User's Manual

Chapter Three Operation
man_905_2.0.doc Page 29
There are several ways of improving a marginal path :-
• Relocate the unit to a better position. If there is an obvious obstruction causing the problem,
then locating the unit to the side or higher will improve the path.
• If it is not practical to improve a marginal path, then the last method is to use another module as
a repeater. A repeater does not have to be between the two modules (although often it is). If
possible, use an existing module in the system which has good radio path to both modules. The
repeater module can be to the side of the two modules, or even behind one of the modules, if
the repeater module is installed at a high location (for example, a tower or mast). Repeater
modules can have their own I/O and act as a “normal” 905U module in the system.
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 905 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 905 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 905 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.