G10. RS485 extra wiring

1. Introduction

In this chapter, the explicit wiring for RS485 is explained. Therefore this chapter is extra for the wiring described in chapter E2 and in chapter B4 and will go into depth on the other possibilities of the electrical installation via RS485.

2. Termination resistors

The termination resistors reduce the electrical noise sensitivity of the Modbus cable.

• Needed: Typically when the cable distance becomes larger than 10 meters.
• Location: 1 resistor at the very end of the trunk line.

• The value of the termination resistor is equal to the cable characteristic’s impedance.
  (For twister pairs: typically 120 ohms).

VPInstruments junction boxes: These boxes feature a jumper that can be used to enable a 120 Ohm resistor. Make sure the 120 Ohm resistor is enabled only in the last junction box of the Daisy Chain:

3. Biasing resistors

When there is no data activity on an RS485 network, the communication lines are “floating” and, thus susceptible to external noise or interference. Receivers on an RS485 network have a built-in hysteresis (200mV differential required to ensure known state). To ensure that the receiver stays in an inactive state when no data is present, bias resistors are required.

Bias resistors are:

• pull-up resistor on Modbus B line
• pull-down resistor on Modbus A-line

The value depends on the number of devices and the supplied voltage. In the case of 1 to 8 VPFlowScope’s, the bias values are per supply voltage:

4. Bus power

The VPFlowScope can be powered via the same trunk line. Two separate wires are used for positive and negative power. Take into account that long wires with multiple slaves will cause voltage drops. The minimum supply voltage for the last VPFlowscope in the daisy chain is 12VDC. Some specialized cables have two extra-large power lines and two thinner data lines inside. Consult the cable supplier or a technician when selecting the cables for the network in order to make the right chose.

5. M12 connector electrical Scheme