Manual

WI-I/O 9-x_WI-I/O-EX-1-S-x 2.16 Page 35
3.2.4 Design for Failures
All well designed systems consider system failure. I/O systems operating on a wire link will fail
eventually, and a radio system is the same. Failures could be short-term (interference on the radio
channel or power supply failure) or long-term (equipment failure).
The modules provide the following features for system failure :-
• Outputs can reset if they do not receive a message within a configured time. If an output should
receive an update or change message every 10 minutes, and it has not received a message within this
time, then some form of failure is likely. If the output is controlling some machinery, then it is good
design to switch off this equipment until communications has been re-established.
The modules provide a “drop outputs on comms fail” time. This is a configurable time value for
each output. If a message has not been received for this output within this time, then the output will
reset (off, in-active, “0”). We suggest that this reset time be a little more than twice the update time
of the input. It is possible to miss one update message because of short-term radio interference,
however if two successive update messages are missed, then long term failure is likely and the
output should be reset. For example, if the input update time is 3 minutes, set the output reset time
to 7 minutes.
• A module can provide an output which activates on communication failure to another module. This
can be used to provide an external alarm that there is a system fault.
3.2.5 Indicating a Communications Problem
There are two ways to provide an indication of communications problems.
Fail-to-transmit alarm. The first is to map the internal CF status to a local output, to generate a “fail-
to-transmit” alarm. The configured output will activate when a comms fail occurs - that is, when the
module attempts to transmit a message five times without an acknowledgement. This method provides
an indication immediately an attempt to transmit a message fails. If you want the radio path to be
“tested” regularly, then you need to configure the update times such that transmissions occur regularly
(however do not overload the radio channel).
Notes regarding this method:
1. Each CF mapping corresponds to only one remote address - you need to make separate mappings
for each remote address. You can map the CF for each remote module to a separate output, or to
the same output.
2. You need to reset the comms fail output using the “reset output” parameter. Select a reset time
which is greater than the effective update time period. For example, if there are four inputs
mapped from module #1 to module #2, each with a 10 minute update, then you would expect at
least four transmissions in each 10 minute period. At module #1, a comms fail for #2 is mapped to
DO1. If you set the “reset time” for DO1 to 10 minutes, then there will be at least four
transmissions made during the reset period - that is, the output will only reset when the
communications has been successful four times.
3. This method will not work for radio links with repeaters. If a repeater is used, you will need to use
the second method described below.