905U Wireless I/O Module User Manual
105S Serial I/O Module
Page 36 © January 2011
the mapping at #2. If #1 fails to receive update messages from #2, then the output will reset,
indicating a communications failure. Notes regarding this method:
1. This method will work with repeaters in the link.
2. The “comms OK” output is fail-safe - if module #1 fails, then the output will reset indicating a
problem.
3. You should use separate outputs to indicate “comms OK” of different remote modules.
4. It is recommended that you set the reset time at #1 to more than twice the update time of the
mapping at #2. This means that the comms OK output will only reset if #1 misses two consecutive
updates from #2.
3.2.6 Testing and Commissioning
We recommend that you set-up and test the system with all of the modules together before you install
the modules. It is much easier to find a configuration problem.
When the system is configured, record the radio signal strength and background noise level for each
radio link. If there are future communications problems, you can compare the present measurements to
the as-commissioned values. This is an effective way of finding problems with antennas, cables and
also changes in the radio path (for example, the erection of new buildings).
3.3 Security Considerations
There are three dimensions of security considerations:
1. Failure to operate when required - or “operational reliability”.
The features discussed above optimize operating reliability. Using an acknowledgement and re-try
protocol ensures that the transmitting module is aware whether the transmitted message has been
transmitted reliably. The “fail to transmit” and “fail to receive” alarms provide indication if the
radio link has failed to operate.
2. Mal-operation, or operating when not requested.
This problem occurs when an output is “triggered” by the wrong radio device. The 905U modules
use frequency encoding and a very secure addressing system to ensure this does not occur. An
additional security level using data encryption can also be selected.
3. Malicious operation, or “hacking”
This is the problem most associated with security concerns - the ability for someone to access
information from a radio system by “listening-in”, or to cause damage by transmitting radio
messages to force outputs.
A security option can be selected during the module configuration to protect against this. The security
option (if selected) adds data encryption to radio messages. Modules in the same system are
automatically configured with the encryption key, such that only these modules can understand each
other. “Foreign” modules will hear the messages, but cannot decrypt the messages. For more
information, refer to section 4.3.7.
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