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Common mistakes made by CAN repeaters
CAN repeaters are widely used in large CAN networks. They help extend the communication distance, allow for changes in network topology, and isolate electrical noise, but they also increase the overall design cost. Some people might wonder: can two CAN transceiver chips alone be used to implement a CAN relay function? Is it really that simple?
First, let’s take a closer look at CAN transceivers. The ISO 11898 standard defines high-speed communication in automotive applications using the CAN bus protocol. A CAN transceiver acts as the interface between the data link layer and the physical layer of the protocol. Common examples include NXP's PCA82C250/251 and TJA1050/1051. Since the pinout and functionality of these chips are similar, we'll use the TJA1050 as an example.
Now, let's analyze why simply connecting two TJA1050s won’t work as a CAN relay. The TJA1050 has eight pins: TXD (transmit data), RXD (receive data), VCC (5V power supply), GND (ground), and CANH/CANL (differential outputs). As shown in Figure 1, when connected in a basic cross-connection setup, it may appear to function, but in reality, this configuration fails due to internal chip behavior.
The issue arises because of the self-feedback mechanism within the TJA1050. When a dominant signal is received by one transceiver, it causes the other to send out a dominant signal as well. This creates a loop where both devices continuously read and transmit the same signal, leading to a failure in communication. Additionally, the TJA1050 includes protection against excessive dominant signals on the TXD pin, which can disable the chip if the threshold is exceeded.
So, what is the correct way to design a CAN repeater? A proper design typically includes an MCU (microcontroller unit), a CAN controller, and a CAN transceiver. The MCU handles data buffering and forwarding, while the CAN controller converts data into the appropriate CAN frame format. The transceiver then sends the data onto the CAN bus. Many MCUs come with built-in CAN controllers, such as the LPC2119 from NXP.
To improve reliability and safety, optocouplers are often used to isolate the CAN controller and transceiver, and isolated DC-DC modules are used for the transceiver driver. This ensures electrical isolation between different parts of the network, enhancing performance and reducing the risk of damage from electrical surges or interference.
Another option is to use an integrated isolated CAN transceiver like the CTM1051KT from Zhiyuan Electronics. It combines signal isolation, DC-DC conversion, and CAN transceiver functions in one package, offering higher integration, better reliability, and lower costs compared to traditional designs.
For more advanced applications, intelligent CAN bridge repeaters, such as those in ZLG Zhiyuan's series, provide additional features. These devices offer multiple isolated CAN interfaces, allowing for network expansion, speed adaptation, and data filtering. They can store and forward data between different CAN networks, support flexible baud rate configurations, and filter out unwanted messages to reduce bus load.
These intelligent repeaters also provide strong immunity to electromagnetic interference, ensuring reliable operation even in harsh environments. By isolating the network, they protect connected devices from potential damage caused by noise or voltage spikes.
In summary, while using two CAN transceiver chips might seem like a simple solution, it doesn't account for the complex behaviors and protections built into modern CAN hardware. A properly designed CAN repeater not only improves network performance but also enhances reliability and safety.