Modbus is an open, serial communications protocol based on the master/slave architecture. Since it is easy to implement on all kinds of serial interfaces, it has gained wide acceptance. The Modbus protocol was originally developed in order to link controllers into a network. However, it has been frequently applied for connecting input/output modules. Because of the low transmission rate, which has a maximum of 38.4 kbaud, Modbus is best employed in applications with less numbers of devices and low demands on response time.
The bus consists of a master station and a number of slave stations. The communication is controlled entirely by the master.
Modbus offers two basic communication mechanisms:
- Question/answer (polling): The master sends an inquiry telegram to any one of the stations and waits for the answer telegram.
- Broadcast: The master sends a command to all the stations on the network. These stations execute the command without providing feedback.
The telegrams allow process data (input/output data) to be written and read, either individually or in groups. The data can be transmitted either in ASCII code or packed into RTU format.
Modbus is used on a variety of transmission media. An implementation based physically on the RS485 bus, with a twisted screened two-wire cable and termination resistors, as in PROFIBUS, is widespread.
The node address is set in the range from 1 to 69 using two decimally coded rotary switches. The transmission rate and other system parameters can be set by the address selection switch or by using the KS2000 software tool through the serial configuration interface on the Fieldbus Box.
The Beckhoff Modbus nodes support the diagnostic functions of the Modbus protocol. The diagnostic messages are transmitted over the bus and collated in the master. 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 for Modbus 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 Modbus Coupler Box gathers the I/O data from the Extension Box modules over the interference-free IP-Link fiber-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.