Specifications
| Specifications | Details |
|---|---|
| Masters Nodes | only 1 |
| Slave Nodes | max 247 (each with a unique slave Id) |
| Max Devices | RS-485 - Max 32 devices |
| Speed/Baud Rate | 9600-19200 (Typical) |
| Network | Daisy Chain |
| Byte Order | Byte order for registers is not defined in the protocol. Some vendors will store the higher byte first... |
Tables
| Address - Start | Stop | Table | Read/Write | Data Type |
|---|---|---|---|---|
| 00001 | 09999 | Coils | read/write | 1 bit representing a bool or on/off |
| 10001 | 19999 | Discrete Inputs | read only | 1 bit representing a bool or on/off |
| 30001 | 39999 | Input Registers | read only | 16 bits / 2 bytes (Int) |
| 40001 | 49999 | Holding Registers | read/write | 16 bits / 2 bytes (Int) |
Function Codes
| Function Code | Action |
|---|---|
| 1 | Read Coil Status |
| 2 | Read Input Status |
| 3 | Read Holding Register |
| 4 | Read Input Register |
| 5 | Write Single Coil Status |
| 6 | Write Single Register |
| 15 | Multiple Coil Write |
| 16 | Multiple Register Write |
Message
| Slave Address/ID | Function Code | Data | CRC |
|---|
CRC - Cyclic Redundancy Check
Response
References
| Reference | URL |
|---|---|
| Understanding Modbus Serial and TCP/IP | https://www.youtube.com/watch?v=k993tAFRLSE&t=327s |
| What is Modbus and How does it Work? | https://www.youtube.com/watch?v=txi2p5_OjKU |
| Modbus RTU | https://en.wikipedia.org/wiki/Modbus#Modbus_RTU_frame_format_(primarily_used_on_asynchronous_serial_data_lines_like_RS-485/EIA-485) |
| Modbus versus Memobus | |
| Metasys™ N2 |