RS-485 / Modbus RTU Commissioning Checklist

Site: ____________________ Date: __________ Technician: ____________________

Tick each item as you go. Always follow the device manufacturer's instructions when they differ from this list.

1. Before you start

• ☐ Register map / Modbus manual for each device model (check the firmware version matches)
• ☐ List of all devices on the bus with planned unit ID, location and model
• ☐ Planned serial settings written down: baud rate, data bits, parity, stop bits
• ☐ Wiring drawing showing the cable route, device order and both ends of the bus
• ☐ USB-to-RS-485 adapter (and its driver) tested on your laptop
• ☐ Know which device is the master (client) — usually only one master per bus

2. Wiring

• ☐ Cable is twisted pair, typically shielded, rated for RS-485 (about 120 ohm impedance)
• ☐ Devices connected in a daisy chain (line), not a star or ring
• ☐ Stubs (drops from the main line to a device) kept as short as possible
• ☐ A / B (or D+ / D-, + / -) consistent on every device — note: vendors label these differently
• ☐ Signal common / reference (0 V, GND, C) connected between devices when they have this terminal
• ☐ Shield continuous along the bus and grounded at one point only (unless the manufacturer says otherwise)
• ☐ Cable kept away from power cables and VFD motor cables where possible
• ☐ Total cable length within limits (typically up to about 1200 m at lower baud rates; less at high baud rates)
• ☐ Number of devices within the driver limit (standard is 32 unit loads; more with low-load transceivers or a repeater)

3. Termination and biasing

• ☐ 120 ohm termination at both ends of the bus only — not at every device
• ☐ Termination switches/jumpers off on all devices in the middle of the bus
• ☐ Biasing (fail-safe) applied at one point only, typically at the master or one end
• ☐ Not too many devices with built-in biasing switched on

4. Serial settings

• ☐ Same baud rate, data bits, parity and stop bits on every device and on the master
• ☐ Modbus specification default: 19200 baud, 8 data bits, Even parity, 1 stop bit (8E1)
• ☐ With no parity, the specification asks for 2 stop bits (8N2) — but many devices actually use 8N1; match what the devices really use
• ☐ Mode is RTU (not ASCII) on all devices
• ☐ Response timeout on the master long enough for the slowest device (typically 500-1000 ms to start)

5. Addressing

• ☐ Every device has a unique unit ID between 1 and 247
• ☐ Unit ID 0 is broadcast (write only, no reply) — do not assign it to a device
• ☐ Register numbering understood: 40001 in the manual = holding register address 0 on the wire (30001 = input register 0)
• ☐ Check if the manual uses 0-based or 1-based addresses (off-by-one is very common)
• ☐ Data types and byte/word order noted for 32-bit values (e.g. ABCD, CDAB)

6. First communication test

• ☐ Connect and test one device at a time if the bus is new or not working
• ☐ Scan unit IDs to confirm which IDs answer
• ☐ Read one known register (e.g. a voltage, temperature or serial number)
• ☐ Compare the value with the device display (check scale, units and sign)
• ☐ Add the remaining devices one by one and repeat the test
• ☐ Leave a poll running for several minutes and watch for timeouts or CRC errors
• ☐ Record final settings, unit IDs and termination positions on the drawing

7. Troubleshooting quick table

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