2-Wire Connection; Host Computer Interface; Pull-Up And Pull-Down Resistors / Polarization - ABB AquaMaster 3 User Manual

AquaMaster 3
Electromagnetic flowmeter

3.4.1 2-wire Connection

AquaMaster 3 MODBUS RS485 uses a 2-wire serial link in accordance with EIA/TIA-485 standard – see
Fig. 3.14.
Line
Terminator
Balanced Pair
R
D
Slave 1
For example, AquaMaster 3
Fig. 3.14 General 2-Wire Topology

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. 3.15 Host Computer Interface
26
D1
D0
R
D
Slave n
Up to 32 slaves
Tx+/Rx+ A D0
Tx–/Rx– B D1
GND Common
Master
D
R
Common
Host Computer
Tx+
'A'
Tx– 'B'
+5V
Pull-up Resistor 560 
Rx+
'A'
Rx–
'B'
Pull-down Resistor 560 
0V
GND
User Guide OI/FET2XX–EN Rev. E
3 Electrical Installation
5 V
Pull Up
Line Terminator
Pull Down