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Common mistakes made by CAN repeaters
CAN repeaters are widely used in large-scale CAN networks. They help extend the transmission distance, allow for changes in network topology, and isolate electrical noise, but they also add to the overall design cost. Some people might wonder: can two CAN transceiver chips be used to implement a basic 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. Commonly used CAN transceiver chips include models like NXP's PCA82C250/251, TJA1050/1051, and others. Since the pin configuration and functionality of the PCA82C250 and TJA1050 are compatible, we’ll use the TJA1050 as an example.
Now, let’s analyze why simply connecting two TJA1050 transceivers won’t work as a relay. The TJA1050 has eight pins: TXD (transmit data), RXD (receive data), VCC (connected to 5V), GND (ground), and CANH/CANL, which are the differential output lines for the CAN bus. In this setup, pins 8 and 5 are used for the CAN bus mode and reference ground, respectively.
In the RS-232 protocol, if you cross-connect the TXD and RXD lines of two devices and share a common ground, communication is possible. This leads some to think that connecting the TXD and RXD of two TJA1050s could create a basic relay. However, this approach doesn't work in practice.
As shown in Figure 2, although the circuit may seem functional on paper, it fails when actually connected to the CAN bus. When a dominant bit is sent from TJA1050(A) to TJA1050(B), the RXD of TJA1050(A) becomes low, and the TXD of TJA1050(B) also goes low. This causes the CAN output of TJA1050(B) to go high, leading to a feedback loop. Due to the built-in protection mechanism on the TXD pin of the TJA1050, the chip will disable itself once the maximum allowable time for a dominant level is exceeded. This makes the simple cross-connection method ineffective.
To properly implement a CAN repeater, a more sophisticated design is required. The hardware typically includes a microcontroller (MCU), a CAN controller, and a CAN transceiver. The MCU handles data buffering and forwarding, while the CAN controller formats the data into CAN frames. The transceiver then sends the data onto the CAN bus. Many MCUs, such as the NXP LPC2119, have integrated CAN controllers, making the design more compact.
For better isolation and reliability, optocouplers are often used to isolate the CAN controller from the transceiver, and isolated DC-DC modules are employed for power supply. This ensures both electrical and signal isolation between different parts of the network. While this increases complexity and cost, it significantly improves performance and safety.
Alternatively, using an isolated CAN transceiver like Zhiyuan Electronics’ CTM1051KT can simplify the design. It integrates isolated DC-DC, signal isolation, and bus protection, offering higher reliability and lower design risk. This makes it a great choice for applications where robustness and integration are key.
ZLG Zhiyuan Electronics offers intelligent CAN bridge repeaters with multiple isolated CAN interfaces. These devices not only act as relays but also support data storage and conversion between CAN networks operating at different speeds. Their main applications include:
1. Expanding the network by increasing the number of nodes and extending communication distance.
2. Configuring different baud rates independently for each channel to connect CAN networks with varying speeds.
3. Providing ID filtering and data conversion features to reduce bus load and eliminate interference.
4. Offering strong anti-interference capabilities to protect equipment from external noise or damage.
These intelligent repeaters are ideal for complex automotive and industrial applications where reliability and performance are critical.