct software is not in use, some SCSI errors should be expected when multiple adapters attempt to access the same SCSI device. Also, each adapter should have a unique address.

High availability or multiple SCSI system checks

If you have a high-availability configuration, or if more than one system is attached to the same SCSI bus, do the following:

Verify that the adapters and devices have unique SCSI addresses. The default SCSI adapter address is always 7. If you have more than one adapter on the bus, change the address of at least one by using SMIT (SMIT Devices > SCSI Adapter > Change/Show characteristics of an adapter). You must make the changes to the database only, then reboot the system in order for the change to take effect.
Note: Diagnostics defaults to using ID 7 (do not use this ID in high availability configurations).
If RAID devices such as the 7135 or 7137 are attached, run the appropriate diagnostics for the device. If problems occur, contact your service support structure for assistance. If the diagnostics are run incorrectly on these devices, misleading SRNs can result.
Diagnostics cannot be run against OEM devices; doing so results in misleading SRNs.
Verify that all cables are connected securely and that both ends of the SCSI bus is terminated correctly.
Verify that the cabling configuration does not exceed the maximum cable length for the adapter in use. See the SCSI Cabling section in the RS/6000® eServer™ pSeries Adapters, Devices, and Cable Information for Multiple Bus Systems for more details on SCSI cabling issues.
Verify that the adapter and devices are at the appropriate microcode levels for the customer situation. If you need assistance with microcode issues, contact your service support structure.

SCSI-2 single-ended adapter PTC failure isolation procedure

Before replacing a SCSI-2 single-ended adapter, use these procedures to determine if a short-circuit condition exists on the SCSI bus. The same positive temperature coefficient (PTC) resistor is used for both the internal and external buses. The PTC protects the SCSI bus from high currents due to short-circuiting on the cable, terminator, or device. It is unlikely that the PTC can be tripped by a defective adapter. Unless instructed to do so by these procedures, do not replace the adapter because of a tripped PTC resistor.

A fault (short-circuit) causes an increase in PTC resistance and temperature. The increase in resistance causes the PTC to halt current flow. The PTC returns to a low resistive and low temperature state when the fault is removed from the SCSI bus or when the system is turned off. Wait 5 minutes for the PTC resistor to fully cool, then retest.

These procedures determine if the PTC resistor is still tripped and then determine if there is a short-circuit somewhere on the SCSI bus.

Determining where to start

Use the following steps to determine the adapter configuration and select the appropriate procedure:

If there are external cables attached to the adapter, start with the External Bus PTC Isolation Procedure for your type of adapter.
If there are no external cables attached, start with the Internal SCSI-2 single-ended bus PTC isolation procedure.
If there is a combination of external and internal cables start with the External Bus PTC Isolation Procedure for your type of adapter. If this procedure does not resolve the problem, continue with the Internal Bus PTC Isolation Procedure for your type of adapter.

External SCSI-2 single-ended bus PTC isolation procedure

Isolate the external SCSI bus PTC fault with the following procedure:

Note: The external bus is of single-ended design.

