© 2017 Van Putten Instruments BV | MAN-VP-SINL-UK | Revision:1701 | Date:22-03-2017
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VPFlowScope in-line
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
last Modbus Junction box in the daisy chain.
Biasing
When there is no data activity on an RS485 network, the communications lines are "floating" and,
thus susceptible to external noise or interference. Receivers on a RS485 network have built in
hysteresis (200mV differential required to insure known state). To insure that a receiver stays in an
inactive state, when no data signal is present, bias resistors are required. Bias resistors are a pull-up
resistor on Modbus B and a pull-down resistor on the Modbus A line. The value of the bias resistor
depends on the number of devices and the supply voltage. The table below shows which resistor
values can be used for different voltage in a chain with 1 to 8 VPFlowScope in-lines.
Supply voltage
Bias pull up
Bias pull down
12 V
5 KΩ
1 KΩ
24 V
10 KΩ
1 KΩ
Bus power
The VPFlowScope in-line 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 in-line in the daisy
chain.
Electrical scheme
