Bently Nevada 3300/01 Series, Operation Manual

Page: 55 / 78

System Monitor
80175-01
16 PROTOCOL SPECIFICATIONS
BACKPLANE DATA SCAN RATES
ACTIVE MODE
When there is no Communications Processor connected to the monitor rack, the Serial Interface
must control scanning o f the 3300 rack backplane t o acquire direct and status values. When
operating in this mode the interface data base is completely updated for an entire rack in 120
msec.
PASSIVE MODE
When a Communications Processor (DDM or TDM) is connected to the 3300 rack and the Cp is
configured, then the interface must release control of the backplane mux control signals and the
interface will acquire its data by observing the mux signals as the Communications Processor
controls them. This means that while operating i n this mode, o n l y those monitors t h a t are
configured i n t h e D a t a Manager a r e scanned. A l s o t h e s c a n r a t e i s controlled b y t h e
Communications Processor.
When a Transient Data Manager (TDM) is connected and configured for a full rack of 12 dual
channel monitors, the scan rate for the status values (Alert, Danger, and not OK) is 83 msec. Since
the TDM Communications Processor scans direct values only once every four seconds, t h e
interface data base update rate for direct values is every four seconds. This means that the direct
data returned over the serial communications link may be up to four seconds old. On the other
hand, status data is guaranteed to be no more than 83 msec old. Data scan rates do not affect
the response rate for communications messages.
When a Dynamic Data Manager (DDM) is connected and configured for a full rack o f 12 dual
channel monitors, the scan rate for both status and direct values is a minimum of 120 msec.
In both the TDM and DDM, static data collection has a lower priority to other more important tasks
which may have to be performed. Servicing of alarms and receiving communications messages
are examples of higher priority tasks. Scan rates may be lengthened if higher priority tasks are
very active.
48