19
4.10 Connecting to an RS-485 network
16131.5 is designed for a two-wire (half-duplex) RS-485 network. In such a network, 16131.5 acts as
a slave, receiving data requests from the master. An example of the connection to an RS-485 two-wire
network is shown in the figure below. 16131.5 is powered from 5 to 30 VDC. The power supply is not
shown in the figure. The VDC [-] power supply ground must be connected to the common line of the
network. [
Modbus over serial line specification and implementation guide V1.02 (
www.modbus.org
).
]
After the last nodes in the network, on both sides, line termination resistors (LT) are required to
eliminate reflections in the network. According to the RS-485 standard, these LT have a typical value
of 120 to 150 Ω. Never place more than two LT on the network and never place the LT on a derivation
cable. To minimise noise on the network when no transmission is occurring, a pull up and pull down
resistor are required. Typical values for both resistors are in the range from 650 to 850 Ω.
Figure 4.10.1
Connecting 16131.5 to a typical RS-485 network
[+] 12 VDC (heater)
[+]
5 to 30 VDC
[
-
]
5 to 30 VDC / [-] 12 VDC (heater)
c
o
m
m
o
n
[+]
d
a
t
a
[
-
]
d
a
t
a,
RS-485 A /
A
’
16131.5
wire
white
black
blue
grey
brown
,
RS-485 B / B’
RS-485 network
shield
shield
