Rockwell Automation 57C423 Common Memory Module - Manual (Page 11)

2Ć3
2.2 Electrical Description
The Common Memory module incorporates the bus arbitration logic
required when there are two or more Processor modules in a rack.
Note that bus arbitration logic is enabled only when the module is in
slot 0. The bus arbitration logic will support up to a maximum of four
Processors located in slots 1Ć4.
The bus arbiter resolves the contention problem that arises when
two or more Processors attempt to access the backplane bus at the
same time. Bus arbitration logic guarantees that every Processor
requesting the bus will be given a turn on the bus before any other
Processor can access it a second time. If two Processors attempt to
use the bus at the same time and neither one has previously
accessed the bus, the Processor module located in the
lowerĆnumbered slot will be permitted to access the bus first.
The superĆcapacitor on the Common Memory module can be
charged to more than 90% of its rated capacity in approximately 8
minutes. It is typically capable of retaining the contents of RAM
memory for approximately 400 minutes should the BAT.OK" light go
out and power is removed from the module. (Removing or replacing
the Common Memory module may affect tasks and variables in the
rack. Appendix E describes the effect on tasks and variables in a
DCS 5000 rack. Appendix F describes the effect on tasks and
variables in an AutoMax rack.)
The module also contains a watchdog timer that is used to detect
Processor failures in a multiĆProcessor system. If a Processor is
unable to reset the watchdog timer, the timer will generate an
interrupt to notify the other Processors in the rack of the failure. A
4.6" will appear on the faceplate of all Processor modules except
the one that timed out. The watchdog timer is enabled only when the
module is in slot 0.
PowerĆup diagnostics for this module are run by a Processor module.
Diagnostics are performed on the RAM memory, control registers,
and watchdog timers.