Ensure the system power and all externally attached device power is turned off. All testing is accomplished with the power off.
Disconnect any internal and external cables from the adapter and remove the adapter from the system.
Verify with a digital ohmmeter that the internal PTC resistor, labeled Z1, (see the illustration after Internal SCSI-2 Single-Ended Bus PTC Isolation Procedure, step 3) is cool and in a low resistance state, typically less than 1/2 Ohm. Measuring across, be sure to probe both sides of the PTC where the solder joints and board come together. The polarity of the test leads is not important. If necessary, allow the PTC resistor to cool and measure again.
Locate Capacitor C1 and measure the resistance across it by using the following procedure:
1. Connect the positive lead to the side of the capacitor where the + is indicated on the board near C1. Be sure to probe at the solder joint where the capacitor and board come together.
2. Connect the negative lead to the opposite side of the capacitor marked GND. Be sure to probe at the solder joint where the capacitor and board come together.
3. If there is no short-circuit present, then the resistance reading is high, typically hundreds of Ohms.
Note: Because this is a measurement across unpowered silicon devices, the reading is a function of the ohmmeter used.
- If there is a fault, the resistance reading is low, typically below 10 Ohms. Because there are no cables attached, the fault is on the adapter. Replace the adapter.
  Note: Some multi-function meters label the leads specifically for voltage measurements. When using this type of meter to measure resistance, the plus lead and negative lead my not be labeled correctly. If you are not sure that your meter leads accurately reflect the polarity for measuring resistance, repeat this step with the leads reversed. If the short-circuit is not indicated with the leads reversed, the SCSI bus is not faulted (short-circuited).
- If the resistance measured was high, proceed to the next step.
Reattach the external cable to the adapter, then do the following:
1. Measure across C1 as previously described.
2. If the resistance is still high, in this case above 10 Ohms, then there is no apparent cause for a PTC failure from this bus. If there are internal cables attached, continue to the Internal SCSI-2 single-ended bus PTC isolation procedure.
3. If the resistance is less than 10 Ohms, there is a possibility of a fault on the external SCSI bus. Troubleshoot the external SCSI bus by disconnecting devices and terminators. Measure across C1 to determine whether the fault has been removed. Replace the failing component. Go to Verifying a repair.

External SCSI-2 single-ended bus probable tripped PTC causes

The following list provides some suggestions of things to check when the PTC is tripped:

A short-circuited terminator or cable. Check for bent pins on each connector and removable terminator.
Intermittent PTC failures can be caused by improperly seated cable connectors. Reseat the connector and flex the cable in an attempt to duplicate the fault condition across C1.
Plugging or unplugging a cable or terminator while the system is turned on (hot plugging).
A short-circuited device.
Differential devices or terminators are attached to the single-ended SCSI bus.
Note: The SCSI-2 Fast/Wide and Ultra PCI Adapters use an onboard electronic terminator on the external SCSI bus. When power is removed from the adapter, as in the case of this procedure, the terminator goes to a high impedance state and the resistance measured cannot be verified, other than it is high. Some external terminators use an electronic terminator, which also goes to a high impedance state when power is removed. Therefore, this procedure is designed to find a short-circuited or low resistance fault as opposed to the presence of a terminator or a missing terminator.

Internal SCSI-2 single-ended bus PTC isolation procedure

Isolate the internal SCSI bus PTC resistor fault with the following procedure:

Note: The internal bus is single-ended.

Ensure that system power and all externally attached device power is turned off.
Disconnect any internal and external cables from the adapter, then remove the adapter from the system.
Verify with a digital ohmmeter, that the internal PTC resistor, labeled Z1, is cool and in a low resistance state, typically less than 1/2 Ohm. Measuring across, be sure to probe both sides of the PTC where the solder joints and board come together. The polarity of the test leads is not important. If necessary, allow the PTC to cool and measure again. See the following illustration.

Note: Ensure that only the probe tips are touching the solder joints. Do not allow the probes to touch any other part of the component.
Locate capacitor C1 and measure the resistance across it using the following procedure:
1. Connect the positive lead to the side of the capacitor where the + is indicated. Be sure to probe at the solder joint where the capacitor and board come together.
2. Connect the negative lead to the opposite side of the capacitor. Be sure to probe at the solder joint where the capacitor and board come together.
3. If there is no short-circuit present, the resistance reading is high, typically hundreds of Ohms.
Note: Because this is a measurement across unpowered silicon devices, the reading is a function of the ohmmeter used.
- If there is a fault, the resistance reading is low, typically below 10 Ohms. Because there are no cables attached, the fault is on the adapter. Replace the adapter.
  Note: Some multi-function meters label the leads specifically for voltage measurements. When using this type of meter to measure resistance, the plus lead and negative lead my not be labeled correctly. If you are not sure that your meter leads accurately reflect the polarity for measuring resistance, repeat this step with the leads reversed. Polarity is important in this measurement to prevent forward-biasing diodes, which lead to a false low resistance reading. If the short circuit is not indicated with the leads reversed, the SCSI bus is not faulted (short-circuited).
- If the resistance is high and there is no internal cable to reattach, there is no apparent cause for the PTC resistor diagnostic failure.
- If the resistance is high and there is an internal cable to reattach, proceed to the next step.
Reattach the internal cable to the adapter, then do the following:
1. Measure across C1 as described previously.
2. If the resistance is still high, above 25 Ohms, there is no apparent cause for a PTC failure.
3. If the resistance is less than 10 Ohms, a fault on the internal SCSI bus is possible. Troubleshoot the internal SCSI bus by disconnecting devices and terminators. Measure across C1 to determine if the fault has been removed.
  Note: Some internal cables have nonremovable terminators.

