What else is there under the hood?
Apart from very high data transfer reliability, the protocol adds several more useful features to communication between the transmitter and the receiver which broadens the range of possible applications. These are:
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1024 unique addresses available for identifying each transmitter-receiver pair of chips. These addresses are being set during manufacturing process and they are not user changeable. Although 1024 might not seem as a particularly large number, it is more than enough to enable tenths of pairs of chips to operate in the same area without interfering with each other. For example, several junior RC enthusiasts will be able to operate their robots on the same school playground without a fight.
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Each transmitter can be configured to work either inside a hendheld remote control unit in which case it spends most of the time in standby mode emitting a comand code only after a button press, or as a sensor monitor in which case it continually emmits data sampled from up to eight attached on/of sensors (i.e. switches). The first mode is suitable to be used in any kind of device that should be operated remotely while the second one is suitable for using transmitter-receiver pairs as components of alarm- and automatic control systems. The mode of operation is set using a single pin on both transmitter and receiver chip.
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There are 256 possible comands that can be transmitted and received. Of them, 16 are most commonly used if transmitter is configured to work inside a hendheld remote control unit. In that case, a pair of buttons at the transmitting side is used to switch each one of 8 output pins in the receiver on and off. There are as well other possible variants of interpreting comands.
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If transmitter is configured so that it continually sends data from the sensing switches, their states are copied on the receiving side in pin-for-pin basis. This means that each one out of 8 output pins on the receiving chip corresponds to a particular one out of 8 input pins on the transmitting chip. These eight bit states are updated simoultaneously approximately 12 times a second. This mode of operation is very convenient for forming arrays of remotely monitored signalling points in alarm systems, home automation systems etc. In this case, a pair of chips forms a bridge between sensors inside the area of interest and the central processing unit. From the standpoint of the central unit, the bridge behaves as if all sensors were directly connected to its own inputs.
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A pair of chips can be configured to use several types of data carrying media, such as copper wires or coaxial cables, IR or visible light, ultrasound, analog telephone lines, unmodulated radio channels or voice communication channels. These modes are set by several pins on both the transmiting and the receiving chip, which influences temporal and frequency content of generated pulses. For example, when using copper wires, it is optimal to transmit bits as short PCM rectangular pulses, whereas when using IR light, it is better to use pulses modulated at a carrier frequency of 38kHz which permits usage of inexpensive integrated IR detectors such as TSOP34838 on the receiving side. When using radio or ultrasonic communication equippment to carry data, there is a possibility to generate long "wake-up" pulses prior to transmitting actual data, the purpose of which is to activate squelch circuits in complex receivers.
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Transmitting chip can be connected to a local receiver that monitors the communications channel in order not to start emitting commands until the channel becomes free. Transmitting chip remembers the last command input by the user and only after the communications channel becomes free to use, will it start emitting data. This feature enables forming of large networks of several transmitters and receivers in which no transmitter can interfere with others, while at the same time no receiver is reacting to commands from transmitters other than the one matching its own address. In additon, it is possible to give the same address to several transmitters and/or receivers so that they form a cluster in which each such transmitter is able to issue commands to all corresponding receivers. For example, a single hendheld remote control unit or command post is able to control several distant devices, there can be more than one such unit in the same cluster, and there could be several mutually non-interfering clusters of devices at the same area sharing the same communications channel.
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Receivers can be configured so that they remember previously set states of output pins, so that they return each of their pins to its previous state after the receiver is being turned off and then on, for example in case of a power blackout. This option is obviously valid only if the corresponding transmitter was configured to work inside a hendheld remote control unit as otherwise states of sensing switches are being updated continually anyway. This feature can be useful in some devices but not so useful in others - for example it might not be clever to have a TV set automatically turning on after plugging its power supply cord into the outlet. It is up to a device designer to figure out what might be the optimal choice. Similarly to other optional functions, this one can also be set using a single pin on the receiving chip.
The next two pages give a preview of several possible applications of receiver-transmitter pairs. Diagrams provided point out the appropriate pin configuration for each of these situations.
