Basics Modbus - Siemens SITRANS LG260 Operating Instructions Manual

Pollution degree
5)
Protection rating (IEC 61010-1)
Approvals
Instruments with approvals can have deviating technical data (depending on the version). For such
instruments, the corresponding approval documents must be noted.

10.2 Basics Modbus

Bus description
The Modbus protocol is a communication protocol for the communication between instruments. It is
based on a Master/Slave or Client/Server architecture. By means of Modbus, a Master and several
Slaves can be connected. Each bus participant has an unambiguous address and can send mes-
sages to the bus. This is initiated by the Master, the addressed Slave answers. For data transfer, the
versions serial (EIA-485) as well as the mode RTU are available. In the RTU and ASCII mode inter-
esting here, data will be transmitted in binary form. The telegram consists generally of the address,
the function, the data as well as the transmission check.
Bus architecture
In the version Modbus RTU, up to 32 participants can be connected to the bus. The length of the
twisted two-wire cable can be up to 1200 m. The bus must be terminated on both sides at the last
bus participant with a terminating resistor of 120 Ohm. The resistor is already integrated in the
SITRANS LG260 and is activated/deactivated via a slide switch.
4
1
Fig. 44: Bus architecture Modbus
1 RTU
2 Connection resistor
3 Bus participant
4 Voltage supply
Protocol description
The SITRANS LG260 is suitable for connection to the following RTUs with Modbus RTU or ASCII
When used with fulfilled housing protection
5)
72
USB
MODBUS
power supply
+ 1 2 off on 3 4 5
( ) (-)
3
SITRANS LG260 - Operating Instructions
4
III
USB
MODBUS
power supply
+ 1 2 off on 3 4 5
( ) (-)
3
USB
MODBUS
power supply
+ 1 2 off on 3 4 5
( ) (-)
3
2
PBD-51041059