The rapid development of digital electronic information technology has significantly enhanced the role of the manufacturing industry globally, making the design of various devices more electronic, digital, and networked. The ECCT product, introduced by Emerson CT, is a specialized PLC controller with CAN bus protocol tailored for the textile industry. It not only meets the basic I/O requirements of textile processes but also seamlessly integrates the CAN bus protocol, enabling users to easily connect and integrate different systems. Devices are connected through the CAN protocol, and this article explores the application of the CAN bus function on the Emerson CT PLC.
**Introduction to CAN Bus Basics**
The CAN bus (Controller Area Network) was first developed by Bosch in Germany. Today, it is widely used in industrial applications due to its robust features. Here are some key characteristics:
1. **Multi-Master Architecture**: Unlike RS-485, which uses a master-slave structure and relies on polling, the CAN bus operates in a multi-master mode. Each node can compete for bus access using a bit-by-bit arbitration mechanism based on message identifiers. This ensures real-time communication and higher reliability.
2. **No Station Address Coding**: Instead of assigning station addresses, the CAN protocol uses message identifiers, allowing an unlimited number of nodes on the network. Adding or removing equipment does not disrupt the system, and multiple nodes can receive the same data simultaneously. This makes the communication highly reliable and flexible.
3. **Robust Physical Layer**: The CAN bus connects via CANH and CANL lines, ensuring that no bus errors occur due to simultaneous transmission, as seen in RS-485. If a serious error occurs, the node automatically shuts down, preventing the entire network from failing.
4. **Built-in Protocol Support**: The CAN protocol is implemented by the controller chip and interface, reducing the complexity of system development and shortening the development cycle. This is a major advantage over RS-485, which requires custom implementation.
5. **High Speed and Long Distance**: CAN bus supports a maximum communication rate of 1 Mbps at 5 Kbps, with a transmission distance of up to 10 km using twisted-pair cables. Its high speed, reliability, and cost-effectiveness make it ideal for many industrial applications.
**CAN Bus vs. RS-485 Comparison**
| Feature | RS-485 Method | CAN Bus |
|----------------------|----------------------|---------------------|
| Topology | Linear topology | Linear topology |
| Transmission Medium | Twisted pair | Twisted pair |
| Hardware Cost | Very low | Higher per node |
| Bus Utilization | Low | High |
| Network Structure | Single master | Multi-master |
| Data Rate | Low | Up to 1 Mbps |
| Fault Tolerance | None | Built-in error handling |
| Communication Failure| High | Extremely low |
| Effect of Node Error | Can crash the whole network | No impact on other nodes |
| Communication Distance | <1.2 km | Up to 10 km (at 5 kbps) |
| Maintenance Cost | Higher | Very low |
**CAN Bus System Structure**
Each CAN node requires a CAN controller chip and appropriate interface circuitry. Nodes are connected via shielded twisted-pair cables. Matching resistors (120 ohms) are placed at the first and last nodes to achieve maximum communication speed. The system structure is as follows:
**CAN Message Format**
The CAN protocol supports two message formats: CAN 2.0A (standard) and CAN 2.0B (extended). The main difference is the length of the identifier (ID), which is 11 bits in standard format and 29 bits in extended format.
- **Standard Format (CAN 2.0A)**:
- Start of Frame (SOF): A dominant bit (0).
- Arbitration Field: 11-bit ID + RTR bit.
- Control Field: Includes IDE, RBO, and DLC.
- Data Field: 0–8 bytes.
- CRC: Cyclic Redundancy Check.
- ACK Field: Acknowledgment bit and delimiter.
- End of Frame (EOF).
- **Extended Format (CAN 2.0B)**:
- Similar structure, with 29-bit ID instead of 11 bits.
**Emerson CT PLC with Integrated CAN Bus Function**
The ECCT PLC supports both CAN 2.0A and 2.0B protocols, with baud rates ranging from 5 to 100 kbps. It can be configured using Controlstar FOR ECCT.
**Steps to Use CAN Communication:**
1. **Basic Settings**: In the Project Manager, double-click “System Block,†select “CAN Port Settings,†choose “Free Protocol,†and set the protocol type (2.0A or 2.0B) and baud rate.
2. **Data Transmission**: Use the CANXMT command to send data. For example, sending four words (16#1122, 16#3344, 16#6789, 16#1234) with ID 5.
3. **Data Reception**: Use the CANRCV instruction or interrupt functions. Interrupts are recommended for beginners.
**Application Example in Textile Machinery**
The ECCT PLC has been successfully applied in textile machinery, improving system stability and communication speed. The overall system architecture includes multiple nodes connected via CAN bus.
**Usage Instructions:**
1. Set the CAN port in the “System Block†as needed.
2. For sending data, use the CANXMT instruction with proper configuration.
3. For receiving data, enable interrupts and write the corresponding program to process received messages.
This implementation has proven effective, leading to better performance and user satisfaction.
**Summary**
Due to its simple protocol and hardware-based implementation, CAN bus programming is straightforward and efficient. After integrating CAN communication into the textile system, the performance improved significantly, and the system became more stable, earning positive feedback from users.
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