Westermo RM-505U-K - Manual (Page 14)

505K Radio Telemetry Module User Manual
    ELPRO Technologies Pty Ltd 2008 Page 14
The Supply Low Volts is treated as a digital signal and its value is transmitted according to
the rules for a digital input.
2.4.9 Supply Failure
The 505K will stop operating if the supply voltage falls below 95% of the Supply LOV value.
Prior to shutting down, the 505K will store the values of the pulse counters in non-volatile
memory. The unit will restart when the supply voltage has increased to the configured high SP
value.
2.4.10 Communications Failure
The 505K cannot provide an indication that its transmitted messages have not been received
successfully.
This indication is however available at the receiving 105U by using the 105U Output Reset
on Comms Fail function. To use this function, map an input that is not being used on the
505K to a spare output on the 105U. The unused input can be an internal input such as the
Low Voltage status or Setpoint status, or even the analogue input as this can also be mapped
to a digital output. Configure the 505K so that the digital output at the 105U is normally on -
you can configure an input to output mapping to be inverted or direct.
If you configure a reset time to the 105U output, then this output will turn off if it has not
received an update message from the 505K within that time. The 105U output is effectively a
"Communications OK" output - on when communications are OK, and off during
communications failure. Note that the maximum output reset time at the 105U is 32 minutes,
so the update time for the 505K input must be less than this. It is generally a good idea to set
the update time to less than half of the reset time. Then, the 105U must fail to receive two
consecutive update messages - it is possible to miss one update message because of random
noise, but two consecutive failures means that there is a system failure.
For example, if you wish to have a failure alarm within 10 minutes of a system failure, set the
output reset time at the 105U to 10 minutes and the update time at the 505K to 4.5 minutes.
2.5 How to Design a Remote Monitoring System
2.5.1 Achieving reliable radio transmission
A system can theoretically have an unlimited number of 505K modules, but in practice, the
number is limited by the amount of radio traffic on one frequency in the system. When a
radio channel becomes unreliable because of radio traffic, then a second radio channel must
be used to increase the size of the system. This limit is not a function of the number of
modules, but the number of radio messages.
A system comprises 505K and 105U modules - each can transmit input signals. The 105U
can "hear" other radio messages, and will hold off transmitting a message until the radio
channel is clear. The 505K module cannot, and there is a possibility that an individual
transmission will clash with another transmission, and both transmissions will be corrupted.
This possibility increases as the density of transmissions increases. Configuring the re-
transmit feature (transmission of each message several times) will increase the chance of each
message being received successfully, but will increase the overall density of radio traffic.
For large systems, a compromise is required between the number of re-transmissions, and the