Communication network faults
Communication network faults are generally prone to occur at the contact bus, local bus, or due to address identification errors.
Node bus failure
The transmission medium of node bus is generally coaxial cable, some use token signal transmission method, and some use multi pass contention bus signal transmission method with conflict detection. Regardless of the method used, any interruption in the trunk line of the bus will cause communication failures among all stations and their sub devices on that bus.
At present, the general method to prevent such faults is to use a dual redundant configuration to avoid the global impact caused by a fault on one bus. However, this cannot fundamentally prevent the occurrence of faults, and once one bus fails, it is very easy to cause another bus fault during processing, and the consequences are very serious. An effective method should be to prevent poor bus contact or open circuit by human hands.
A relatively successful system node bus layout method. The connection of the coaxial cable is not in front of the communication module, but behind the module. This way, when dealing with communication module faults during system operation, it can avoid accidentally touching the coaxial cable and causing network cable disconnection. At the same time, the coaxial cable will not be touched at any time except for special inspection, which can prevent loosening caused by multiple plugging and unplugging of the coaxial cable plug, increasing the possibility of its failure. In addition, a management system for coaxial cable inspection and replacement should be established, and replacement or handling should be carried out before the contact resistance increases to the extent that it affects communication.
Local bus failure
Local bus or fieldbus generally refers to a data communication network composed of twisted pair cables. Due to the fact that the connected equipment is a primary component or control device directly connected to the production process, the working environment is harsh, the failure rate is high, and it is easy to be affected by the misoperation of maintenance personnel and affect the production process. In addition, the bus itself can also cause communication failures for various reasons. The effective method to prevent such faults is to first properly handle the connection points between the local bus and the local equipment. When disassembling and assembling the equipment, it should not affect the normal operation of the bus, and the bus branch should be installed in a place that is not easy to touch. Meanwhile, it is best to use dual redundant configuration for the local bus to improve communication reliability.
Error in address identification
Whether it is a local component or a bus interface, once its address identification is incorrect, it will inevitably cause communication network disorder. So, it is necessary to prevent address identification failures of each component and prevent human error and alteration. When expanding the system, it should generally be done when the system stops running. Especially for systems that use token based communication, any addition or reduction of components must be reported to the network during system shutdown to avoid unpredictable consequences.
Hardware malfunction
According to the different functions of each hardware, the faults of the DCS system can be divided into human-machine interface faults and process channel faults. The human-machine interface mainly refers to the engineer station, operator station, printer, keyboard, mouse, etc. used to achieve human-machine communication functions; Process channels mainly refer to local buses, I/O channels, process processors, primary components or control equipment, etc. The human-machine interface consists of multiple workstations with the same function. When one of them malfunctions, as long as it is handled promptly, it generally does not affect the monitoring operation of the system. When a process channel failure occurs on the local bus or primary equipment, it directly affects the control or detection function, and therefore the consequences are relatively serious.
Human machine interface failure
Common human-machine interface failures include ball mark operation failure, control operation failure, operator station crash, abnormal film keyboard function, printer malfunction, etc.
Abnormal operation of the ball marker is generally due to long-term aging or contamination of the internal mechanical device, which makes the contact points unreliable to make or break, or the lack of communication with the host due to loose cable connections. In this case, it is only necessary to replace and inspect it.
The failure of control operation is due to the inability of the operation signal of the ball mark to change the state of the process channel. On the one hand, it may be a hardware failure of the process channel itself, and on the other hand, it may be a software defect of the operator station itself. When the equipment is overloaded or there are too many open process windows, it may not respond. After checking that the channel function is normal during the inspection process, the operator station should be checked and restarted if necessary to initialize the operator station.
There are many reasons for the operator station to crash, which may be due to hard disk or card failures, software defects, cooling fan failures causing the host to overheat, or excessive load. You can first check the temperature rise of the host itself, and then use alternative methods to check the hard disk, host card components, etc. to determine the faulty part.
Thin film keyboards are widely used on most operator stations. Its main function is to quickly retrieve process graphics, making it easy for operators to quickly monitor process parameters. When the membrane keyboard is not fully started due to configuration errors, poor keyboard contact, loose signal cables, or accidental keyboard movement during host startup, it can all lead to abnormal functions. Different situations should be addressed accordingly.
The printer’s malfunction is usually due to configuration reasons. At the same time, blocking the printer will also prevent the printing function from working. In addition, hardware failures of the printer itself can cause some or all of its functions to be abnormal. The printer’s settings and hardware should be rechecked to ensure they are functioning properly and addressed.