Wireless Profibus

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In automation technology, wireless Profibus refers to the wireless transmission of data packets from the Profibus fieldbus via a radio interface (instead of the usual transmission via a cable interface).

Technically speaking, the radio link behaves like a cable. With Profibus DP, a Profibus master such as a programmable logic controller (PLC), a programming device or an operator panel transmits the data cyclically to connected Profisbus slaves.

The technical challenge in radio transmission is that the Profibus data is transmitted on the cable interface at 9.6kbit / s to 12 Mbit / s. Exact adherence to the timing is essential for trouble-free operation. With a simple transmission of the RS485 level, radio links can not guarantee that the exact timing is adhered to. The principle of keying the RS485 physics to another medium only works with a light barrier .

There are three technical solutions for the Profibus radio transmission.

Radio modem as a gateway

In approach 1, the Profibus telegrams are not transmitted via radio; there is a Profibus slave function in the radio.

The integration in the master takes place via a GSD file. In this description file is u. a. the data framework is defined. Only the net data is then exchanged via the radio channel. This approach has the disadvantage that the various monitoring mechanisms of Profibus transmission cannot be used. An interruption in the radio link does not lead to a bus error in which the connected subscribers recognize the interruption of the connection and can react according to a specified fault behavior. The advantage for the manufacturer is a simple technical implementation in that the timing is decoupled from the radio link.

Transparent radio transmission with changes in timing

In approach 2, the cyclical Profibus transmission is slowed down considerably by inserting long pauses after a telegram until the next telegram.

This enables a much longer response time. With this approach, the Profibus can be operated from 9.6 kbit / s to 93.75 kbit / s. For this purpose, the TSLOT time is increased to the maximum value of 16000 bit times. However, no wired participants can be connected to the same master that also operates a radio link. In practice, a data packet can still be transmitted cyclically every 150 ms.

For this variant, the use of very simple, cheap radio modules is possible. However, interruptions in the radio link of 100 ms are sufficient to trigger a Profibus error on the master.

Transparent radio transmission without changing the timing

The third approach is to accept the data on the radio module with a very high data rate at the cable interface, to remove the cyclical redundant data and only to transfer the changes via radio. This makes it possible to operate the Profibus master at a very high data rate on the cable interface in such a way that the radio link functions like a cable without changing the settings in the Profibus master compared to a cable interface. All Profibus diagnostic functions are retained and any Profibus master and slaves can be coupled via the radio link.

The advantage for the user is that the PLC is programmed like a cable connection, all Profibus functions are retained and no Profibus errors are triggered in the event of timing fluctuations in the radio link. A time monitoring function in the radio system only triggers the interruption after a freely parameterizable time, comparable to a cable break.

Individual evidence

  1. ↑ EloktronikPraxis: Profibus DP: More than a replacement (describes solutions for Wireless Profibus)

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