The Controller Area Network (CAN) is a communication protocol developed by BOSCH in the mid-1980s primarily for the automotive industry. Initially designed to facilitate data transmission between electronic control units in vehicles, CAN has gradually expanded to various other industries, including industrial automation, medical devices, and shipbuilding. CAN provides a low-cost, efficient, flexible, and reliable communication infrastructure with several advanced features.
Key Features of the CAN Protocol
Multimaster Support
Devices connected to a CAN network can send data simultaneously, without being dependent on a specific order. This means each device has the right to transmit data independently. If multiple devices attempt to send data simultaneously, a priority order is determined. In the CAN protocol, priority is based on the message's ID value. The message with the lowest ID is transmitted first. This mechanism prevents conflicts during data transmission and ensures efficient network management.
Message Transmission and Collision Resolution
CAN ensures that data is transmitted in a specific format. Each message consists of a header, data field, and a control field. When a device begins transmitting data, other devices in the network "hear" this data. If two devices attempt to send data simultaneously, the priority of the message is determined by the ID value. This prevents collisions and guarantees that the data is transmitted in the correct order. CAN uses Carrier Sense Multiple Access with Collision Resolution (CSMA/CR), providing fair sharing of the transmission medium.
Flexibility and System Design
In a CAN network, devices can communicate with each other without affecting the functioning of other devices when a device is added or removed. This makes the CAN protocol highly flexible. Adding or removing a device in the network does not require changing the software, hardware, or network applications of other devices. This feature is especially beneficial in rapidly changing industries such as automotive.
Speed and Performance
CAN offers different speed options for data transmission. All devices in the CAN network must transmit data at the same speed. As the speed increases, the number of devices that can be connected to the network decreases. The CAN protocol can achieve transmission speeds of up to 1 Mbps. However, as the number of devices in the network increases, the transmission speed decreases because more devices transmitting data can cause delays in communication.
Error Management and Recovery
The CAN protocol has mechanisms for detecting communication errors, reporting errors, and recovering from errors. Errors can be caused by bardware and software issues. When an error is detected, the device that detects titnotifies all other devices, and data transmission is repeated until the error is corrected. This ensures the integrity of the data in the network. Additionally, devices that continuously generate errors are excluded from the network, ensuring that communication proceeds smoothly for other devices.
Connectivity and Topology
The CAN protocol allows multiple devices to be connected in the network. This connection is typically configured in a bus topology, where devices transmit data over two main lines: CAN_H (high) and CAN_L (low). These two lines enable the transmission of the data in a differential signal format. The CAN protocol offers a flexible architecture based on the number of devices in the network, but a balance must be struck between the number of devices and the transmission speed.
The Importance of Termination Resistors in CAN Protocol
Termination Resistors
Termination resistors, placed at both ends of the network, are crucial for the proper functioning of a CAN network. Their primary function is to prevent reflection issues. Reflection occurs when signals are not transmitted correctly during data transmission and bounce back, causing data loss in the network. If termination resistors are not placed correctly, these reflections can disrupt communication, thereby preventing the network from operating properly.
In general, 120-ohm termination resistors should be placed at both ends of the network. This ensures that signal reflections are avoided and data transmission accuracy is maintained. The importance of termination resistors increases as the length of the network cable increases. If these resistors are missing or incorrectly placed, errors will occur in the network, leading to data loss between devices.
Termination Resistor Configuration
Correct configuration of termination resistors is essential. Placing 120-ohm resistors at both ends of the network is typically sufficient for most CAN networks. However, factors such as network topology and length may require additional resistors or alternative configurations. Additional termination resistors may be necessary for long cables and extensive networks to prevent signal degradation.
The CAN protocol provides a reliable and flexible communication infrastructure used across various industries, from automotive to industrial automation. With multimaster support, message prioritization, flexibility, and robust error management, CAN ensures high performance and reliability in data transmission. Moreover, correctly placed termination resistors prevent data loss and signal reflections, contributing to the smooth operation of the network. These features have made the CAN protocol one of the most important communication solutions in various industries today.