Internal SCSI-2 single-ended bus probable tripped PTC resistor causes

The following list provides some suggestions of things to check when the PTC is tripped:

A short-circuited terminator or cable. Check for bent pins on each connector and removable terminator.
Intermittent PTC failures can be caused by incorrectly seated cable connectors. Reseat the connector and flex the cable in an attempt to duplicate the fault condition across C1.
A short-circuited device.
On some systems, the terminator is fixed to the internal cable and cannot be removed. If all devices are removed from the cable and the resistance is still low, then the cable should be replaced.
Note: The SCSI-2 Fast/Wide and Ultra PCI adapters use an onboard electronic terminator on the internal SCSI bus. When power is removed from the adapter, as in the case of this procedure, the terminator goes to a high impedance state and the resistance measured cannot be verified, other than it is high. Some internal terminators use an electronic terminator, which also goes to a high impedance state when power is removed. Therefore, this procedure is designed to find a short-circuit or low resistance fault as opposed to the presence of a terminator or a missing terminator.

SCSI-2 differential adapter PTC failure isolation procedure

Use this procedure when SRN xxx-240 or xxx-800 has been indicated.

The differential adapter can be identified by the 4-B or 4-L on the external bracket plate.

Before replacing a SCSI-2 differential adapter, use these procedures to determine if a short-circuit condition exists on the SCSI Bus. The PTC protects the SCSI bus from high currents due to short-circuits on the cable, terminator, or device. It is unlikely that the PTC can be tripped by a defective adapter. Unless instructed to do so by these procedures, do not replace the adapter because of a tripped PTC resistor.

These procedures determine if the PTC resistor is still tripped and then determine if there is a short-circuit somewhere on the SCSI bus.

External SCSI-2 differential adapter bus PTC isolation procedure

Isolate the external SCSI bus PTC fault with the following procedure:

Notes:

Ensure that only the probe tips are touching the solder joints. Do not allow the probes to touch any other part of the component.
The external bus is differential.

Ensure that system power and all externally attached device power is turned off.
Check to ensure all devices are marked SCSI Differential and that the terminator on the end of the SCSI bus is also marked differential. If not, you may have a single-ended SCSI device or terminator on the differential SCSI bus. Single-ended devices do not work on a differential SCSI bus and may cause a PTC type error to be reported. The entire SCSI bus may appear to be intermittent. After ensuring the system is completely differential, continue.
Disconnect the external cables from the adapter and remove the adapter from the system.
Verify with a digital ohmmeter that the internal PTC resistor, labeled Z1, is cool and in a low resistance state, typically less than 1/2 Ohm. See the following illustration. Measuring across, be sure to probe both sides of the PTC resistor where the solder joints and board come together. The polarity of the test leads is not important. If necessary, allow the PTC resistor to cool and measure again.
Locate capacitor C1 and measure the resistance across it using the following procedure:
1. Connect the negative lead to the side of the capacitor marked GND. Be sure to probe at the solder joint where the capacitor and board come together.
2. Connect the positive lead to the side of the capacitor marked Cathode D1 on the board near C1. Be sure to probe at the solder joint where the capacitor and board come together.
  - If there is no fault present, then the resistance reading is 25 to 35 Ohms. The adapter is not faulty. Continue to the next step.
  - If the resistance measured is higher than 35 Ohms, check to see if RN1, RN2, and RN3 are plugged into their sockets. If these sockets are empty, you are working with a Multi-Initiators or High-Availability system. With these sockets empty, a resistive reading across C1 cannot be verified other than it measures a high resistance (not a short-circuit). If the resistance measurement is not low enough to be suspected as a fault (lower than 10 Ohms), continue to the next step.
  - If the resistance is high and there is no external cable to reattach, there is no apparent cause for the PTC diagnostic failure.
  - If the resistance reading is low, typically below 10 Ohms, there is a fault. Because there are no cables attached, the fault is on the adapter. Replace the adapter.
  - If the resistance measured was high and there is an external cable to reattach, proceed to the next step.
Reattach the external cable to the adapter.
1. Measure across C1 as previously described.
2. If the resistance is between 10 to 20 Ohms, there is no apparent cause for a PTC resistor failure.
3. If the resistance is less than 10 Ohms, there is a possibility of a fault on the external SCSI bus. Troubleshoot the external SCSI bus by disconnecting devices and terminators. Measure across C1 to determine if the fault has been removed.

