PLC communication interface failure and solution——Xinjie

1. Common problem phenomena

When the RS-485 port of the PLC is connected to the computer through the non-blocking PC/PPI cable, the connection between the PLC and the PLC, or the communication between the PLC and the inverter, touch screen, etc., the communication port is damaged, and the more common damage conditions are as follows:

(1) R1 or R2 is burned out, Z1, Z2 and SN75176 are not missing. This is because there is a large transient disturbance current through R1 or R2, bridge rectification, Z1 or Z1 to the ground, Z1, Z2 can accept the impact of a maximum current of 10A, and the transient power of the current on R1 or R2 is: 102×10=1000W, of course, it will be burned off.

(2) SN75176 is damaged, R1, R2 and Z1 and Z2 are not missing. This is mainly due to electrostatic shock or transient overvoltage speed faster than the action speed of Z1 and Z2, static electricity is ubiquitous, and static electricity of ± 15kV can occur only in the human body model.

(3) Z1 or Z2 and SN75176 are damaged, and R1 and R2 are not missing. This may be due to the transient disturbance voltage of high voltage and low current, which breaks down Z1 or Z2 and SN75176, because the current is small and the occurrence time is short, so R1 and R2 will not heat up and burn out.

PLC communication interface failure and solution——Xinjie

PLC communication interface failure and solution——Xinjie

2. Analysis of the cause of the problem

From the analysis in 1, it is known that the primary reason for the damage of the PLC interface is due to the formation of transient overvoltage and static electricity, and the reasons for the occurrence of transient overvoltage and static electricity are many and complex, such as because the 24V power supply and 5V power supply in the PLC are grounded, and the output terminals of the 24V power supply L+ and M are mixed with other equipment, which may lead to the change of ground potential, and then the formation of common mode voltage beyond the allowable range. Therefore, the EIA-485 specification requires that the signal ground of each RS485 interface be connected together with a low-resistance wire to ensure that the ground potential of each node is equal, eliminating the ground circulation.

(1) When the unblocked connecting cable is plugged and unplugged live, because there are many inductors, capacitors and other equipment in the circuit with unequal potentials at both ends, transient overvoltage or overcurrent will inevitably occur at the moment of plugging and unplugging. Based on this consideration, when plugging and unplugging the communication connector, try to keep the device in a power-off condition.

(2) The transient overvoltage or overcurrent of other equipment connected to the RS-485 bus will also flow into the PLC, and the more equipment stations connected to the bus, the more transient overvoltage factors will occur.

(3) When the communication line is long or there are outdoor overhead lines, lightning is a disturbance that must be considered. Lightning is the primary source of natural disturbances, and the disturbances caused by lightning can be transmitted thousands of kilometers away. The time-domain waveform of lightning disturbance is superimposed into a large spike pulse on the background of a random pulse, and this spike pulse with huge energy will inevitably form an overvoltage on the line, resulting in damage to the equipment connected to the communication network such as PLC, which should be prevented or reduced to reduce the damage and reduce the loss.

3. Solution

1. Consider from within the PLC

(1) The DC/DC with blocking is used to block the 24V power supply and the 5V power supply, as is the case with the Profibus interfaces of Mitsubishi, Omron, Schneider PLC and Siemens

(2) Select high-grade RS-485 chips with perfect maintenance measures such as electrostatic maintenance, overheating maintenance, input failure maintenance, etc., such as: SN65HVD1176D, MAX3468ESA, etc., the price of these chips is generally more than ten yuan to dozens of yuan, while the price of SN75176 is only 1.5 yuan.

(3) Choose a new type of maintenance equipment with faster response speed and greater transient power TVS or BL surge absorber, such as P6KE6.8CA stress voltage of 6.8V, transient power of 500W, BL equipment can resist high current impact of more than 4000A. If you use a chip with no faulty maintenance, such as SN75176, you can do some processing on the software to prevent abnormal communication. That is, before entering the normal data communication, the host pre-drives the bus to be greater than +200mV, and insists on it for a period of time, so that the receivers of all nodes have a high-level output. In this way, when announcing useful data, all receivers can correctly receive the start bit and thus good data.

(4) R1 and R2 use positive temperature coefficient of self-recovery and stable PTC, such as JK60-010, the resistance value under normal conditions is 5 ohms, and does not affect normal communication, when the surge impact, the high current flows through the PTC and maintenance equipment TVS (or BL), the resistance value of PTC will increase suddenly, so that the surge current is quickly reduced.

2. Consider from outside the PLC

(1) Use blocked PC/PPI cables, and try not to use cheap non-blocked cables (especially in industrial sites). The PC/PPI cable (6ES7901-3BF00-0xa0) produced by Siemens in the early stage is non-blocking, and now it has also been changed to a barrier cable.

(2) When the RS-485 port of the PLC is networked, the blocked bus connector, such as PFB-G, has a rate of 0-1.5Mbps and is automatically adapted, and the shape and application method are the same as those of Siemens' non-blocked bus connection.

(3) The third-party equipment networked with the PLC, such as the RS-485 port of the inverter and touch screen, is blocked by the RS-485 blocker BH-485G, so that there is no "electrical" connection between the RS-485 nodes, and there is no ground circulation, even if one node is damaged, it will not be damaged with other nodes.

(4) Good grounding is an important condition for the safe and reliable operation of the industrial control system, especially for the industrial communication network. In an industrial communication network, there are at least three separate ground wires, which are grounded by a point. The first is the ground wire of the low-level circuit (i.e., the signal ground), including digital ground, analog ground, signal ground and DC ground, etc.; The second is the noise ground wire, that is, the ground wire of relays, motors, and high-power circuits; The third is the grounding point of the casing, which is specially used for the mechanical shell, fuselage, rack, and ground, and the ground wire should be connected to the ground wire of the AC power supply. The AC power ground wire should be connected to the maintenance ground wire to prevent disturbance caused by the uneven position of each point of the announced ground wire. The RS-485 communication cable is shielded with a special shield cable for the PROFIBUS bus, which ensures that the shield is connected to the housing of each device and finally to the ground.

(5) Regarding the system with overhead lines, it is best to set up special lightning protection equipment on the bus.

PLC communication interface failure and solution——Xinjie

Sections viewed

Contact

working hours