S+S Regeltechnik PREMASGARD 814 Series Operating And Mounting Instructions Manual page 27

G U
Modbus | Installation
General layout of bus structure
LA
Line
termination
Stub line
Bus topology with terminating and bias resistors
General layout of bus structure
Line
termination
R
AB
Terminating resistor may only be installed at the ends of the bus line.
In networks with repeaters not more than two line terminations are allowed.
Bus topology with terminating and bias resistors
Line termination at the device can be activated via DIP switch 6.
The bias resistors for bus level definition in the resting state are usually activated at the Modbus master / repeater.
The maximum number of subscribers per Modbus segment is 32 devices.
When the number of subscribers is greater, the bus must be subdivided into several segments separated by repeaters.
The subscriber address can be set from 1 to 247.
For the bus line, a twisted-pair cable data line / power supply line and copper mesh wire shield must be used.
Therefore, the line capacitance should be less than 100 pF / m (e.g. Profibus cable).
Passive Active
branch branch
Slave 1 Slave 2
Slave 1 Slave 2
MODBUS
RTU-Master
RS-485
Trunk line
Stub line:
shorter 20m
MODBUS
RTU-Master
5 V
A (D1) D+
B (D0) D–
Common
(GND)
27
LA
Line
termination
Slave n
Multiple branch n:
shorter 40m/n
Pull-up /
bias resistor
R
BIAS
Line
termination
R
AB
Pull-down /
bias resistor
R
BIAS