Operational Power Considerations - IBM 1 Series Manual

Operational Power Considerations
2-48 GA34-0033
Refer to Figure 2-20. Each processor unit and I/O expansion unit on the
I/O channel has a self-contained power-on reset. These unit power-on
resets are not electrically connected among processor or I/O expansion
units.
For example, assume that a system consists of an IBM 4955 Processor,
without the repower feature (#1565), and an IBM 4959 I/O Expansion
Unit. If a loss of power occurs on the I/O expansion unit, outbound
isolation is not provided. The I/O channel is inoperative and errors may
occur.
When the channel repower feature is installed in a unit, the feature uses
the 'power onreset' signal from the unit immediately outbound ofit to
degate all inbound tags and buses to the processor whenever the outboard
unit is powered downor actively generating a 'power-on reset' signal. For
example, if power goes down on I/O expansion unit 2, the 'power-on
reset' signal from unit 2 is sent across the cable to the repower feature
plugged into unit 1. This causes the repower feature in unit 1 to degate
any inboundsignals from unit 2 and all other units outboard of unit 2.
The 'power-on reset' of the unit containing the repower card is used only
to initialize certain latches on the repower card; outboard units are not
directly affected by this signal. If I/O processing is taking place at the
time of the power loss, an error may occur. Indications of the errorare:
e An unexpected condition code to an Operate I/O instruction
e An TJ/O interrupt with a condition code of 2 or 6
e A machine-check class interrupt with I/O check on
e A machine-check class interrupt with I/O check and the Sequence
indicator on.
The error results because the processor may continue to send outbound
tags out, but inbound tags are degated. If the inbound tags are degated,
this may allow the channel to time-out and cause the machine check. An
indication of a power loss on the I/O channelis the reporting of condition
code 0 to Operate I/O instructions that are directed to devices in the
powered-down expansion unit or outboard from the powered-down unit.
In order to power the system up and down without I/O checks, the
following sequences must be observed:
Power-up: 1. All external devices, such as terminals, printers, disk
storage units, etc.
2. All 4959 I/O expansion units in any sequence.
3. The processor (if a two-channel switch system, power
up the B-processor prior to the A-processor).
Power-down: 1. The processor unit.
I/O expansion units in any order.
If it is necessary to power-down the I/O expansion unit, but leave the
processor powered-up without incurring I/O checks, press the system
Reset key before powering down the I/O expansion units. This procedure
is valid only if the channel repower feature is installed in the processor.