User's Manual

Appendix A System Example
man_905_2.1.doc Page 59
digital outputs. In the event of a system failure, the digital outputs will stay at their last correct
status, but the analogue outputs will reset to 0 mA.
System Failure Alarm
After the system had been running for some time, the operators wanted a "system failure" output at
the control station, to warn the operators that there was a fault with the system.
The following configuration was added :
At #2 (tank station), NOT DI4 → Out4 at 96 via 1 ; DI4 Update time = 1 minute
At #96 (control station), DO4 Comms fail reset time = 3.5 min
At the control station, DO4 was a "system OK" signal. It was normally active - if the signal reset,
then this represented a system failure. At the tank station, there is no signal wired to DI4. By
mapping NOT DI4 to DO4 at the control station, a message is transmitted every minute to this
output to activate it. The message is transmitted via the radio link to #1, and then by the serial link
to #96. If anything happened to either module #2 or module #1, or the radio link, or the serial link,
then the update messages for DO4 will not be received at the control station module. After 3.5
Minutes, DO4 will reset indicating a problem.
The time of 3.5 minutes was selected as this means that 3 successive update messages have to be
missed before a system alarm occurs. Also note, that if module #96 fails, DO4 will reset and give
an alarm signal.