1 Introduction
The emergence of fieldbus in the late 1980s and early 1990s simplified the connections between production process sites and control equipment, as well as between fieldbus and higher control and management levels. It also offers advantages such as strong fault tolerance, high security, and low cost. This integrated technology, primarily based on intelligent sensing, control, computer, and digital communication technologies, has garnered global attention, become a hot topic in automation technology development, and has been widely applied in the field of automation control.
2. Comparison of Profibus fieldbus and conventional wiring methods
Before the advent of the PROFIBUS fieldbus, the connection between field control sensors, I/O modules, solenoid valves, and other input/output devices and the controller and computer was mainly achieved through hardwiring. However, with the increase in the number of field inputs and outputs, the number of control lines has multiplied. Especially when the field input/output components are far from the controller, not only are there many control lines and long distances, but there is also the problem of signal instability due to interference on the control lines, making the drawbacks even more prominent.
Unlike hard-wired field connections, the Profibus fieldbus uses solid bare copper wire conductors, twisted in pairs, with red and green wires, aluminum foil and a layer of reinforced bare metal wire for better shielding. It features a purple PVC outer sheath. In automation systems, the Profibus bus connects the automation control system to field signal units. It can also directly connect to transmitters, actuators, drives, and other field instruments and equipment with interfaces for signal acquisition and monitoring. Replacing numerous traditional transmission cables with just one pair of twisted-pair wires significantly reduces cable costs, simplifies field control cabling, and correspondingly saves time and costs during construction, commissioning, and post-installation maintenance.
3 Common Bus Architecture Forms
Common bus connection methods are shown in Figure 1.
The PLC's Profibus-DP port leads to a communication line via a Profibus communication connector. Each station (the SEW servo drive in the diagram) has a communication S-connector. Stations communicate with each other via Profibus communication lines. Generally, large equipment has more stations.
4 Common Profibus Faults
(1) If an alarm occurs at a single station, and the alarm is specific to a particular station, the problem is most likely with the station itself. Check the station's communication settings and whether there is a problem with the station's Profibus communication board. This can be done by checking the communication board.
(2) The entire network experiences random site drops. This type of abnormal failure is a tricky problem for maintenance personnel, but it can be completely resolved through careful analysis.
Figure 1 Common bus connection methods
• Problem with the Profibus communication connector
A Profibus network may have a dozen or more communication heads. These communication heads have a bus terminating resistor switch marked on/off, and the internal circuitry is shown in Figure 2.
Generally, when the plug is at the end position (beginning or end), the bus termination resistor switch is in the ON position, and the station provides voltage to the network. When the plug is in the middle position of the network, the bus termination resistor switch is in the OFF position, meaning A1 and A2, and B1 and B2 are short-circuited. Because the bus termination resistor switch itself has contacts, these contacts can oxidize over time, causing resistance between A1A2 or B1B2. Poor contact leads to poor signal transmission quality, resulting in irregular station alarms throughout the communication network.
Currently, there is an instrument on the market called BC-700-BP for monitoring communication quality. This instrument can be directly plugged into the network to display the communication quality of each site in a bar chart (Figure 4).
The horizontal axis displays the Profibus address of each station, while the vertical axis represents the signal quality. Normal signal quality values are greater than 2.5, shown in green in the graph. Values below 2.5 are shown in yellow, indicating poor signal quality. Improvements can be made to stations with poor signal quality based on the test results.
This instrument can also measure and detect network changes over a period of time by recording trends. While network malfunctions may not necessarily trigger site alarms, monitoring network trends allows us to assess network health and proactively address any issues, preventing abnormal site alarms. Trend monitoring is illustrated in Figure 5.
In trend monitoring chart 5, green indicates normal stations, while red dots indicate stations experiencing frame errors at that moment. This instrument can record up to 90 hours of monitoring data. If the trend signal is entirely green within a certain time period, the network is healthy and no station alarms will occur during operation. If red dots indicating abnormal frame errors appear in the trend record, the network is unhealthy. When the intensity of the red dots exceeds a certain value, a station alarm will occur, requiring preventative improvements to the stations until no red dots appear in the trend record, indicating that the network is operating normally and stably.
Generally, we improve networks by replacing communication plugs and cables. A malfunction in any network plug or cable can cause random site alarms across the entire network.
• The control system CPU malfunctioned
When a site alarm anomaly occurs on the network, we first check the communication plug,
Figure 2 Internal wiring
Figure 3 Bus Connection Diagram
Figure 4. Communication quality diagram of BC-700-BP instrument monitoring
Figure 5 Trend Monitoring Chart
Inspect and troubleshoot peripheral components such as communication lines. If the external network is confirmed to be normal, but the problem persists, the next step is to consider CPU hardware issues in the control system. CPU problems can also cause irregular site alarms across the network. We can back up the control system program in advance, replace the CPU, download the backed-up control program, and then observe the network quality trend records. If there are no abnormal frame error red dots, it indicates that the entire network is healthy and the problem has been completely resolved.
5. Conclusion
The emergence of the PROFIBUS bus control system replaced the original control system.
Hardwiring connecting to field I/O modules, servo drives, and other input/output devices greatly simplifies the field cable distribution structure and brings many advantages; however, poor maintenance of the Profibus bus network can lead to irregular site alarms. To ensure stable equipment operation, it is necessary to regularly monitor and prevent network signal issues using instruments. A sufficiently unhealthy network manifests as abnormal site alarms. Addressing these issues only after they occur can result in prolonged downtime and production losses. Therefore, regularly monitoring the network status to ensure its healthy operation is an essential trend in equipment maintenance.
