What Is Modbus - ABB EQmatic Product Manual

ABB EQmatic
Planning and application

What is Modbus?

Modbus is a serial communication protocol that was developed and published for use with
programmable logic controllers (PLC). It is a communication method for the transmission of
information via serial cables between electronic devices. The device that requests the information is
termed the Modbus master. The devices that send the information are Modbus slaves. In a standard
Modbus network there is one master and up to 247 slaves each with a unique slave address from
1 to 247.
Bus principle RS-485
The Modbus standard uses the RS-485 standard. This standard defines the physical layer of the
Modbus interface. The data are transmitted in serial form via a 2-wire bus (RS-485).
The RS-485 standard is based on the master – slave method and defines the bus cable as a cable
with a start and an end that are each terminated using an EOL resistor RT (T=Termination).
Master = level converter (e.g. QA/S 4.xx.1 Energy Analyzer)
Slave = Modbus device/meter (e.g. ABB electricity meter from the A and B series, water meter, heat
meter, gas meter, etc., with Modbus RTU interface).
Transmission speed
300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200 baud
Polarity
Attention must be paid to the correct polarity of the core pairs during installation because incorrect
poling will invert the data signals. Particularly if there are difficulties in relation to the installation of
new devices, any troubleshooting should start with a check on the bus polarity.
Topology
The optimal cable topology for the Modbus RTU is a purely linear structure. Droplines to individual
devices with a maximum length of 1 m are allowed. These droplines are not terminated.
QA/S x.yy.1 | 2CDC512090D0202 101