Did you experience occasional communication errors while debugging with a CAN or RS-485 bus? Or can't receive data? Bus that has been used normally, suddenly appeared a wide range of errors, or node damage? Are you still overwhelmed with these questions and puzzled? Using bus isolation may help you solve problems easily.
In actual bus applications, have you encountered the following problems:
1. Equipment and Personal Safety - Potential High Voltage Hazards: The use environment of the CAN and RS-485 buses is very complicated, and high voltages may be present in some harsh applications. It is extremely prone to electric shock and endangers the safety of people or equipment.
2. The remote can not receive data - ground potential difference exists: In many practical applications, the communication distance can reach several kilometers, and the distance between nodes is very long. Designers often directly connect the reference point of each node to the local earth as the return of the signal. It seems like a normal and reliable practice, but there is a great hidden danger! Even if you debug a normal system, you may experience various problems after a period of use.
The problem that is often overlooked is that there may be a large potential difference between the two nodes! ! ! The actual earth is not the ideal "0" potential. The earth is also a conductor and there is also an impedance. When a large current flows through the earth, there is also a potential difference across the ground where the current flows. As shown in Figure 1.
figure 1
If the distant communication nodes are directly connected to their respective local grounds, the ground potential difference will be superimposed on the output of the bus transmitter in the form of a common-mode voltage. The signal after superposition may far exceed the total capacity that the receiver can bear. The input voltage range of the module is too high to properly receive the signal, which can seriously damage the transceiver. The common CAN and RS-485 transceivers have a small common-mode input range. For example, the SN65HVD251 and SP3085 transceivers only support the -7~+12V common-mode input range. The earth flows through large-scale equipment and injects large currents. The resulting ground potential difference can be as high as several volts, tens of volts, or even hundreds of volts, far beyond the voltage range that the transceiver can withstand.
3. Data error without indication, or device damage - ground loop effect
Since there is a potential difference between the nodes in the earth, is it possible to reconnect the two nodes directly with a single line? Wrong! This will only make the situation worse, and this long wire will form an enormous loop with the earth!
It is believed that everyone knew in their student days that a closed coil would generate current in a changing magnetic field. 50 Hz AC power lines, large motors, etc. are all sources of AC magnetic fields. If the bus is close to or passing through these places, the ground loop can generate currents of up to several amps or even hundreds of amps. The common-mode voltage generated by the current flowing through the ground loop affects the normal communication of the bus.
In addition to a stable source of magnetic fields, transients such as surges, lightning strikes, and high-frequency noise on some power lines may be picked up by this giant "loop antenna" and cause communication anomalies.
figure 2
4. How to do?
Replace the CAN and RS-485 transceivers you are using with isolated CAN and RS-485 transceivers!
The isolated transceiver can electrically isolate the bus from the control circuit and block the high voltage outside the control system, which can effectively guarantee the safety of the operator's person and system. Not only that, isolation can suppress various common-mode interferences caused by ground potential differences and ground loops, ensuring uninterrupted, error-free operation of the bus under severe interference and other system-level noise conditions. As shown in Figure 3, using isolated transceivers can effectively prevent the formation of ground loops. The bus reference ground can fluctuate following the fluctuation of the common-mode voltage. The common-mode voltages are all borne by the isolation band, and the common-mode voltage becomes the bus signal. No longer visible, thus ensuring a stable and reliable bus communication.
image 3
5. How to achieve isolation?
There are two main methods to achieve bus isolation. One is to use isolation DC-DC, optocoupler and other discrete devices to achieve isolation between the node and the transceiver. Using this method, the circuit is relatively complex and large, and it is difficult to meet the high integration requirements of current electronic products. The other is the direct use of isolated transceivers, single products, simple design and high integration.
ZLG Zhiyuan Electronics focuses on the research and development of isolated power supplies and isolated communication products. Its CTM and RSM series isolated transceivers have been widely recognized by the market. These products retain the design concept of traditional isolation circuits, including power isolation and electrical isolation. More importantly, the perfect testing system and advanced technology ensure the consistency of the product, and it has many advantages such as waterproof, shockproof, and prolonged service life. The product adopts potting technology and can continuously protect the circuit board and electronic components from moisture, vibration, overheating, corrosion and radiation, and prolong the life of the product. The use of an integrated communications isolation module brings more value to customers, as shown in Table 1.
Table 1 Comparison of plans
ZLG Zhiyuan Electronics provides you with high quality and reliable isolated CAN transceiver, isolated RS-485 transceiver, power supply, signal isolation, isolation pressure up to 2500VDC and above, can protect your CAN, RS-485 bus!
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