505U-K Radio Telemetry Module
Page 16
© April 2002
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
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:
