AquaMaster 3 FET200
Electromagnetic flowmeter Transmitter
3 Electrical Installation
OI/FET200–EN Rev. I
23
3.4.1 2-wire Connection
AquaMaster3 MODBUS RS485 uses a 2-wire serial link in accordance with EIA/TIA-485 standard – see
Fig. 3.14.
3.4.2 Host Computer Interface
An RS485 communications driver must be fitted to the host computer. It is strongly recommended that the
interface has galvanic isolation to protect the computer from lightning damage and increase signal immunity
to noise pick-up if the data is to be taken over long distances.
3.4.3 Pull-up and Pull-down Resistors / Polarization
To prevent false triggering of slaves when the master (host computer) is inactive, pull-up and pull-down
resistors must be fitted to the RS485 interface at the host computer – see Fig. 3.15.
Fig. 3.14 General 2-Wire Topology
Fig. 3.15 Host Computer Interface
Master
5 V
Slave n
Slave 1
Pull Up
Pull Down
Line Terminator
D0
D1
Common
Balanced Pair
Line
Terminator
Up to 32 slaves
For example, AquaMaster3
D
R
D
R
D
R
Host Computer
’A’
’B’
'A'
0V
'B'
Pull-down Resistor 560
+5V
Pull-up Resistor 560
Tx+
Tx–
Rx+
Rx–
GND
Tx+/Rx+ A
Tx–/Rx– B
GND
D0
D1
Common
