1, common fault phenomena When the rs-485 port of the PLC is connected to the computer via non-isolated pc/ppi cable, the connection between PLC and PLC or the communication between PLC and inverter, touch screen, etc., the communication port is damaged. The common damages are as follows: (1) r1 or r2 is blown, and z1, z2 and sn75176 are intact. This is due to the large transient interference current through r1 or r2, bridge rectification, z1 or z1 to ground, z1, z2 can withstand the impact of the maximum 10a current, and the transient power generated by this current on r1 or r2 It is: 102 × 10 = 1000w, of course, it will be blown. (2) sn75176 is damaged, r1, r2 and z1, z2 are intact. This may be caused by electrostatic shock or transient overvoltages faster than z1, z2. Static electricity is ubiquitous, and only human body mode produces ±15kv of static electricity. (3) z1 or z2, sn75176 is damaged, r1 and r2 are intact. This may be due to the high voltage and low current transient interference voltage to break down z1 or z2 and sn75176. Since the current is small and the generation time is short, r1 and r2 are not burnt. 2, the cause of the failure analysis The main reason for the PLC interface damage is that the transient overvoltage and static electricity are caused by transient overvoltage and static electricity. Many reasons are also complicated, such as 24v power supply and 5v power supply in the PLC, 24v. The output terminals l+, m of the power supply may cause the ground potential to change due to the hybrid power supply of other devices, thereby causing the common mode voltage to exceed the allowable range. Therefore, the eia-485 standard requires that the signal ground of each rs485 interface be connected together with a low-resistance wire to ensure that the ground potentials of the nodes are equal and the ground loop is eliminated. (1) When the unconnected connecting cable is plugged and unplugged, there are many devices such as inductors and capacitors in the circuit with unequal potentials at both ends. Transient overvoltage or overcurrent will inevitably occur during plugging and unplugging. Based on this consideration, when the communication connector is plugged and unplugged, the device is in a power-off state as much as possible. (2) Transient overvoltages or overcurrents generated by other devices connected to the rs-485 bus will also flow into the PLC. The more devices connected to the bus, the more transient overvoltages are generated. (3) When the communication line is long or there is an outdoor overhead line, lightning is a disturbance that must be considered. Lightning is the main source of natural interference, and interference from lightning can be transmitted thousands of kilometers away. The time domain waveform of the lightning interference is superimposed into a large spike on the background of the random pulse. This huge spike of energy will inevitably cause overvoltage on the line, causing damage to the equipment connected to the communication network such as PLC, which should be avoided or reduced. Degree, reduce losses. 3, the solution Considered from within the PLC (1) Isolating the 24v power supply from the 5v power supply using isolated dc/dc. We analyzed the Mitsubishi, Omron, Schneider PLC and Siemens profibus interfaces. (2) Select high-stop rs-485 chips with static protection, overheat protection, input failure protection and other protection measures, such as: sn65hvd1176d, max3468esa, etc. The price of these chips is generally between ten yuan and tens of yuan, while the sn75176 The price is only 1.5 yuan. (3) Using a new type of protection device with faster response and greater transient power, tvs or bl surge absorbers, such as p6ke6.8ca, with a clamping voltage of 6.8v, with a transient power of 500w, bl devices can fight High current impact above 4000a. If you use a chip without fault protection, such as sn75176, you can do some processing on the software to avoid communication anomalies. That is, before entering normal data communication, the host drives the bus to be greater than +200 mv in advance and keeps it for a period of time, so that the receivers of all nodes generate a high level output. Thus, when valid data is issued, all receivers can correctly receive the start bit and receive the complete data. (4) r1 and r2 use self-recovery insurance ptc with positive temperature coefficient, such as jk60-010. Under normal conditions, the resistance value is 5 ohms, which does not affect normal communication. When subjected to surge, large current flows through ptc and Protection device tvs (or bl), the resistance value of ptc will suddenly increase, so that the inrush current is rapidly reduced. Considering from outside the PLC (1) Use isolated pc/ppi cables and try not to use inexpensive non-isolated cables (especially at industrial sites). The pc/ppi cable (6es7901-3bf00-0 xa0) produced by Siemens in the early days is not isolated and is now changed to an isolated cable. (2) PLC rs-485 port is connected with isolated bus connector, such as pfb-g, the rate is automatically adjusted from 0 to 1.5 mbps, the shape and usage are the same as the non-isolated bus connection of Siemens. (3) Third-party equipment connected with PLC, such as inverter, touch screen and other rs-485 ports are isolated by using rs-485 isolator bh-485g, so there is no "electric" connection between each rs-485 node. There is no ground loop circulation, even if a node is damaged, it will not be damaged by other nodes. (4) Good grounding is an important condition for the safe and reliable operation of industrial control systems, especially for industrial communication networks. In an industrial communication network, there are at least three separate ground lines that are grounded at one point. The first one is the low-level circuit ground (ie, the signal ground), including digital ground, analog ground, signal ground, and DC ground; the second is the noise ground, that is, the ground of the relay, motor, and high-power circuit. The third is the chassis grounding point, which is used exclusively for the mechanical casing, fuselage, frame, and ground. This ground wire should be connected to the ground wire of the AC power supply. The AC power ground wire should be connected to the PGND cable to avoid interference caused by uneven points on the bulletin ground. The rs-485 communication line uses a dedicated shielded cable for the profibus bus to ensure that the shield is connected to the outer casing of each device and finally connected to the ground. (5) For systems with overhead lines, it is best to set up special lightning protection facilities on the bus. Small Gear Motor,Customized Dc Motor,Customized Dc Gear Motor,Dc Permanent Magnet Motor NingBo BeiLun HengFeng Electromotor Manufacture Co.,Ltd. , https://www.hengfengmotor.com