1. Introduction With the development of automation technology, the design of asphalt mixing plant control systems is becoming increasingly intelligent and user-friendly. Modern control systems place higher demands on the skills of engineering service personnel. Simultaneously, to adapt to the market environment and road construction environment of asphalt mixing plants, manufacturers inevitably face the challenge of providing diagnostic and maintenance services for control systems of asphalt mixing plants scattered across domestic and international locations. This often requires significant investment of manpower and resources by the company's engineering service departments, which substantially increases equipment production costs. To address the limitations imposed by geographical location and the need for comprehensive technical expertise among maintenance personnel, remote diagnostic and maintenance services are currently an effective method. In the currently popular centralized asphalt mixing plant control systems, the "Programmable Logic Controller (PLC)" is the core of the control. Most mainstream PLCs on the market support remote access, so diagnosing the PLC can enable the diagnosis of faults in the entire equipment control system. This paper takes Siemens PLC as an example and discusses in detail Teleservice remote access based on modem dialing. 2. Remote Service System Configuration Remote access based on a modem is the standard configuration for Siemens PLC remote access. This means that the engineer station (ES) and the remote PLC station are connected via a modem dial-up connection. This requires placing a modem at each end and connecting it to the PLC CPU's MPI port via a Siemens TS-Adapter, as shown in Figure 1 below. [align=center]Figure 1 TeleService Network Configuration Diagram Based on Modem Dial-up[/align] 2.1 Hardware Configuration As shown in Figure 1, the following can be summarized: 1) One PC with a serial port 2) Two Modems with serial ports 3) Two telephone lines 4) One TS Adapter 2.2 Software Configuration 1) Step7 v 5.x 2) Siemens Teleservice v5.2 (or higher) 3) PC Operating System WIN XP SP2/WIN 2K SP4 2.3 Hardware Connection The hardware connection in the system is very simple. The following points should be noted: 1) The connection between the telephone line and the Modem requires the telephone to be a direct-dial telephone. That is, there should be no separator or shielding between the telephone exchange and the telephone line terminal. Some organizations have installed IP splitters at the telephone line terminal; connecting the Modem through this splitter is not allowed. 2) Connection between PLC and Modem The connection between the PLC and the Modem needs to be through the TS Adapter. There is a communication rate selection switch on the TS Adapter, which is set to 19.2K by default. 3) Connection between TS Adapter and PLC The adapter must be connected to the PLC's MPI communication port. If the PLC is connected to a host computer or touchscreen, the bus connector connecting the PLC must also be a programmable bus connector, and the terminating resistor switch should be in the "off" position. 2.4 Software Settings After the hardware connection at one end of the remote PLC station is complete, no further software or hardware settings are required. Only the engineer station needs to configure the communication connection between Teleservice and PC/PG. 2.4.1 Remote Phone Number Settings Enter the Teleservice main interface, create a new "folder" in the left folder list, and name it "beijing". As shown in Figure 2 [align=center] Figure 2 Teleservice [/align] Select the folder "beijing" and create a new "plant" in the right plant window. The "plant properties" prompt box shown in Figure 3 will pop up: [align=center] Figure 3 plant properties [/align] Figure 3 contains two parts of information. The plant data includes plant name, company, address, etc. The plant name field is required, and the others can be left blank. The phone number field is the remote phone number setting item. The country code, area code, and phone number must be filled in completely. 2.4.2 Local Connection Settings In Figure 2, click the menu "option" and select "customize" to bring up the screen shown in Figure 4. From Figure 4, we can see the connected modem, monitoring time selection, and other information. Generally, we choose the default settings. Local connection settings are in the "location" option. Click the "dialing properties" button to bring up the telephone and modem options window shown on the left in Figure 5. In this window, select the "edit" button to bring up the window shown on the right in Figure 5. In this window, set the local dialing rules. This includes the country, area code, local phone line number, and long-distance phone line number. Leave other options as default. When setting the dialing rules, pay attention to whether the company's external phone number needs to be preceded by 0 or 9. If there is such a restriction, fill in the number to be preceded in the "local phone line number" option, as shown in Figure 5. If there is no such restriction, leave the "local phone line number" field blank. The format of the edit location window may be slightly different in newer versions of the Teleservice software, but the content is basically the same. [align=center] Figure 4 customize Figure 5 Telephone and Modem Options[/align] 2.4.3 PG/PC Options Settings The PG/PC options are for selecting the communication method between the PC and PLC. There are several ways to access the PG/PC options. You can directly select "Enter Options" from the PC control panel; if Siemens STEP7 is enabled, you can also access the options through the STEP7 menu "options\set PG/PC interface". Regardless of the path used to access the PG/PC options, the screen shown in Figure 6 will appear. [align=center]Figure 6 PG/PC Settings[/align] After successfully installing the Teleservice software, "TS Adapter" will appear in the interface options list. Select this interface to configure its properties. The steps are: select "TS Adapter", click the properties button in the upper right corner, and a dialog box will pop up, as shown in Figure 7. [align=center]Figure 7 TS Adapter Properties[/align] The dialog box in Figure 7 has three items: Network, Local Area Connection, and Local Modem. Here we only need to configure the Local Area Connection item. As shown in Figure 7, select "Modem Connection" in the Local Area Connection. 3 Dial-up After all hardware and software settings are complete, you can establish a dial-up connection. There are two ways to do this: one is to select the newly created "Test plant" from the menu in the Teleservice main interface and then navigate to the path: connection\establish to enter the dial-up screen; the other is to double-click "Test plant" to enter the dial-up screen. The dial-up screen is shown in Figure 8: [align=center] Figure 8 Dial-up[/align] In Figure 8, click "Dial" to make the dial-up connection. The connection information will be displayed in the "status" column. After a successful dial-up connection, you can begin accessing the remote PLC. The process of remotely accessing the PLC is the same as locally accessing it. You can use STEP7 to debug the PLC, such as downloading user programs and monitoring programs online. 4 Conclusion Remote access to the PLC allows for online monitoring of the entire asphalt mixing plant control system, which is quick and effective in identifying and resolving problems. For asphalt mixing plant manufacturers, remote maintenance can significantly reduce long-distance business trips, provide more effective services to customers, and quickly diagnose faults in customer equipment. It can improve global service capabilities and reduce personnel and equipment costs. For customers, remote maintenance can diagnose and resolve faults faster, reduce downtime and maintenance time, and reduce production schedule losses. Currently, the Modem-based remote access scheme for the asphalt mixing plant control system PLC has been successfully applied in the equipment maintenance service of Beijing Deji Machinery in Australia. In the application of the scheme, it was also found that the speed of remote access based on Modem is limited, and the speed is only the speed of dial-up Internet access, and connection interruption is easy to occur. However, because the scheme is simple to configure, inexpensive, and does not require additional hardware cards, it has received special attention in actual engineering services. References: [1] Fan Liuqun Research on the establishment of remote service system Tongji University CIMS Research Center 1999/04 [2] Siemens TeleService for S7, C7 and M7 – manual 2002