Modbus / RS485 Specifications
Aegir Ultrasonic Meters support communication over Modbus/RS485 protocol. Modbus RTU is typically used for serial communication (RS-485) interface for seamless integration with external devices.
1. Modbus Registry Details
| Register | Data Type | Description |
|---|
| 40001 | U16 | A sequentially increasing counter that resets after 9999. Useful for detecting out of sequence reads. |
| 40002 | bit-wise | 16-Trickle, 15-Zero Flow,14-High Flow (above 60LPM), 13-Empty or Error, 12 to 1 - Unused |
| 40003 | U16 | Flow Rate as LPM × 10 |
| 40004 | U32 | Cumulative flow reading, in Litres |
| 40006 | U32 | Flow reading since last restart, in Litres |
| 40008 | U32 | Unique ID of this device |
| 40010 | U32 | Manufacturing Date (days since 01-01-2020) |
Communication Details
- Baud Rate: 9600
- RS485 [Modbus] ID: 1
- Bits: 8
- Parity: N
- Stop Bits: 1
2. RS-485 Electrical Specifications
| Feature | Description |
|---|
| Standard | EIA/TIA-485-A |
| Signaling Type | Differential (balanced) |
| Wires | 4 wires (A, B, VCC and GND) |
| Voltage Levels | Logic ‘1’ and ‘0’ represented by the voltage difference between A and B (typically ±1.5V to ±5V) |
| Maximum Nodes | 32 per channel |
| Maximum Cable Length | Up to 100 meters |
| Baud Rate | Typically 9600, 19200, 38400, 57600, or 115200 bps |
| Topology | Bus (daisy chain); no star connection |
| Termination | 120-ohm resistors at both ends of the bus |
| Common Mode Range | −7 V to +12 V |
| Connector Type | M8 |
3. Modbus Protocol Specifications
| Property | Description |
|---|
| Protocol Type | Controller & Nodes (Gateway & Meters) |
| Physical Layer | RS-485 (two-wire differential) |
| Data Framing | Modbus RTU or Modbus ASCII |
| Address Range | Slave addresses: 1–247 (0 is broadcast) |
| Data Bits | 8 bits |
| Parity | None / Even / Odd (default: None) |
| Stop Bits | 1 or 2 (default: 1) |
| Error Checking | CRC-16 (for RTU) or LRC (for ASCII) |
| Function Codes | Define operations like read/write (e.g., 01, 03, 04, 05, 06, 15, 16) |
| Registers | 16-bit data blocks (Holding Registers, Input Registers, Coils, etc.) |
4. Modbus RTU Frame Structure
A Modbus RTU message is transmitted as binary bytes:
| Field | Size (bytes) | Description |
|---|
| Slave Address | 1 | Identifies the slave device |
| Function Code | 1 | Defines the type of action |
| Data | N | Depends on function |
| CRC-16 | 2 | Error check |
Silent intervals:
- 3.5 character times = Start of frame
- 3.5 character times = End of frame
5. Wiring Example
4-wire RS-485 bus:
Controller (A,B,+,-) → Node1 (A,B,+,-) → Node2 (A,B,+,-) → … → Termination (120Ω)
6. Summary
| Category | RS-485 | Modbus |
|---|
| Type | Electrical standard | Communication protocol |
| Signaling | Differential | Binary (RTU) or ASCII |
| Role | Defines how bits travel | Defines how data is formatted |
| Layers | Physical | Application |
| Used Together | For robust long-distance serial communication | |