IBM 4381 Manual page 120

Functional Operation
To further improve reliability, all power connectors used in 4381 Processors are the
positive retention type (fewer than half of those in a 4341 Processor have positive
retention). To reduce field upgrade times, all the power supplies required for a
43 81 Processor are part of the basic 43 81 Processor. Significant functional
improvements that will aid serviceability have also been made for 4381 Processors,
as discussed below.
Once the support processor is powered on via microprocessor-controlled
sequencing, the power-on sequence for the rest of the 4381 Processor system is
microcode-controlled and handled by the support processor. The hardwired power
sequencing in 4381 Processors is controlled by a single-card maintenance bias
controller (MBC) instead of a controller implemented on one board with five cards,
as in 4341 Processors. The MBC improves reliability, reduces cost, and aids
serviceability by providing reference codes that are used to automatically isolate
many of the faults that can occur during initial power-on sequencing of the support
processor. In 4341 Processors, such fault isolation has to be done via manual
probing.
In addition, the switching power regulators in 4381 Processors have internal
sensors to detect the presence and level of the externally supplied bias voltages. If
a bias voltage is out of specification or missing, a digital status line to the MBC is
activated and the power microcode receives control to generate the appropriate
reference code. This facility improves fault detection capability for the case in
which intermittent errors are caused by cabling.
After the support processor power sequencing is completed, the power diagnostic
program is executed as part of the power-on procedure to test the operation of the
power controller adapter. If an error is found during these tests, the power-on
procedure is terminated and a reference code that identifies the power failure is
displayed on the operator console.
During the IML procedure, the power feature table is generated based on the 4381
Processor configuration specified on functional diskette 1. The table contains
configuration-dependent power sensor and control data information in the form of
strings and masks. This table is used by the power monitor program.
The power monitor program is resident in storage of the support processor during
system operation. Once a power-on is successfully completed, the power monitor
program is executed periodically.
The power monitor program reads all the analog and digital sense points to
determine whether any power or thermal fault conditions exist. The sense data
read is compared against the mask in the power feature table. If the comparison
indicates no fault exists, execution of the power monitor program is terminated. If
the comparison detects a fault, the power monitor program rereads all the sense
points and repeats the comparison to suppress spurious faults. If the readings still
show a fault, the power monitor program determines whether the readings indicate
power or thermal conditions are critical enough to warrant a power-down of the
processor. If so, a reference code is generated, a power logout record is written to
functional diskette 1 that includes the reference code, the reference code is
displayed on the operator console, and a power-down of the processor is initiated.
Neither the operating system nor the operator is notified that a power-down is to
Section 60: Reliability, Availability, and Serviceability (RAS)
111