Description of the Fieldbus System

Modbus is an open serial communication protocol, which is based on a Master/Slave architecture. Since it is quite easy to install on any serial interface, it has found a wide usage. Originally it was developed for the connection of controllers. But due to its spread of use, it is also used for I/O communication. The relatively low transmission rate of 38.4 kBaud admits Modbus for applications with a low number of stations or slow real time requirements.

The systems consists of a Master and several Slave stations, where the communication is purely controlled by the Master. Modbus uses two basic communication mechanisms:

  • Polling - the Master sends a telegram to any station and waits for an answer
  • Broadcast - the Master sends a telegram to all stations without waiting for an answer

The telegrams allows the reading and writing of process data (in / output data) either individually or  group wise. The data can be coded in ASCII or in RTU format.

Modbus is used on different physical layers. The most common one is the RS485 physics, a (shielded) twisted pair of wires with an end termination like PROFIBUS.


The station address can be defined with two decimal rotary switches from 1 to 69. Transmission rate and other features can be set with the KS2000 configuration software via the serial interface or with the address switch 69 -99.


All BECKHOFF Modbus stations support the diagnostic functions of the Modbus protocol. All diagnostic messages are transmitted via the bus and are collected in the Master. The status of the network, the status of the device and the status of the in and outputs and the power are displayed with LEDs.

Cord sets

Since Modbus uses the RS485 transmission like PROFIBUS, all PROFIBUS cord sets can be used. Using the premoulded BECKHOFF cord sets simplifies the installation and helps avoiding mistakes.  The delivery program covers fieldbus cord sets, power supply cord sets and sensor cord sets as well as termination resistors and field wire able connectors.