SCSI-2 differential adapter probable tripped PTC causes

The following list provides some suggestions of things to check when the PTC is tripped:

A short-circuited terminator or cable. Check for bent pins on each connector and removable terminator.
Intermittent PTC failures can be caused by incorrectly seated cable connectors. Reseat the connector and flex the cable in an attempt to duplicate the fault condition across C1.
Plugging or unplugging a cable or terminator while the system is turned on (hot-plugging).
A short-circuited device.
Single-ended devices are attached to the differential SCSI bus.

Dual-channel ultra SCSI adapter PTC failure isolation procedure

Use the following procedures if diagnostics testing indicates a potential positive temperature coefficient (PTC) resistor fault or the TERMPWR short-circuited LED is lit.

This procedure is used for SRNs 637-240 and 637-800 on the Dual-Channel Ultra SCSI Adapter. If the TERMPWR short-circuited LED is lit, use this procedure to help isolate the source of the problem on the failing channel.

Identify the adapter by its label of 4-R on the external bracket. Then, determine if the failure is on channel A or channel B.
The same PTC is used for both the internal and external buses. The PTC protects the SCSI bus from high currents due to short-circuits on the cable, terminator, or device. It is unlikely that the PTC can be tripped by a defective adapter. A fault (short-circuit) causes an increase in PTC resistance and temperature. The increase in resistance causes the PTC to halt current flow. The PTC returns to a low resistive and low temperature state when the fault is removed from the SCSI bus or when the system is turned off.
Wait 5 minutes for the PTC resistor to fully cool, then retest.
If this same error persists, or the TERMPWR short-circuited LED is lit, replace the components of the failing channel in the following order (wait five minutes between steps):
1. If the failure is on the external cable, replace the following:
  1. Cable
  2. Device
  3. Attached subsystem
  4. Adapter
2. If the failure is on the internal cable, replace the following:
  1. Cable
  2. Device
  3. Backplane
  4. Adapter
3. If the failure persists, verify that the parts exchanged are in the correct channel (internal or external, A or B).

64-bit PCI-X dual channel SCSI adapter PTC failure isolation procedure

Use the following procedures if diagnostics testing indicates a potential self-resetting thermal fuse problem. This procedure is used for SRN 2524-702 on the integrated dual-channel SCSI adapter in a 7039/651 system.

Identify the adapter as the one embedded in the system board. Then, determine if the failure is on channel 0 or channel 1.
The thermal fuse protects the SCSI bus from high currents due to short-circuits on the terminator, cable, or device. It is unlikely that the thermal fuse can be tripped by a defective adapter. A fault (short-circuit) causes an increase in resistance and temperature of the thermal fuse. The increase in temperature causes the thermal fuse to halt current flow. The thermal fuse returns to a low resistive and low temperature state when the fault is removed from the SCSI bus or when the system is turned off.
Wait 10 seconds for the thermal fuse to reset itself and recover, then retest.
If the same error persists, replace the components of the failing channel in the following order. Wait 10 seconds for the thermal fuse to reset itself between steps.
1. Cable
2. Device
3. DASD backplane (if present)
4. System board (adapter)
If the failure persists, verify that the parts exchanged are in the correct channel (0 or 1). If the errors are still occurring, contact your next level of support.

Last updated: Wed, June 19, 2019