Discussion on PLC failures

If only phenomena 1 and 2 are considered, there is a suspicion that it is a programming issue – an error in the PLC program or the host computer script caused the data to be written to the AI’s address； After reading point 3, this possibility cannot be completely ruled out; it needs to be checked carefully ； After reading point 3, I added a few more aspects for analysis and troubleshooting: 1. Has the output from this AO been sent to the control terminals of the inverter, or has it been routed along with the high-voltage power cables? High-order harmonics from inverters causing all or some of the AI values to jump is a common occurrence ； It was normal before; perhaps some grounding or shielding layer has been damaged ； This is difficult to determine; if you’re lucky, keep the AO in a floating state and use a multimeter to check whether there is an AC component between the two poles and ground ； But high frequencies cannot be measured, so it’s a matter of luck ； 2. AO issue: The AO hardware varies from manufacturer to manufacturer. Theoretically, when the AO output is open-circuited, the operating principle of the constant current source causes the output voltage to become abnormally high, which in turn raises a certain reference potential inside the PLC and leads to problems with the AI function ； This can only be analyzed on the hardware PCB, and we often cannot see such information or the definitions of the backplane buses ； Disconnect the original wiring at the AO output and try using a resistor of around 250 ohms instead ； 3. Power supply: Some models of PLCs require external power for both AI and AO functions. Check whether different 24V power supplies are being used, as this could result in a reference potential difference ； 4. Other issues: The poster’s problem is not just related to an AI component; it requires checking various aspects of the entire project. The matter is complex, and many tests and analyses are needed to diagnose it. I personally am not a fan of answering questions by reposting existing content, but the information provided below will offer a clear approach for the poster to conduct an analysis; see the third comment

Interference resistance issues in DCS control system applications 1. Sources of interference and their general classification The sources of interference that affect DCS control systems are, similar to those that affect industrial control equipment, mostly found in areas where current or voltage changes sharply. These areas of rapid charge movement are the sources of noise, that is, the sources of interference. Interference types are usually classified based on the cause of the interference, the pattern of noise interference, and the waveform characteristics of the noise. Among them: depending on the cause of the noise, it is classified into discharge noise, surge noise, high-frequency oscillation noise, etc ； Based on the waveform and nature of the noise, it is classified into continuous noise, sporadic noise, etc ； Depending on the noise interference pattern, it is divided into common-mode interference and differential-mode interference. Common-mode interference and differential-mode interference are relatively common classification methods. Common-mode interference is the potential difference of a signal with respect to ground, which is primarily caused by the intrusion of electrical grid signals, differences in ground potential, and the common-mode (in the same direction) voltages induced on the signal lines by spatial electromagnetic radiation. The common-mode voltage can sometimes be quite high, especially in supply rooms equipped with distributors that have poor isolation capabilities. The common-mode voltage of the transmitter’s output signal is generally high, and in some cases it can exceed 130 V. The common-mode voltage can be converted into differential-mode voltage through an asymmetric circuit, directly affecting the measurement and control signals and causing damage to components (this is the main reason for the high failure rate of I/O modules in some systems). Such common-mode interference can be either DC or AC. Differential mode interference refers to the interference voltage that acts between the two poles of a signal; it is primarily caused by the coupling and induction of spatial electromagnetic fields between signals, as well as by voltages generated by unbalanced circuits that convert common mode interference. This interference is added directly to the signal, affecting the accuracy of measurement and control. 2 Main sources of electromagnetic interference in DCS control systems 2.1 Radiative interference from space The radiative electromagnetic fields (EMI) in space are primarily generated by power grids, transient processes in electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc. This type of interference is commonly referred to as radiative interference, and its distribution is extremely complex. If the DCS system is placed within an RF field, it will experience radiation interference, the effects of which occur through two pathways: one is direct radiation inside the DCS, causing interference due to circuit induction ； Rather, it is the radiation from the internal network of DCS communication that causes interference due to induction from the communication lines. Radiation interference is related to the layout of on-site equipment and the magnitude of the electromagnetic fields generated by the equipment, especially the frequency; protection is generally achieved by using shielded cables, local DCS shielding, and high-voltage discharge components. 