Chapter Three Operation
man_905-105_2.16 Page 35
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.
Fail-to-receive alarm. The second method is to set up a “comms OK” output using the “Reset
Outputs” function. The output is normally on, indicating “comms OK”, and will reset if the module
does not receive a message from the remote module within the configured reset time.
Consider a link between module #1 and #2, and assume that you want a “comms OK” output at #1. At
#2, map an unused input to an output at #1 such that the output is normally active (‘on”). If there is no
spare inputs at #2, you can use an internal input such as “low voltage status”. You will need to invert
the mappings such that the output is normally on (because the input is normally off).
At #1, configure a reset time for the output. The reset time should be greater than the update time for
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