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| Home Principle of Operation Streetlight Monitor Street Light Monitoring System Emergency Light Monitoring System |
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| Principals of Operations |
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The principal of operation is that each Slave not only receives commands from the Master, but passes them on; the same happens with the returned response. Provided at least one Slave is within range of another, a chain of communication is established throughout the network. It does not rely on line of sight nor does the slave need to be within range of the master unit; as long as there is at least one slave within range, messages will get through. If a slave receives a message that is not for itself it will re-transmit it into the network. When the master unit receives the message it will acknowledge it. The slave will retain the message and periodically re-transmit it until it has an acknowledgement from the master unit.
The Master-Slave radio link uses licence-exempt 433MHz transceivers that can bi-directionally transfer serial data over a range of up to 250 metres Line Of Sight (LOS). The transceiver operates on the Pan-European 433MHz frequency band from a 3.6V supply. These are type-approved modules with CE approval. There is no requirement for licensing.
Communication is digital, a packet structure transferring the data securely, with in-built error detection. Each Slave has an "address" which is entered on a database at the time of installation, so the Host is able to determine the location and type of Slave.
The message protocol is "self-healing", in that interference or corruption of data will delay, but not prevent, communication.
It uses a sophisticated Cyclic redundancy checksum (as used by B.T. in their packet switching networks) to offer a high degree of protection for the data.
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The Master module is normally mains-powered (230vac, 20VA) and may be situated in a remote equipment, within a building etc. as appropriate. Normally the serial version would suffice where a single building is being monitored but the GSM version would be required to monitor a number of sites or for remote communications. The master unit can be network enabled with its own IP address by means of an additional Serial-to-Ethernet Device Server.
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The master unit can control up to 1024 slave units.
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Slaveunits are installed in or near the devices they monitor or control; examples of such devices are street lamps, emergency lighting, fridges pumps, compressors etc. A slave monitors the device and when it detects a fault condition it communicates this to the master via its internal radio frequency (RF) transmitter. |
Slave specification depends upon function. Emergency Lamp monitors would normally fit within, or on, the lamp enclosure, and again monitor lamp current. In essence, the Slave detects either current or voltage, to suit the equipment being monitored. Usually, the Slave would be mains-powered (230vac, 5VA).
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In addition to monitoring the equipment operation, a self-test function is included. When commanded by the Master, the Slave carries out a self-test. This will usually involve switching the power to the equipment off, then testing the monitored parameter (eg. lamp current) and then reporting the result, pass or fail. This therefore tests the communication link, and the monitoring capability.
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