Modbus — the main industrial PLC protocol
Modbus is a communication protocol linking PLCs with VFDs, sensors, energy meters, HMI panels, and other equipment. It exists in two variants: RTU (via RS-485) and TCP (via Ethernet). Created in 1979, it remains the most widespread industrial protocol — simple, open, universally supported.
Modbus RTU
- Physical layer: RS-485, up to 1,200 m
- Speed: 9600-115200 baud
- Max devices: 247 (practically 16-20)
Typical setup: Delta DVP PLC polls VFDs and pressure sensors via RS-485.
Modbus TCP
- Physical layer: Ethernet RJ-45
- Port: TCP 502
- Speed: 10/100 Mbps
Supported by Siemens S7-1200, Mitsubishi FX5U, Delta DVP with Ethernet module.
Modbus registers
| Type | Address | R/W | Example |
|---|---|---|---|
| Coils | 00001-09999 | R/W | Relays, outputs |
| Discrete Inputs | 10001-19999 | R | Inputs |
| Input Registers | 30001-39999 | R | Measurements |
| Holding Registers | 40001-49999 | R/W | Setpoints, commands |
RS-485 wiring tips
- Shielded twisted pair, 0.5-0.75 mm2
- 120 ohm terminators on end devices
- Ground shield on one end only
- Do not route with power cables
- Same settings on ALL devices
Frequently Asked Questions
RTU or TCP?
RTU — cheaper, up to 1,200 m. TCP — faster, better for IT integration. Budget: RTU. New Ethernet projects: TCP.
How many devices per bus?
Protocol limit: 247. Stable: 16-20. Beyond that, use repeaters.
How to connect a Delta VFD?
RS-485 from PLC COM2 to VFD S+/S- terminals. Address P09-00, speed P09-01, Modbus control P00-02=3.
Can RTU and TCP be mixed?
Yes, via a Modbus RTU-TCP gateway.
Modbus vs PROFINET?
Modbus — open, multi-vendor. PROFINET — Siemens only, faster, deterministic. Mixed systems: Modbus.
Summary
Modbus is the simplest way to link PLCs with peripherals. Delta DVP, Siemens S7-1200, Mitsubishi FX5U — all support it.
