EMH ITZ - Electrical Data Interface RS485

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6.4.3 Electrical data interface RS485
The electrical interface (galvanically de-coupled) is found at the additional terminals
under the sealable terminal cover (see connection diagram).
The electrical interface RS485 is a symmetrical two wire interface and is designed
in accordance with TIA/EIA-485 / ITU-T V.11. Up to 32 meters can be connected
to a meter modem with RS485-interface. The distance between the readout device and
the meter may not exceed 1,000 m.
Features
Number of connected meters
up to 32
Maximum cable length
up to 1,000 m
Data transmission rate
300 ... 9600 baud
Signal according to
TIA/EIA-485 / ITU-T V.11
logical “1”
-0.3 V to -6 V
logical “0”
+0.3 V to + 6 V
Table 4: Specifications of RS485
RS485 Bus
Up to 32 devices can be operated by one RS485 Bus. Usually in a bus system the first
and last device should be terminated with a terminating resistor between wire „A“ and
„B“ in order to eliminate conduction reflections.
Bus structure:
Figure 8: System layout - RS485-two wire bus
Detail:
Please note:
The terminating resistor may only be
installed at the first and last device.
See detail
See detail
B
A
B
A
B
A
. . .
Device 1 Device 2 Device 32
RS485-two wire bus
RS485
23 24 25
A
B
R
TERM
= 120
Shield