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4.2
Electronic output 09-V – Interface RS485 Modbus-RTU
The Modbus protocol is a communication protocol, that base on a master/slave architecture.
All devices are connected by two data wires (A+ / B-) and by one COMMON-wire (C/L-).
An original RS485 allows the connection of 32 slaves within one segment. The device has a load of only 1/8 of the standard
load (Rin ≥ 96 kΩ), thus up to 256 of the devices can be theoretically operated within one network segment. However the
number is limited to 247 due to the Modbus address space.
The both termination resistors RT prevent reflections on the data wires. The optimum resistor value depends on the wave
impedance of the used cable, but a value of 120 Ohm is a popular choice.
The polarisation network is necessary, to ensure suitable potentials, if none of the devices transmits and thus the wires
A+ and B- are undefined (high impedance). The value for RP depends e.g. on bus load or the termination resistors.
Recommended values are between 450 Ohm and 650 Ohm.
The use of a polarisation network is recommended, to ensure a stable network. Usually the polarisation resistors are
implemented within the master device or they are connectible.
Connect the device at bus topology (line). A stub line must be avoided.
Use a cable 4-core acc. to the EIA485 recommendations:
Impedance
Cable capacity
Cable diameter
Cable cross section
Loop resistance
Shielding
Cable length
Observe maximum permitted supply voltage Us at the terminals L+/L-:
•
Us = 6...35VDC
8
Y O U R P A R T N E R F O R M E A S U R E M E N T & A U T O M A T I O N
RT
120 Ohm
R
D
D
Slave 1
Slave n
Master
D
R
5 V
RP
650 Ohm
A+
B-
C / L-
RP
650 Ohm
R
RD
L+
BN
A(+)
WH
B(-)
BK
L-
YE/GN
135...165Ω @ 3...20Mhz
< 30pF/m
> 0,64mm
0,34 mm2 / AWG 22
< 110Ω/km
Braided shield /shield foil
38400 Baud ≤ 1200m
RT
120 Ohm
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