Wiring a Multi-Node RS-485 Modbus Network

Deployed a 12-node sensor network over RS-485 using Modbus RTU. The bus runs 80 meters through a warehouse with significant EMI from motor drives.

Bus topology: star topology is incorrect for RS-485 — it causes reflections. Use a daisy-chain from master to each node in sequence with a 120-ohm termination resistor at each end. Twisted pair cable (CAT5 with one pair used for A/B, another pair for GND) gives excellent common-mode rejection.

Ground: each node must share bus GND but this creates ground loops. The solution is a common bus ground reference (one wire through the cable) while each node's mains isolation is maintained. Isolated RS-485 transceivers (ADM2587E) eliminate ground loop issues entirely and are worth the $5 premium per node in an industrial environment.