Two-channel communication systems

A safety control system comprising:

• a first control unit arranged to control a controlled system,
• a second control unit arranged to detect a fault with the controlled system and arranged to transmit messages wirelessly to the first control unit,
• wherein the second control unit comprises:
• a first controller arranged to detect a condition of the controlled system and output messages indicative of whether or not the condition has been detected;
• a transmitter arranged to transmit wirelessly to the first control unit;
• a second controller also arranged to detect said condition of the controlled system and output messages indicative of whether or not said condition has been detected;
• and
• a multiplexer arranged to connect each of the first and second controllers in turn to the transmitter so that messages from each of the first and second controllers can be transmitted to the first control unit;
• wherein each of the first and second controllers is arranged to perform a series of processing cycles, and in each of the processing cycles to:
• perform a check to determine whether the condition has occurred in the controlled system;
• and then to output at least one message relating to the results of the check;
• and
• each of the first and second controllers is arranged, when it has output the at least one message, to generate an output to the multiplexer, the output being arranged to cause the multiplexer to connect the other of the first and second controllers to the transmitter.