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© 2020 Van Putten Instruments BV | MAN-VP-V-UK | Revision: 2000 | Date: 29-7-2020
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RS485 connection terminal
The VPVision M features 1 pre wired RS485 interface. The RS485 is connected to COM3 of the
VPVision computer. It contains the following pins:
Pin
Description
1
Data B
2
Data A
3
Power -
4
Power + (24VDC)
3 more COM ports are available on the VPVision computer. A Modbus extension module can add
additional RS485 screw terminals and power supply for one of these 3 COM ports. This enables you
to create 3 more daisy chains.
RS485 wiring
RS485 is a differential balanced line over twisted pair. It can span relatively large distances up to
1200 meter | 4000 feet. The wires should be connected as a point-to-point configuration, or also
called daisy chain. Do not install as star or ring network! The trunk line goes from the master to all
devices making a drop down to each device. The cable length from the trunk line to the Modbus
device needs to be as small as possible. Junction boxes are used to make the T junction.
Shielded twisted pair should be used. For Modbus field cabling only use the
sized and shielded in line with the specific situation in your plant. Connection of a third wire between
the master and slave should be done to limit the common mode voltage that can be impressed on
the slaves inputs. The cable shielding should only be connected at the earth rail in the VPVision
cabinet. The cable shielding should not be connected to any other earth connection point.
Bus power
The VPFlowScope probe can be powered via the same trunk line. 2 separate wires are used for
power + and power -. Take in account that long wires with multiple slaves will cause voltage drops.
The minimum supply voltage is 12VDC measured at the last VPFlowScope probe in the daisy chain.
Termination resistor
Termination resistors reduce electrical noise sensitivity. They need to be added to the installation
when cable distances become longer then 10 meter. The value of each termination resistor should be
equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs).
There can only be one termination resistor at the very end of the trunk line. The VPInstruments
junction box features a jumper that can be used to enable a 120 Ohm resistor. When using the
VPInstruments Modbus Junction boxes make sure that the 120 Ohm resistor is only enabled in the