It was once commonly said that 'the AS-Interface does not work for distances greater than 100 m without a repeater' - which is also what the specification stated. In the future however, the limit will be between 200 and 300 m (depending on the circumstances).
The reason for the limitation of the network length is rooted in the principle of data transmission in the AS-Interface network. All the data between master and slave is transmitted as an analog signal, which is superimposed on the DC-supply voltage. As the transmitted current pulses are transformed to voltage pulses by an inductance at the receiving end, the condition of the detected signal is strongly influenced by the net impedance of the entire system. So, accordingly, all components are specified for only 100 m networks - allowing for any topology.
Tuning solution
Innomatic is offering two Bihl + Wiedemann solutions that can significantly extend the useable bus length. These include a fixed bus termination and an active 'AS-i Tuner', which adjusts the impedance of a network to the conditions specific to the network. Greater flexibility of the system is made possible by adjusting the impedance with a suitable bus termination. Generally, it is dependent on the kind and number of slaves and on the network topology.
Repeated trials have now shown that a fixed, passive bus termination can alter the network length limit without repeater significantly towards 200 m. Bihl + Wiedemann have taken the next logical step: besides the (passive) bus termination, the company also offers an (active) self-adjusting AS-i Tuner. In a teach-in-phase, the unit analyses the number of telegram repetitions and adjusts its own impedance so that the number of repetitions is minimised. Thus the optimal value can be achieved for each network. The network is 'individualised' by the AS-i Tuner so that the 100 m limit can be deliberately exceeded in a controlled manner. The tuner signals with three LEDs whether a satisfactory configuration has been reached.
Detailed trials have been unable to produce a network topology that would fail under full load with 31 certified slaves at network lengths of 200 m. Most topologies were expandable up to 250 m, some to 300 m. This substitutes the use of one or two repeaters or a complete redesign with more complex solutions. As repeaters can nevertheless be used, two slaves being installed 900 m apart with the master in their midst can be connected to one network (two repeaters and three tuners).
The controlled adjustment of the network impedance brings additional security for the availability of the network in which repetitions occur: in all critical applications the number of telegram repetitions which are triggered by impedance errors, can be minimised with the AS-i Tuner. The use of the AS-i Tuner is also interesting in all cases in which unexpected problems occur, eg, when using a non-AS-i specified cable, a malfunctioning slave, or when the network is extended after its initial installation. Picture shows the AS-i Tuner on the left, with the fixed bus termination on the right.
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