2.2 Interference from leads outside the system is mainly introduced through power and signal lines, and is commonly referred to as conductive interference. This kind of interference is quite severe in industrial sites in our country. (1) Interference from the power supply. Experience has shown that many DCS control system failures are caused by interference introduced by the power supply; the author encountered such a problem during the commissioning of a project, and it was only after replacing the DCS power supply with one that had better isolation capabilities that the issue was resolved. The normal power supply for DCS systems comes from the grid. Due to its wide coverage, the power grid is subject to all spatial electromagnetic interference, which induces voltages and currents in the lines. In particular, changes within the power grid—such as surge currents caused by switch operations, startup and shutdown of large electrical equipment, harmonics generated by AC/DC drive systems, and transient impacts from grid short circuits—all are transmitted to the primary side of the power source through the transmission lines. DCS power supplies typically use isolated power supplies, but due to their design and manufacturing processes, their isolation performance is not ideal. In fact, absolute isolation is impossible due to the presence of distributed parameters, especially distributed capacitance. (2) Interference introduced from signal lines: Various signal transmission lines connected to the DCS control system, in addition to transmitting useful information, are always subject to the intrusion of external interference signals. There are mainly two ways such interference occurs: one is grid interference that enters through the power supply of the transmitter or the power supply shared by the signal instruments, and this is often overlooked ； Secondly, the signal lines are disturbed by spatial electromagnetic radiation, that is, external induced interference on the signal lines, and this is quite serious. Interference introduced by signals can cause abnormal operation of I/O signals and a **reduction in measurement accuracy; in severe cases, it can lead to damage to components. In systems with poor isolation performance, this also leads to interference between signals, resulting in backflow in the common-ground system bus, which causes changes in logical data, malfunctioning, and system crashes. The DCS control system suffers from quite serious damage to its I/O modules due to interference introduced by signals, which leads to numerous system failures. (3) Interference from chaotic grounding systems. Grounding is one of the effective methods for improving the electromagnetic compatibility (EMC) of electronic devices. Proper grounding can both suppress the effects of electromagnetic interference and prevent the equipment from emitting interference ； Incorrect grounding, on the other hand, can introduce severe interference signals, preventing the DCS system from functioning properly. The ground wires of a DCS control system include system ground, shielding ground, AC ground, and protection ground, etc. The interference caused by a chaotic grounding system on the DCS system is primarily due to uneven potential distribution at various grounding points; voltage differences exist between these points, which generate ground loop currents and affect the proper operation of the system. For example, the cable shielding layer must be grounded at only one point; if both ends A and B of the shielding layer are grounded, a potential difference across the ground will arise, causing current to flow through the shielding layer. In the event of an abnormal condition such as a lightning strike, the current in the grounding wire will be even greater. Furthermore, the shielding layer, ground wire, and earth may form a closed loop; under the influence of a changing magnetic field, induced currents can arise within the shielding layer, and through the coupling between the shielding layer and the core wire, these currents can interfere with the signal circuit. If it is systematically mixed up with other grounding treatments, the resulting ground loop currents can cause uneven potential distributions on the ground wires, affecting the proper operation of the logic circuits and analog circuits within the DCS. The DCS has a low tolerance to interference in logical voltages; disturbances in the distribution of logical voltages can easily affect its logical operations and data storage, leading to data corruption, abnormal program execution, or system crashes. The distribution of simulated earth potentials will lead to a decrease in measurement accuracy, causing severe distortion and malfunction in signal measurement and control. 