RS-485 wiring and supply voltage drop

Translated wiring guidance relevant to the device manuals.

Bus wiring

RS-485 uses A and B for differential data and a common reference conductor. Connect A to A, B to B and the relevant GND terminals. A/B must be the two conductors of one twisted pair. A is usually labelled + and B −, but check the equipment labels. A reversed pair prevents communication.

Use a linear bus with 100–120 Ω termination at its physical ends; avoid stars and rings. Disable a controller's built-in termination if it is in the middle of the bus. At up to 115200 bit/s, branches up to 50 m may be possible depending on the installation; a linear bus remains the recommended topology.

Use shielded twisted-pair cable where interference is expected and bond its shield at one point. For short runs around 10 m without interference sources, shielding may be unnecessary. CAT5 cable is also suitable. There is no universal wire-colour convention.

A second pair can carry V+/GND, provided its cross-section and voltage drop are adequate. For an isolated RS-485 port, use that port's isolated reference terminal.

Peak-load voltage drop

Size supply wiring for peak consumption, cable resistance and supply voltage. A module may reboot during relay switching even when average operation appears normal.

The source example uses five MSW2 sensors and two MRM2-mini modules on 50 m of 0.6 mm-diameter conductors with a 12 V supply. Peak power is 7.5 W versus about 3 W average; cable loop resistance is 6 Ω. At 0.25 A average and about 0.6 A peak, the drop is 1.5 V and 3.6 V. The resulting 8.4 V peak-load supply is below the required 9 V.

Use larger conductors or a suitable 24 V supply. At the same power, 24 V roughly halves the current and cable drop while increasing voltage margin.

Connections

Use crimped wire-end ferrules for reliable terminal connections. A break disconnects downstream devices; a short can stop the entire bus. For large installations, do not depend on controller Vout terminals without checking their current limits; the source specifies a combined 1 A limit for the two outputs it describes.

See GND and iGND connections for grounding arrangements.