L-INX User Manual
235
LOYTEC
Version 4.0
LOYTEC electronics GmbH
11 Modbus
11.1 Introduction
The Modbus is a de facto industrial standard which was initially intended for the
communication between PLCs. In the meantime the Modbus has become an important
interface for automatic meter reading applications and industrial applications. As the
Modbus is an open protocol a large number of automation devices providing Modbus
communication is available.
Two communication methods are available for Modbus. Modbus TCP utilizes Ethernet for
the Modbus communication. Modbus RTU uses a RS485 bus for Modbus data transfer.
Modbus RTU uses a serial bus, which is controlled by a single master. This master can
request data from several slave devices connected to the network.
11.2 Modbus Network
Modbus TCP utilizes the Ethernet for the communication. Several Modbus slave devices
can be connected. In Modbus TCP a unit identifier is used instead of the slave address.
Modbus slaves may have the same unit identifier. This unifier is usually used to
communicate with bridges, routers and gateways.
The Modbus RTU network utilizes an RS485 connection. Several Modbus slave devices are
connected in parallel to the transmission medium. Each Modbus device has a unique slave
address in the range from 1 to 255. The Baud rate of the RS485 can be configured to use
1200, 2400, 4800, 9600, 19200, and 38400 Baud. The parity is always none. The LOYTEC
Modbus interface supports 8N1 only. Parity and stop bits are not configurable.
Modbus devices use function codes for the specification of the desired data. The following
function codes for read/write actions are supported by the LOYTEC device:
02: Read discrete inputs.
01: Read coils.
05: Write coil.
04: Read input register
03: Read holding registers
06: Write holding registers
