05. MODBUS Basics

What you'll learn

• 1Explain the MODBUS Master(Client)↔Slave(Server) request/response structure
• 2Compare Modbus RTU vs TCP physical layer·error checking (HIWIN RTU=RS485)
• 3Distinguish the four registers' bit width·permission·value range
• 4Map Function Codes (01–06,15,16) to registers

Introduction

In MODBUS, a Master(Client) sends a Demand and a Slave(Server) sends a Response. Every slave receives the command, but only the device with the addressed ID executes and replies. A HIWIN robot can be master (controlling a gripper·sensor) or slave (controlled by a PLC).

Key concepts

1) RTU vs TCP

2) The four data registers

3) Function Codes & the 4-part data

01=read Coil·02=read DI·03=read HR·04=read IR·05=write single Coil·06=write single HR·15=write multiple Coil·16=write multiple HR. Rule: 01/02=bit read, 03/04=word read, 05/06=single write, 15/16=multiple write. Read-only (DI·IR) have no write codes. A command has 4 parts: ① Address ② Function code ③ Data ④ Error check (CRC, omitted on TCP).

Core example

With RobotMaster, read each register type (02/01/04/03), then write to the R/W ones (Holding 06·Coil 05) and read back to confirm.

with RobotMaster(host, port, unit) as m:
    di = m.read_discrete(0, 8)   # FC02 Discrete Input
    co = m.read_coils(0, 8)       # FC01 Coil
    ir = m.read_input(524, 1)[0]  # FC04 Input Register (motion state)
    hr = m.read_holding(100, 1)[0]# FC03 Holding Register (speed setting)

    m.write_holding(100, 50)      # FC06 Holding write
    assert m.read_holding(100, 1) == [50]
    m.write_coil(300, 1)          # FC05 Coil write (DO1 ON)

Common mistakes

Q. ConnectionError: failed to connect to slave.

A. Terminal A's slave isn't up or the port differs. Confirm robot_server_sim.py --port 1502 is running and the master also uses --port 1502.

Q. I want to write to an Input Register but there's no method.

A. By design, not a bug. Input Register·Discrete Input are read-only, so there's no write Function Code and no write_input/write_discrete in RobotMaster.

Q. read_holding(100,1) gives [50] which compares unequal to 50.

A. read_* always returns a list. Take the single value with m.read_holding(100,1)[0] before comparing.

Summary

• MODBUS is Master(demand)↔Slave(response); a command has Address·Function code·Data·CRC(omitted on TCP)
• RTU=RS485+binary+CRC required, TCP=Ethernet+encapsulation+TCP checks. HIWIN uses RS485 for RTU
• The four registers split into bit (DI=R, Coil=R/W) and word (IR=R, HR=R/W); permission is enforced
• Function Codes: 01/02=bit read, 03/04=word read, 05/06=single write, 15/16=multiple write

Exercises

1. Read all four registers with 02/01/04/03 and confirm the output matches the Function Codes
2. Write to Holding(06)·Coil(05), read back ✅, and confirm Input Register has no write method
3. Explain which of RTU/TCP needs CRC and why