Data in a Modbus device is referenced by register numbers. What each register contains, and the format for analog registers, is defined by the device developer.
| Register Type | Decimal | Hex |
|---|
| Start | End | Start | End |
|---|
| Discrete Output Coils | 1 | 9999 | 0 | 270E |
| Discrete Input Contacts | 1 | 9999 | 0 | 270E |
| Analog Input Registers | 1 | 9999 | 0 | 270E |
| Analog Output Holding Registers | 1 | 9999 | 0 | 270E |
- Note that the addresses are offset by 1, because register 1 has address 0.
- Some devices support extended registers, with the maximum register number being 65536 = 2^16.