3.7.4 General ModBus RTU RS-485 wiring considerations
Devices connected on the Modbus RS-485 wire, including the Child, converter(s) and other instrumentation, must be wired as follows:
• Connect the shield of each cable segment to ground at one end only.
• Isolate cables, as much as possible, from sources of electrical noise.
• Install a ¼ Watt termination resistor (RT) between the (+) and (-) terminals of the device at each end point of a
straight-line bus. The resistor should match the nominal impedance of the RS-485 cable, which is typically 120
ohms (consult the manufacturer’s documentation for the cable’s impedance value).
RS-485 Connection methods to avoid
Any device connection that causes a branch in the main RS-485 ModBus cable should be avoided. This includes star and tee (T)
methods. These wiring methods cause signal reflections that may result in interference. No more than two cables should be
connected at any connection point on the RS-485 ModBus daisy chain. This includes connection points on instruments, converters,
and terminal strips. Following these guidelines ensures that both star and tee connections are avoided.
Figure 17. Serial modbus network wiring example
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