Weidmuller WI-I/O 9-K: Wireless I/O Transmitter v1.9 - Manual (Page 18)

WI-I/O 9-K v1.9
Page 18
This indication is however available at the receiving WI-I/O 9-x by using the WI-I/O 9-x Output
Reset on Comms Fail function. To use this function, map an input that is not being used on the
WI-I/O K to a spare output on the WI-I/O 9-x. 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 WI-I/O K so that the digital output at the WI-I/O 9-x is normally
on - you can configure an input to output mapping to be inverted or direct.
If you configure a reset time to the WI-I/O 9-x output, then this output will turn off if it has not
received an update message from the WI-I/O K within that time. The WI-I/O 9-x 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 WI-I/O 9-x is 32
minutes, so the update time for the WI-I/O 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 WI-I/O 9-x 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 WI-I/O 9-x to 10 minutes and the update time at the WI-I/O K 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 WI-I/O 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 WI-I/O K and WI-I/O 9-x modules - each can transmit input signals. The
WI-I/O 9-x can "hear" other radio messages, and will hold off transmitting a message until the
radio channel is clear. The WI-I/O 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