User's Manual

905U-K Radio Telemetry Module User Manual
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ELPRO Technologies Pty Ltd 2001 Page 14
If the battery is not replaced at the point of Low Voltage Alarm, at a point of 95% of the set
point value, the unit will be permenently put into ‘sleep’ mode to prevent damage from over
discharge to lead acid batteries.
2.3.6 Communications Failure
The 905U-K cannot provide an indication that its transmitted messages have not been
received successfully.
This indication is however available at the
receiving 905U
by using the 905U Output Reset
on Comms Fail function. To use this function, map an input that is not being used on the
905U-K to a spare output on the 905U. The unused input can be an internal input such as the
Low Battery status or Setpoint status, or even the analogue input as this can also be mapped
to a digital output. Configure the 905U-K so that the digital output at the 905U is normally on
- you can configure an input to output mapping to be inverted or direct.
If you configure a reset time to the 905U output, then this output will turn off if it has not
received an update message from the 905U-K within that time. The 905U 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 905U is 32
minutes, so the update time for the 905U-K 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 905U 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 905U to 10 minutes and the update time at the 905U-K to 4.5 minutes.
2.4 How to Design a Remote Monitoring System
2.4.1 Achieving reliable radio transmission
A system can theoretically have an unlimited number of 905U-K 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 905U-K and 905U modules - each can transmit input signals. The 905U
can "hear" other radio messages, and will hold off transmitting a message until the radio
channel is clear. The 905U-K 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
update times for each input. High priority inputs should have shorter update times than lower
priority inputs.
The peak transmission density should be calculated for large systems. These values are
calculated by determining the number of transmissions from inputs changing value and the
number of update transmissions per hour.
The probability of success for an individual message depends on the transmission density and
the number of re-transmissions for each message. This is shown in the following graph: