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System /
application
6d
10d
L1
L2
L3
4-wire
N
3-phase
asymmetric
6d
10d
load
u
x
X
U
L1
L2
L3
N
6d
14d
4-wire
3-phase
asymmetric
L1
load,
L2
open-Y-
connection
L3
N
Low-voltage system
5. Connecting devices to the bus
The RS 485 interface of the DME 440 is galvanically isolated
from all other circuits. For an optimal data transmission the
devices are connected via a 3-wire cable, consisting of a
twisted pair cable (for data lines) and a shield. There is no
termination required. A shield both prevents the coupling of
external noise to the bus and limits emissions from the bus.
The shield must be connected to solid ground.
You can connect up to 32 members to the bus (including
master). Basically devices of different manufacturers can be
connected to the bus, if they use the standard MODBUS
protocol. Devices without galvanically isolated bus interface
are not allowed to be connected to the shield.
34
14d
2z
1
2
3
4
5
14d
2z
1
2
3
4
5
u
u
x
x
X
X
k
l
k
l
U
U
K
L
K
L
K
2z
1
2
3
4
5
k
l
k
l
K
L
k
K
L
K
Measuring inputs
Plug wiring
6d
6
L1
L2
L3
N
6
k
l
3 single-pole insulated voltage transformers
L
in high-voltage system
6
u
u
x
x
X
X
U
U
L1
l
L2
L3
L
N
2 single-pole insulated voltage transformers
in high-voltage system
The optimal topology for the bus is the daisy chain connec-
tion from node 1 to node 2 to node n. The bus must form
a single continuous path, and the nodes in the middle of
the bus must have short stubs. Longer stubs would have
a negative impact on signal quality (reflexion at the end). A
star or even ring topology is not allowed.
There is no bus termination required due to low data rate. If
you got problems when using long cables you can termina-
te the bus at both ends with the characteristic impedance
of the cable (normally about 120 Ω). Interface converters
RS 232 ⇔ RS 485 or RS 485 interface cards often have a
®
built-in termination network which can be connected to the
bus. The second impedance then can be connected directly
between the bus terminals of the device far most.
10d
14d
2z
1
2
3
4
k
l
k
l
K
L
K
1
6d
2
3
4
5
k
k
k
l
l
K
L
L
K
L
K
5
6
k
l
L
K
L
14d
6
2z
l
L
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