User's Manual

Chapter Three Operation
man_905_2.1.doc Page 27
3.1.4 Start-up Poll
After a module has completed its initial diagnostics following power up, it will transmit update
messages to remote modules based on the values of the module’s inputs. The module’s outputs will
remain in the reset/off/zero condition until it receives update or “change-of-state” messages from the
remote modules.
The module can transmit a special “start-up poll” message to another module. The remote module
will then immediately send update messages to this module such that its outputs can be set to the
correct value. Start-up polls will only occur if they are configured. It is necessary to configure a
start-up poll to each remote module which controls the module’s outputs. For further information,
refer to Chapter 4 Configuration.
3.1.5 Communications Failure (CF)
The internal communications failure (CF) status is set if a module does not receive an
acknowledgement message after five attempts at transmitting a message. The CF status may be
configured to set a local digital output for an external alarm.
Although the CF status can set an output, it will not reset the output. That is, once communications
is re-established (and the CF status is reset), the output will stay “on”. The Reset Output feature
(see below) is used to reset the output.
The output will reset only when no communications failures occur within the configured “Reset
Output Time” for the output that CF status is mapped to. Note that if the reset output time is not
enabled, the CF status will remain set forever, once an unsuccessful transmission occurs. See
Chapter 4 Configuration for further details.
3.1.6 Resetting Outputs
Each digital and analogue output may be individually configured to reset if that output has not
received a change-of-state or an update message within a certain time period. Generally this time is
set to twice the update period, so at least one update can be missed before an output is reset.
In most cases it is desirable to reset outputs which are controlling equipment if there is a system
failure, however alarm or indication outputs are not reset so the last valid indication remains shown.
See Chapter 4 Configuration for further details.
3.2 System Design Tips
The following tips will help to ensure that your system operates reliably.
3.2.1 System Dynamics
It is important to be aware of the dynamics of the system. Digital inputs have a minimum
“debounce” delay of 0.5 sec - that is, a change message will not be sent for 0.5 sec after a change
has occurred. Analogue inputs and outputs have time delays of 1 to 2 seconds.