The motor power supply input common (GND) is connected internally to the common of the communication port. When two or more units are connected in the party mode, or multi-drop configuration, a “ground loop” is created.
This occurs because the common (communication ground) which connects the communication ports of all connected devices is in parallel with the system power common. This can allow the motor power current to flow through the communication port of another device and vice versa.
Figure 1 shows this recirculation where I
1
represents the total current, I
2
is return current through the power ground path, and I
3
is the current recirculating through the communication ground path.
Figure 1: Recirculating currents in RS-422 multipoint system
Solution
Eliminating the recirculating current in this system is accomplished quite easily by:
Using an isolated communication converter such as the MD-CC40x-001.*
Connecting the isolated ground of the communication converter direct to power ground.
Leave the communication ground pin on the device open. Communications will still be grounded via power common without the path for recirculating currents.
Figure 2 shows an example of this connection scheme.
Figure 2: Recirculating currents eliminated
