CANopen bus interface circuit principle and design considerations
CAN bus is a serial communication network that effectively supports distributed control and real-time control. It has been widely used in the field of automatic control for its high performance and high reliability. In order to improve the system's drive capability and increase the communication distance, Philips 82C250 is used in practical applications as the interface between the CAN controller and the physical bus, that is, the CAN transceiver to enhance the differential transmission capability of the bus and the CAN control. The differential reception capability of the device. In order to further enhance the anti-interference ability, an opto-isolation circuit is often set up between the CAN controller and the transceiver. The typical CAN bus interface circuit principle is shown as in Fig. 1.
Fig.1 Typical CAN bus Interface Circuit Principle Drawing
1 Key Issues in Interface Circuit Design
1.1 Optical isolation circuit
Although the opto-isolated circuit can enhance the anti-interference ability of the system, it will also increase the transmission delay time of the effective loop signal of the CAN bus, resulting in a reduction in the communication rate or distance. The 82C250 and other models of CAN transceivers are capable of instantaneous immunity, reduced radio frequency interference (RFI), and thermal protection. Current limiting circuits also provide additional bus protection. Therefore, if the field transmission distance is short and electromagnetic interference is small, optical isolation may not be adopted so that the system can reach the maximum communication rate or distance, and the interface circuit can be simplified. If the field environment requires opto-isolation, high-speed opto-isolators should be used to reduce the propagation delay time of the CAN bus's effective loop signal. For example, the high-speed optocoupler 6N137 has a short propagation delay of 48 ns, which is close to the TTL circuit. The level of delay time.
1.2 Power Supply Isolation
The power supply Vdd and Vcc used on both sides of the optoelectronic isolation device must be completely isolated. Otherwise, the optoelectronic isolation will lose its proper function. The isolation of the power supply can be achieved by a low-power DC/DC power supply isolation module, such as a 5 V dual-isolated low power DC/DC module with DIP-14 standard pinout.
1.3 Pull-up resistor
The transmission data input terminal TXD of the CAN transceiver 82C250 in FIG. 1 is connected to the output terminal OUT of the photocoupler 6N137. Note that the TXD must be connected to the pull-up resistor R3 at the same time. On the one hand, R3 ensures that the phototransistor in the 6N137 outputs a low level when it is turned on, and outputs a high level when it is off. On the other hand, this is also a requirement of the CAN bus. Specifically, the status of the 82C250's TXD terminal determines the status of the high and low CAN voltage input/output terminals CANH, CANL (see Table 1). The CAN bus specification states that the bus should be recessive during idle periods. That is, the default state of the nodes in the CAN network is recessive. This requires that the default state of the TXD side of the 82C25O is logic 1 (high level). For this reason, it must be ensured through R3 that the status of the TXD terminal is logic 1 (high level) when no data is transmitted or an abnormal condition occurs.
|TXD status||CANH Level(V)||CANL Level(V)||CAN Bus Status|
1.4 Bus Impedance Matching
Two 120Ω resistors must be connected to the end of the CAN bus. They play an important role in bus impedance matching and cannot be omitted. Otherwise, the reliability and anti-interference of the bus data communication will be greatly reduced, and even communication may not be possible.
1.5 Other anti-jamming measures
To improve the interference immunity of the interface circuit, consider the following measures:
(1) Connect two 30 pF small capacitors in parallel between the CANH and CANL terminals of the 82C25O and the ground to filter out high-frequency interference on the bus and prevent electromagnetic radiation.
(2) Connect a 5Ω resistor in series between the CANH and CANL terminals of the 82C250 and the CAN bus to limit the current and protect the 82C250 from overcurrent.
(3) Add a 100 nF decoupling capacitor between the power supply terminal of the 82C25O, 6N137 and other integrated circuits and the ground to reduce interference.
The interface circuit is an important part of the CAN bus network. Its reliability and security directly affect the operation of the entire communication network. This article summarizes several key issues that should be noted in the design of CAN interface circuits. Only by grasping the key in the design can we improve the quality and performance of multiple interface circuits and ensure that the CAN bus network operates safely and reliably.