Chapter One
Introduction
MAN_105G_1.16
Page 26
When this feature is configured, all output transmissions are stopped if communications with
a remote module fails for a short period. They will start again when an input message from
this module is received. If the 105G determines that a output message should be sent to an
output which is disabled because of this feature, then the output message will not be sent and
the comms fail status of that output is set ("on" or 1).
If it is desired to use this function with a remote 105U module, but there are no inputs from
this module being used, then it is easy to configure an unused input or an internal input (mains
fail or low battery voltage etc). It is the comms fail status for the input, which is used, not the
input itself.
2.6.1
Monitoring Communications Failure
The host device can monitor the communications status of an I/O point by reading the status
register for this point as a binary/discrete register. Modbus, and many other protocols, will
convert a 16 bit register value to a binary/discrete value by returning the most significant bit -
for the status register, this corresponds to the comms status bit.
For example, to monitor the comms status of I/O register 1045, perform a binary/discrete read
on register 6045 (the status register for 1045). A value of “1” will be returned if this I/O point
is in comms fail, and a “0” returned if the status is normal.
If it is desired to monitor the comms status of all I/O points, it is more efficient to only
monitor the comms status of one I/O point at each remote module (if this point is in comms
fail, then all points at the remote module will be in comms fail). If this point is an input, then
the comms fail time for this input can be made short, to give an early warning of a comms
problem (this means that the corresponding update time for the input at the 105U will need to
be short). If the point is an output, then the update time for the output should be made short.
2.7
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 acknowledgment
and re-try protocol ensures that the transmitting module is aware whether the transmitted
message has been transmitted reliably. The “comms fail” 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 105G
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