Both CAN and 485 are commonly used field buses in industrial communication. All engineers must be very familiar with the bus isolation scheme, but they may encounter the situation that the bus adopts the isolation scheme and still has abnormal communication. This article will take you to discuss the bus isolation scheme. How to ground?
1. Preface In order to ensure the communication stability of the bus network, the communication interface is usually isolated. The main purpose of isolation:
l Safety considerations: protect equipment and personal safety, and isolate potential high-voltage hazards;
l Improve the stability of communication: eliminate the influence of ground potential difference;
l Improve the reliability of the device: eliminate the influence of the ground loop;
l Low coupling: improve compatibility between systems;
At present, there are two schemes to achieve bus isolation: using discrete components to build or using integrated modules.
Second, the principle of isolation and grounding The addition of isolation to the bus can ensure stable and reliable communication of the bus, but the equipment with an isolated communication interface will show completely different ESD characteristics in a complex environment or installation state. Understand the effect of ESD on the interface. The impact mechanism can be targeted to increase protection devices and improve the ESD capability of the isolation interface. Taking the communication interface with isolated CAN or RS-485 as an example, the action mechanism of ESD in common equipment states is analyzed, and corresponding improvement measures are proposed.
1. In this state, the bus side is floating, the device control side has access protection ground (PE), the bus side reference ground is floating, and there is no connection with PE, as shown in the figure below.
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Next is the analysis:
l Assuming that sufficient protection measures are taken on the control side, when the interface on the control side is subjected to electrostatic discharge, the energy is discharged to the PE through the protector on the control side, which basically has no effect on the isolated communication interface, as shown in the left figure below.
l When the bus interface is subjected to electrostatic discharge, since the bus side is floating, the energy can only be discharged through the equivalent capacitance Ciso of the isolation barrier. Since Ciso is very small, only a few picofarads to ten picofarads, Ciso is rapidly charged, The voltage Viso at both ends will be very high, almost equal to the discharge voltage. The voltage is all applied to the isolation barrier of the isolation interface module. If the voltage exceeds the voltage withstand range of the isolation barrier, the internal isolation barrier will be damaged, as shown in the right figure below. Note: For the general isolation interface module, the electrostatic discharge voltage that the isolation barrier can withstand is only 4kV. For higher-level 6kV or 8kV static electricity, it is very fragile and easily damaged.
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2. The device control test is suspended In this state, the reference ground of the device control side is suspended without any connection with PE, and the bus side has access to the protective ground (PE), as shown in the figure below.
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Next is the analysis:
l When the bus-side interface is subjected to electrostatic discharge, the electrostatic energy is discharged to the PE through the internal bus-side device of the isolation interface module. However, if the ESD energy exceeds the ESD immunity of the internal bus-side device of the interface module, the bus interface may be damaged, as follows: Picture on the left.
l When the control side interface is subjected to electrostatic discharge, the energy can only be discharged through the equivalent capacitance Ciso of the isolation barrier because the control side is suspended. Since Ciso is very small, the voltage Viso at both ends will be very high, and the voltage is all applied to the isolation interface module. If the voltage exceeds the voltage withstand range of the isolation barrier, the internal isolation barrier will be damaged, as shown in the right figure below.
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3. Improvement measures For the above two situations, the isolation interface module needs to have effective electrostatic protection. It is recommended to increase the Cp, Rp and TVS when designing the isolation interface to improve the ESD immunity of the isolation interface.
l Function of capacitor Cp: reduce the pressure on the isolation barrier and provide a low-impedance path for electrostatic energy. Most of the electrostatic energy is discharged through this capacitor. In order to achieve good results, the value of Cp should be much larger than Ciso, and it is recommended to take 100pF~1000pF between.
l The role of TVS tube: For static electricity on the bus side, the electrostatic energy will be discharged through the protective device. Note: its on-voltage must be less than the maximum voltage that the isolation interface can withstand, and at the same time greater than the signal voltage; when the communication speed is high, or the number of nodes When there are many, it is also necessary to pay attention to select devices with small equivalent capacitance as much as possible, so as not to affect the normal communication of the bus.
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Note: If the product has no safety requirements, a large-value bleeder resistor, such as 1M, can be connected in parallel with Cp to prevent electrostatic accumulation; if there are safety requirements, it is generally necessary to remove the bleeder resistor and select a safety capacitor.
3. The perfect bus interface protection circuit has only analyzed the mechanism of ESD, but as industrial products have higher and higher requirements on the EMC level of the communication interface. Many applications require IEC61000-4-2 electrostatic discharge level 4, IEC61000-4-5 surge immunity level 4 requirements. The ESD and surge protection levels of general transceivers are relatively low. For example, the isolation withstand voltage of the CTM1051M isolated CAN transceiver is 2500VDC. In the case of bare metal, the ESD and surge levels are both low. Therefore, it is necessary to increase the peripheral circuit and improve the EMC level of the communication port.

Taking CAN bus as an example, the above picture shows a perfect peripheral recommended circuit. The GDT is placed at the forefront to provide first-level protection. When lightning strikes and surges occur, the GDT instantly reaches a low-resistance state, providing a relief channel for the instantaneous large current, and clamping the voltage between CAN_H and CAN_L within the range of 20 volts . The actual value can be adjusted according to the comprehensive consideration of protection level and device cost. It is recommended to use PTC for R3 and R4, and fast recovery diodes are recommended for D1~D6. The parameter table is as follows.
Table 1 Description of the recommended parameter table:

In addition, another solution is to use ZLG's SP00S12 surge protection module, which can be used in various signal transmission systems to suppress harmful signals such as lightning strikes, surges, and overvoltages, and protect the device signal ports. With ZLG's fully isolated CTM or SC series of isolated CAN transceivers, as shown below. It can greatly improve the integration of the product, and at the same time greatly shorten the development cycle.
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Fourth, the necessity of grounding the resistance-capacitance loop The principle of grounding after bus isolation and the recommended circuit are described above. I think everyone is very clear. At the scene, many customers will mention why resistance-capacitance grounding is required after bus isolation? Here is a brief description for you:

l Capacitance: From the perspective of EMS (Electromagnetic Immunity), this capacitor is to reduce the possible impact (the impact of high-frequency interference signals based on the ground level on the circuit) under the premise that PE is well connected to the ground. It is to suppress the transient common mode voltage difference between the circuit and the interference source. In fact, it is best to connect GND directly to PE, but direct connection may not be operational or safe. From the perspective of EMI (electromagnetic interference), if there is a metal casing connected to the PE, there is this high-frequency path, which can also avoid the radiation of high-frequency signals.
l 1M resistor: This is for ESD (Electrostatic Discharge) testing. Because of this kind of system that connects PE and GND with capacitors (floating system), during the ESD test, the charge injected into the circuit under test has nowhere to be released, and it will gradually accumulate, raising or lowering the level of GND relative to PE, accumulating To a certain extent, beyond the voltage range that can be tolerated at the weakest point of insulation between PE and the circuit, there will be discharge between GND and PE. In a few nanoseconds, tens to hundreds of amperes will be generated on the PCB. current, which is enough to shut down any circuit with an EMP (electromagnetic pulse), or damage the device at the signal connection where the insulation between the PE and the circuit is weakest. But sometimes PE and GND cannot be directly connected, so use a 1~2M resistor to slowly release the charge to eliminate the voltage difference between the two. Of course, the value of 1~2M is selected according to the ESD test standard, because the maximum number of repetitions specified in IEC61000 is only 10 times/second. If you do a non-standard ESD discharge of 1000 times/second, then the resistance of 1~2M I think The accumulated charge cannot be released.