2.3 Interferences originating from within the DCS system are primarily caused by the mutual electromagnetic radiation between components and circuits within the system, such as radiation between logic circuits and its impact on analog circuits, the interaction between analog grounds and logic grounds, as well as mismatches in the use of various components. All of these are part of the electromagnetic compatibility design carried out by DCS manufacturers within their systems; it is a complex process, and the application departments cannot make any changes to it, so there is no need to worry too much about it. However, it is important to choose systems that have a track record of successful use or have been proven reliable. III. Anti-interference design for the engineering applications of DCS control systems. To ensure that the system is protected from or experiences less internal and external electromagnetic interference in industrial electromagnetic environments, it is necessary to adopt suppression measures in three aspects from the design stage: suppressing the sources of interference ； Cut off or attenuate the pathways through which electromagnetic interference propagates ； Improve the anti-interference capability of devices and systems. These three points are the basic principles for suppressing electromagnetic interference. The interference resistance of DCS control systems is a systematic endeavor that requires manufacturers to design and produce products with strong interference resistance. It also depends on the users to take comprehensive considerations into account during engineering design, installation, construction, and operation and maintenance, and to carry out integrated design based on specific circumstances, in order to ensure the electromagnetic compatibility and operational reliability of the system. When carrying out anti-interference design for specific projects, focus should be placed on the following two aspects. 1 Equipment Selection: When selecting equipment, it is necessary to choose products with high interference resistance, which includes electromagnetic compatibility (EMC), particularly the ability to resist external interference; this is achieved through the use of floating ground technology and DCS systems with excellent isolation properties ； Secondly, it is also necessary to understand the interference resistance specifications provided by the manufacturer, such as the common-mode-to-signal ratio and differential-mode-to-signal ratio, as well as the voltage tolerance, and the range of electric field strengths and magnetic field strengths under which operation is permitted ； Another aspect is to assess its performance in similar tasks. When choosing imported products from abroad, it is important to note that China uses a power grid system with 220V and high internal resistance, while Europe and the United States use a power grid system with 110V and low internal resistance. Due to the high internal resistance of China’s power grids, significant drift in zero potential, and large variations in ground potential, electromagnetic interference at industrial sites is at least 4 times higher than in Europe and the United States. This requires higher standards for the system’s resistance to interference; DCS products that function properly abroad may not necessarily operate reliably in Chinese industries. Therefore, when using foreign products, it is necessary to select them appropriately in accordance with China’s standards (GB/T13926). 2 Comprehensive anti-interference design mainly focuses on several suppression measures against interference from outside the system. The main contents include: shielding the DCS system and external leads to prevent electromagnetic interference from space radiation ； Isolate and filter the external leads, especially those related to the power supply cables; arrange them in layers to prevent the introduction of conductive electromagnetic interference through these external leads ； Properly design the grounding points and grounding devices to improve the grounding system. In addition, software solutions must also be utilized to further enhance the security and reliability of the system. IV. Main anti-interference measures 1. Use power supplies with excellent performance to suppress interference introduced from the power grid. In DCS control systems, power supplies play an extremely important role. Grid interference intruding into DCS control systems enters primarily through the power supplies of the DCS system (such as CPU power supplies, I/O power supplies, etc.), the power supplies for transmitters, and the power supplies for instruments that are in direct electrical connection with the DCS system. Currently, for the power supplies that power the DCS system, power supplies with good isolation performance are generally used. However, insufficient attention is paid to the power supplies that power transmitters and those that supply power to instruments that are in direct electrical connection with the DCS system. Although certain isolation measures have been taken, they are often not sufficient; this is mainly because the isolation transformers used have large distributed parameters, resulting in poor ability to suppress interference, which allows common-mode and differential-mode interference to be introduced through power coupling. Therefore, for the power supply of transmitters and shared signal instruments, it is necessary to use power distribution devices with low distributed capacitance and a wide suppression band (such as those that employ multiple isolation and shielding methods as well as leakage inductance techniques) in order to reduce interference in the DCS system. Furthermore, to ensure uninterrupted power supply to the grid feed point, an online uninterruptible power supply (UPS) can be used to enhance the safety and reliability of the power supply. Moreover, UPS also has strong interference isolation capabilities, making it an ideal power supply for DCS control systems. 