WIKA GDT-20 Mode D'emploi page 35

5. Commissioning, operation
5.2.2 Pin assignment
Circular connector M12 x 1 (5-pin)
1
2
2 1
3
5
3 4
4
5
5.2.3 Requirements for shielding and grounding
Only use shielded cables and connect the shield on one side to the read-out unit.
■
The transmitter must be grounded via the process connection.
■
Ensure that no earth circuits can occur.
■
In case of Modbus
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termination resistor at the physical start and end of the bus. The resistor can be activa-
ted via the software in the transmitters while at the gateway it is inserted mechanically
between A and B.
5.2.4 RS-485
The physical layer for the Modbus
TIA-485. The differential signal between pins 4 and 5 (A and B) is evaluated with a 2-wire
system (half-duplex). The mutual reference potential for the signals is on pin 1 (C).
5.3 Modbus

The Modbus communication protocol is based on a master/slave architecture.
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The protocol implemented in the model GDT-20 and model GDHT-20 transmitters is
Modbus RTU with serial transmission via a 2-wire RS-485 interface.
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The Modbus protocol is a single-master protocol. This master controls the entire data
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transfer and monitors any possible timeouts (no reply from the addressed instrument).
The connected instruments may only send telegrams after request by means of the
master.
Modbus RTU (RTU: Remote Terminal Unit) transmits the data in binary form, guaran-
®
teeing a good data throughput.
Detailed information on the protocol under www.Modbus.org
5.4 Modbus start-up kit
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The transmitter with the optionally available startup kit (order no. 14075896) can be
configured at the measuring location for operation.
WIKA operating instructions transmitter, models GDT-20 and GDHT-20
- -
U Supply voltage
+
B
U Ground
-
B
A Signal RS-485
B Signal RS-485
communication issues, it is recommended to install the 120 Ohm
protocol is the serial RS-485 interface per EIA/
®
EN
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