MODBUS Serial
Industrial Communication: Modbus Serial (RTU/ASCII)
Developed by Modicon in 1979, Modbus Serial is the most enduring and widely used communication protocol in industrial history. From simple sensors to complex variable frequency drives (VFDs), it remains the "universal language" of the factory floor.
1. Architecture: Master-Slave Hierarchy
Modbus Serial operates on a strict request-response basis. Communication is always initiated by a single Master unit.
- The Master: Only one per network. It polls specific slaves for data or sends control commands.
- The Slaves: Up to 247 devices. They remain silent on the bus until they receive a request matching their unique Slave ID.
- The Transaction: If a slave receives a request, it processes the command and returns a response. If the CRC (Cyclic Redundancy Check) fails, the request is ignored.
2. Transmission Modes: RTU vs. ASCII
Depending on the hardware capability and diagnostic needs, data can be encoded in two ways:
| Feature | Modbus RTU | Modbus ASCII |
|---|---|---|
| Data Encoding | Binary (8-bit) | Hexadecimal ASCII (readable) |
| Efficiency | High (faster, denser packets) | Low (roughly 50% slower) |
| Error Check | CRC (Cyclic Redundancy Check) | LRC (Longitudinal Redundancy Check) |
3. Physical Layer: RS-485 and RS-232
The choice of physical media determines the network's distance and capacity:
- RS-485: The gold standard for industry. Supports multi-drop (many devices on one line) and distances up to 1200 meters using differential signaling.
- RS-232: Limited to point-to-point communication (one master to one slave) over short distances, typically under 15 meters.
4. Data Structures: The Register Map
Modbus treats all data as simple bits or 16-bit integers stored in specific memory tables:
| Data Type | Access | Typical Application |
|---|---|---|
| Coils | Read/Write | Digital outputs (Relay state, Solenoid valve) |
| Discrete Inputs | Read Only | Digital inputs (Limit switch, Push button) |
| Input Registers | Read Only | Analog data (Current temperature, Pressure) |
| Holding Registers | Read/Write | Configuration parameters (Speed setpoint, PID values) |
5. Hubbox & Modbus: Bringing Legacy to the Cloud
Hubbox Connect serves as the bridge that connects these 40-year-old serial protocols to 21st-century IT infrastructures.
- Node-RED Logic: Map your register addresses visually. Convert raw 16-bit values into floating-point numbers or scaled engineering units in seconds.
- Diagnostic Monitoring: Monitor bus health in real-time. Hubbox can alert you if specific Slaves are timing out or if electrical noise is causing CRC errors.
- Remote Engineering: Use the Hubbox secure tunnel to connect your PLC programming software to the field serial bus as if you were standing next to the machine.
Why Modbus Serial Persists
- Universal Adoption: Virtually every industrial device supports it, ensuring brand interoperability.
- Minimal Cost: Requires no expensive Ethernet chips in sensors, only a simple transceiver.
- Simplicity: Easy to troubleshoot with basic tools like a multimeter or a simple serial sniffer.
For Modbus register mapping templates and connection diagrams: www.hubbox.io
