Fieldbus Grounding; Fieldbus Shield; End Termination; Pull Up/Pull Down Resistors - ABB ACS310 Quick Start Manual

Quickstart Guide
ABB PLC and drives integration using Modbus RTU
Hardware physical connection
In the following chapter, physical connection of the hardware will be explained.
Fieldbus grounding
For AC500, which has opto‐isolated COM interfaces, it is recommended not to connect the ground. For AC500‐
eCo, where the COM interfaces are not opto‐isolated, it is recommended to connect the ground at one place,
typically in the bus master. For connection examples, see chapter "Installation examples".
Fieldbus shield
The shield should always be connected in a Modbus RTU line. The purpose of the shield is to protect against EMC
disturbances and reflection on the bus line. For connection examples, see chapter "Installation examples".
End termination

Purpose: To avoid signal reflections.

Resistor size: 120 Ohms, ¼ W.

Where to connect: One resistor in each end of the bus line (first and last node).
Pull up/pull down resistors

Purpose: To keep a defined high level on the bus while there is no data exchange.

Resistor size: Depends on bus master (COM) voltage level. 470 Ω for AC500 (5 V output) and 270 Ω for
AC500‐eCo (3.3 V output).

Where to connect: Activate pull‐up and the pull‐down resistors, but only once on every bus line, at the bus
master.
Modbus RTU ‐ 6 ‐ PLC and drives integration