The RS485 input/output modules in the Fieldbus Box series use a simple, open serial communication protocol based on a master/slave architecture. It is quick and easy to implement for any serial interface, since only a few functions are required. Because of the low transmission rate, which has a maximum of 38.4 kbaud, this network is best employed in circumstances where low demands are placed on response time.
In an RS485 system, the bus consists of a master station and of a number of slave stations. The communication is controlled entirely by the master. The master requests the data from the slaves cyclically (polling). When data is exchanged with the bus nodes, the entire process image is always transmitted. In other words, the master sends all of the output data to the Fieldbus Box, after which it receives all of the input data in an answer telegram.
At 38.4 kbaud, 20 ms are needed to exchange 30 bytes of process data. The data is transmitted in a fixed format binary string secured by a checksum.
As an additional operating mode, autonomous master/slave communication can be established between two serial Fieldbus Box modules. The input data from one device are copied directly to the outputs of the other, without the aid of an additional master – and vice versa. This kind of communication does not require any extensive configuration – it is only necessary for the node addresses to be appropriately selected.
The node address is set in the range from 1 to 69 using two decimally coded rotary switches. The transmission rate is set to 38,400 baud by default. Like the other system parameters, it can be altered if required using the KS2000 software tool through the serial configuration interface of the Fieldbus Box.
A status byte is transmitted with each telegram, providing information about the node and communication states. The status of the network connection, the device status, the status of the inputs and outputs and of the power supply are displayed by LEDs.
Compact Box modules with serial interfaces are available for all relevant industrial signals. In addition to digital and analog input and output modules including thermocouple and RTD inputs, there are also incremental encoder interfaces available for displacement and angle measurement in addition to serial interfaces to solve a large number of communication tasks.
The serial Coupler Box gathers the I/O data from the Extension Box modules over the interference-free IP-Link fibre optic cable. It detects the connected modules and automatically allocates the input and output data to the process image. Both data consistency and a clear separation of input and output data are ensured. The Coupler Box has four digital inputs and four digital outputs. Other kinds of signals are available in the Extension Box.