19
WIKA operating instructions, model GDM-100-T
EN
14535893.01 01/2022 EN/DE
5.3.2 Pin assignment of attached, digital transmitter (model GD-20-D)
Circular connector M12 x 1 (5-pin)
1 -
-
2 U
+
Supply voltage
3 U
-
Ground
4 A
Signal RS-485
5 B
Signal RS-485
5.3.3 Pin assignment of integrated, digital transmitter (model GD-20-D)
Modbus
®
-RTU via RS-485 interface in rear cable socket
Rear cable socket
1
-
-
2
U
+
DC 10 ... 30 V
3
U
-
Ground
4
A
RS-485 signal
5
B
RS-485 signal
6
-
-
5.3.4 Requirements for shielding and grounding
■
Only use shielded cables and connect the shield on one side to the read-out unit.
■
The gas density sensor is grounded via the process connection of the mechanical
base instrument.
■
Ensure that no earth circuits can occur.
5.3.5 RS-485
The physical layer for the Modbus
®
protocol is the serial RS-485 interface per EIA/
TIA-485. The differential signal between pins 4 and 5 (A and B) is evaluated with a
2-wire system (half-duplex).
5.4 Modbus
®
The Modbus
®
communication protocol is based on a master/slave architecture. The
protocol implemented in the model GD-20 gas density sensor is Modbus
®
-RTU with serial
transmission via a 2-wire RS-485 interface.
The Modbus
®
protocol is a single-master protocol. This master controls the entire data
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.
5. Commissioning, operation