2 Cable selection and installation: To reduce the electromagnetic interference generated by power cables, especially those used for feeding frequency conversion devices. In a certain project, the author used copper tape-armed shielded power cables, which reduced the electromagnetic interference generated by the power lines; satisfactory results were achieved after the project was put into operation. Different types of signals are transmitted via separate cables. Signal cables should be arranged in layers according to the type of signal they transmit. It is strictly prohibited to use different wires within the same cable to transmit both power supply and signals simultaneously. Signal cables should also not be laid close to power cables in order to reduce electromagnetic interference. 3 Hardware filtering and software anti-interference measures: Before the signal is fed into the computer, a capacitor is connected in parallel between the signal line and ground to reduce common-mode interference ； Installing a filter between the two poles of the signal can reduce differential mode interference. Due to the complexity of electromagnetic interference, it is impossible to completely eliminate its effects. Therefore, when designing and configuring the software for DCS control systems, anti-interference measures must also be implemented at the software level in order to further improve the reliability of the system. Some commonly used measures include digital filtering and power-frequency shaping sampling, which can effectively eliminate periodic interference ； Regularly adjusting the reference point potential and using a dynamic zero point can effectively prevent potential drift ； Use information redundancy techniques to design corresponding software flag bits ； Use indirect jumps and set up software traps to improve the reliability of the software structure. 4 Choose the grounding point properly and improve the grounding system. There are usually two purposes for grounding: one is for safety, and the other is to suppress interference. A proper grounding system is one of the important measures to protect DCS control systems from electromagnetic interference. The system grounding methods include floating ground, direct grounding, and capacitive grounding. For DCS control systems, which are high-speed and low-level control devices, direct grounding should be employed. Due to factors such as the distributed capacitance of signal cables and the filtering in input devices, the signal exchange frequency between devices is generally below 1 MHz; therefore, the DCS control system uses either single-point grounding or series single-point grounding for its grounding wires. A centrally arranged DCS system is suitable for a parallel single-point grounding scheme, with the central grounding points of each device’s cabinet connected to the grounding electrode via separate grounding wires. If the distance between devices is large, a series single-point grounding method should be adopted. Connect the central grounding points of the cabinets of each device using a large-cross-section copper busbar (or insulated cable), and then connect the grounding busbar directly to the grounding electrode. The grounding wire is made of copper conductors with a cross-sectional area greater than 22 mm2, while the main busbars are made of copper bars with a cross-sectional area greater than 60 mm2. The grounding resistance of the grounding electrode should be less than 2Ω. It is advisable to bury the grounding electrode at a distance of 10 to 15 meters from the building, and the grounding point of the DCS system must be at least 10 meters away from the grounding points of high-voltage equipment. When the signal source is grounded, the shielding layer should be grounded on the signal side ； When not grounded, it should be grounded on the DCS side ； When there are connectors in the middle of the signal cable, the shielding layer should be securely connected and insulated; multiple grounding points must be avoided at all costs ； When the shielded twisted-pair wires from multiple measurement points are connected to a multi-core twisted shielded cable, the shielding layers should be properly connected to one another and insulated. Select an appropriate grounding point as a single contact point. V. Conclusion Interference in DCS control systems is a highly complex issue; therefore, comprehensive consideration of various factors should be given in anti-interference design to effectively suppress such interference. For certain types of interference, specific analysis is required, and targeted measures must be taken to ensure the proper functioning of the DCS control system.

A very good example for learning*. . . Scientific and standardized construction of electrical instruments is truly very helpful for future fault analysis.

In some systems, the 24V negative terminal does not need to be grounded. However, when there are multiple 24V power supplies in the same system, the reference voltage of their 24V negative terminals may become unreliable, leading to instability in the system. So, in industries such as the petrochemical sector, there seems to be a mandatory requirement for the 24V negative terminal to be reliably grounded……

2# Icetide Thanks to ICETIDE for the guidance: 1. There are only these three AO outputs, with no other interference issues； 2. The AO card was replaced, but it had no effect, which indicates that the AO card is working properly ； 3. PLCs powered by 24V are the same as DCS ； The issue has been resolved now, but the exact cause remains unclear. I suspect that perhaps there was no voltage difference initially, but during that period (when it was raining heavily), the voltage difference changed, which may have led to the malfunction.