User manual

905K Radio Telemetry Module User Manual
 ELPRO Technologies Pty Ltd 2010 Page 18
spare output on the 905U. The unused input can be an internal input such as the Low Voltage
status or Setpoint status, or even the analog input as this can also be mapped to a digital output.
Configure the 905K 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 905K 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 905K 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 905K 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 905K 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 905K 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 905K 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: