The single most common failure is of a hybrid line driver on a single SSP, or in the cable segment ancillary logic. This is a known problem with these line drivers which will hopefully be corrected by the next generation of line drivers. In the meantime, do the following.
Look at the cable segment display to see if any lights except the GA lines are lit; sometimes a bit or bits stuck on can be detected this way. The hybrid line drivers drive four bits, like 0, 4, 8, and 12 in the SSP. The pattern in the Cable Segment Ancillary Logic card is quite different-- see the table below.
Table 1: Cable Segment Ancillary Logic hybrid assignment. It is very
unlikely that you need to replace any of these.
Measure the differential voltage on the cable segment. There is a box with a 60 pin connector and a rotary switch which is usually under the FASTBUS test crate (at this writing between the CCD racks). Put the DVM on the 300 mV scale for accurate measurement.
Go to the SSP with the cable segment terminators (as of this writing, the ADC1 FASTBUS crate) and remove the terminators, and insert the 60 pin cable from the switch box. Each of the 30 pairs should measure about 225 mV, with excursions from about 200 to 250 mV. An exception to that rule is the SR (Service Request) line, which may be may be asserted (front panel lights on the SSP's), so SR will not have the nominal value; just hit the reset switch on any SSP with SR asserted on the cable. In all other cases, you should be measuring the ``off'' state of the line. If some line is out of tolerance, you need to search for the SSP which is driving the line.
Turn off the power to the SSP's one at a time and observe what happens to the differential voltage. If you are lucky, there will be one SSP which is holding down the offending line. If you aren't, you will have to identify the offending SSP by removing SSP's from the cable segment until you find the offending modules. See the section describing this tedious operation.