S7-200 PLCRS485 Interface

I. RS485 interface circuit diagram inside the S7-200PLC: see the attachment for circuit diagram
In the figure, R1 and R2 are common resistance with a resistance of 10 euro. Their function is to prevent the generation of an over-current burned-out chip when the RS485 signal D + and D-short circuit are used. Z1 and Z2 are the 6 V clamping voltage, zonea diode with a maximum current of 10A. The 24 V power supply and the 5 V power supply are not isolated. When the common mode dry disturbance voltage on the D + or D-line is input, by the bridge rectification circuit and Z1, Z2 can clamp the common mode voltage in + 6. 7 V to protect the RS485 chip SN75176 (the allowed common-mode input voltage range of the RS485 chip is-7V ~ + 12 V ). The protection circuit can withstand a common-mode interference voltage of 60 W. The protection circuit and the chip have no anti-static measures.

Ii. Analysis of Common Failures:
When the RS485 port of the PLC is connected to the computer through an unisolated PC/PPI cable, the connection between the PLC and the PLC, or the communication between the PLC and the inverter, touch screen, and so on, the communication port is damaged, common damages are as follows:
● R1 or R2 is burned out, and Z1, Z1, and SN75176 are in good condition. This is because there is a large transient interference current through R1 or R2, bridge rectification, Z1 or Z1 to the ground, Z1, Z2 can withstand the impact of the maximum 10A current, the transient power generated by this current on R1 or R2 is: 102 × 10 = 1000 W, which of course will be burned out.
● The SN75176 is damaged, and R1, R2, Z1, and Z2 are in good condition. This may be caused by electrostatic impact or transient overvoltage speed faster than Z1 and Z2. static electricity is everywhere, and only human mode will generate ± 15kV static electricity.
● Z1, Z2, SN75176 are damaged, and R1 and R2 are in good condition. This may be due to the transient interference voltage of high voltage and low current, which breaks down Z1, Z2, and SN75176. Due to the low current and short occurrence time, R1 and R2 will not generate or burn out heat.
The above analysis shows that the main cause of the damage to the PLC interface is the transient overvoltage and static electricity. Many reasons for the generation of transient overvoltage and static electricity are also complicated, for example, because the PLC has 24 V power supply and 5 V power supply, the output terminal L + and M of 24 V power supply may change the ground potential due to the hybrid power supply of other devices, resulting in the common mode voltage exceeding the permitted range. Therefore, the EIA-485 standard requirements of each RS485 interface signal with a low resistance wire connected together to ensure that the ground potential of each node is equal, eliminate the ground circulation!
When the unisolated connection cable is plugged in, due to the inconsistent potential at both ends of the circuit, there are many inductor, capacitor and other devices, transient overvoltage or overcurrent is inevitable.
The transient overvoltage or overcurrent generated by other devices connected to the RS485 BUS also flows into the PLC. The more devices connected to the bus, the more factors that generate the transient overvoltage.
When a communication line is long or there is an outdoor air line, lightning will inevitably cause overvoltage on the road. Its energy is often huge, and users often say with frustration: "dozens of connected PLCs are all broken! ".
Iii. solution:
1. Consider from the PLC:
● The 24 V power supply and 5 V power supply are isolated from each other using an isolated DC/DC. The same is true for the PROFIBUS interfaces of Mitsubishi, OMRON, Schneider PLC, and Siemens.
● High-speed RS485 chips with complete protection measures, such as electrostatic protection, overheating protection, and input failure protection, are used, such as SN65HVD1176D and maxcompute Esa. These chips are generally priced at dozens of Yuan to dozens of Yuan, the price of SN75176 is only 1.5 yuan.
● Use a new protection device TVS or BL Surge Absorber with faster response speed and higher transient power. For example, the clamping voltage of P6KE6.8CA is 6.8 V, and the transient power is 500 W, the BL device can withstand large current impacts above.
● R1 and R2 adopt positive temperature coefficient self-recovery insurance PTC, such as JK60-010, under normal circumstances the resistance value is 5 euro, does not affect normal communication, when the surge impact, when the large current flows through PTC and the protection device TVS (or BL), the PTC resistance value will suddenly increase, rapidly reducing the surge current.
2. external considerations from the PLC:
● Use isolated PC/PPI cables to avoid cheap non-isolated cables (especially in industrial scenarios ). The early PC/PPI cables (6ES7 901-3BF00-0XA0) produced by Siemens are not isolated and are now changed to isolated cables!
● The RS485 port of PLC is connected with an isolated bus connector.
● Connected with the PLC third-party equipment, such as inverter, touch screen and other RS485 ports are isolated by RS485 isolator BH-485G, so that the RS485 nodes are not "Electric" contact, there is no ground circulation, even if a node is damaged, it will not be damaged by other nodes.
● The RS485 communication line uses a dedicated PROFIBUS shield cable to ensure that the shield layer is connected to the housing of each device and finally to the Earth.
● For systems with air cables, it is best to set up special lightning protection facilities on the bus.

Find a solution to the S7-200 communication port damage

In our large number of S7-200PLC, from time to time there is a communication port damage, resulting in no connection to the PC or can not communicate, in the disintegration of the PLC found that there is a chip in the PLC communication port-75176, this is the communication interface chip. There are 5 FB around the chip, marking FB1 ~ FB5 is actually five insurances. When communication fails, one of these five insurances is burned out. It can be replaced by the same type of insurance and can be directly short-circuited by wires. Generally, the problem can be solved. But pay attention to the replacement, because the patch is very small, the space is also small, so do not short circuit during welding.