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Today, for most equipment manufacturers and system integrators, the timeliness and efficiency of winning market and after-sales service responses have become crucial in the face of increasingly competitive markets. However, the traditional after-sales service model means that once the equipment or system fails, the professional engineer needs to go to the customer's site to perform equipment maintenance, which will inevitably consume a lot of labor costs and time costs, which indirectly will increase the cost of the equipment or system. Therefore, to control the cost of engineering network, in order to save costs, while providing customers with faster service, reduce customer losses, localized remote diagnosis and maintenance has now become an urgent problem for users to solve.
In fact, with the rapid development of network communication technology, from the early use of the Public Switched Telephone Network (PSTN network) to today's Internet-based solutions, the current solution for remote diagnosis and maintenance has gradually improved and matured. This article will be based on the specific application of the situation, put forward several solutions based on hardware devices to achieve PLC remote diagnosis and maintenance solutions.
Solution One: Through the use of Modem, the PSTN network is used to establish the remote connection telephone network PSTN is the most popular and lowest cost public communications network in the world. PSTN is a circuit-switched method for data transmission. Before the data transmission, an end-to-end link circuit is established. Once the link is established, all the data will be transmitted along the same path and sequence. Therefore, using PSTN network to achieve remote access is a feasible solution, which is also one of the solutions widely used in remote diagnosis and maintenance of PLC. The specific implementation is shown in Fig.1. The connection between the PC terminal of the engineer station and the remote PLC station is dialed directly through the Modem. Hardware configuration requires only one modem at each end. When you need to access the PLC remotely, you can establish a connection by directly dialing the remote modem's phone number using the AT command or the dial-up software provided by the Modem manufacturer. For network support, both the engineer station and the PLC remote station need the access of the telephone network. For software, you only need to continue to use the original PLC configuration software. 
When the scheme is implemented, the compatibility, universality, reliability, and access security of the modem must be fully considered when selecting the modem, so as to ensure reliable and stable data transmission. For the interconnection of the PSTN network, it is usually possible to use ordinary dial-up telephone lines to access the network and lease the telephone line to the network. The ordinary dial-up telephone line connection method is more economical, and the charge price is the same as the ordinary telephone charge; the leased telephone line can provide higher communication rate and data transmission quality, but the corresponding cost is also higher than the former method. . The use of a dedicated line access mode is not much different from the use of an ordinary dial-up line, but it eliminates the dial-up connection process. In general, when deciding to use a dedicated line, the user must submit an application to the local telecommunications bureau. The telecommunications bureau is responsible for erection and opening. When the above hardware devices and PSTN are ready, remote diagnosis and maintenance of the PLC can be easily realized.
Obviously, the solution is very simple to configure, and hardware equipment is cheap. However, this solution also has certain application limitations: the connection speed of the PSTN network is limited, the transmission rate is only 33,600 bit/s, and the phenomenon of connection interruption is likely to occur. Therefore, this solution is particularly suitable for remote maintenance of some small PLC systems or simple remote maintenance tasks such as memory failure reading or program error correction, but it is worth mentioning that this is sufficient for most engineering applications. For larger PLC systems and large data transmission applications, more appropriate solutions need to be sought.
Solution II. Using VPN routers to establish remote connections based on the Internet Because PSTN networks have the above limitations, with the development of Ethernet technology in the field of industrial automation, many users have begun to consider the use of Internet technology to achieve remote access to PLCs and other industrial control devices. Of course, this also benefits from the reduction in the cost of establishing and operating the global Internet. Undoubtedly, remote access based on the Internet can greatly increase the data transmission rate, meet the needs of engineers online monitoring, high-speed upload and download procedures, and can also collect data and system monitoring in real time. At present, this solution has been applied in PLC devices and systems using Ethernet communication protocols. The specific implementation is shown in Figure 2. Each router at the engineering station and the remote PLC station is configured with a router. The routers are respectively connected to the Internet. That is, a point-to-point connection between two routers is established to realize the remote connection between the PC and the PLC. 
The key to the implementation of this solution is to solve the problem of fixed (static) IP addresses. To access a certain device on the Internet, you need to know the IP address of the device. There are two types of IP addresses, fixed IP addresses and dynamic IP addresses. The IP address required for the PLC device referred to here is a fixed IP address. Fixed IP addresses need to be obtained from your local ISP. However, due to limited resources, application and use fees are relatively high. For example, the cost of IP address usage in a customer application in Nanjing is RMB 1,000/month (not quoted, for reference only). Therefore, it is obviously uneconomical to use a fixed IP address to remotely access a large number of PLC devices (of course, this method also has its application environment, such as real-time monitoring of large-scale systems). This article proposes two ways of obtaining IP addresses: one is using ADSL broadband access to the Internet; the other is using DDNS dynamic domain name resolution.
1) ADSL broadband access: Internet access through ADSL Modem, once ADSL
After the modem is connected, an IP address can be obtained. If the network is not interrupted, the IP address remains unchanged, and thus it can be regarded as a fixed IP address. In actual implementation, the router's WAN port and ADSL
Modem connection, the IP address read by the router is a fixed IP address.
2) Dynamic Domain Name Resolution Service, referred to as DDNS (DynamicDomainName)
Server) maps the user's dynamic IP address to a fixed domain name resolution service. Each time the user connects to the network, the client program sends the host's dynamic IP address to the service provider host through information transfer. The server program and service program are responsible for providing DNS services and implementing dynamic domain name resolution. That is to say, DDNS captures the user's changed IP address each time, and then associates it with the domain name, so that the domain name can always be resolved to a non-fixed IP server. Some DDNS service providers, DynDNS.org, can provide free dynamic domain names. Users can apply for free domain names online.
After solving the fixed IP address acquisition problem, the built-in NAT function of the router maps the internal network address of the PLC device to the router's external port, and the PC can easily remotely access the PLC for remote diagnosis and maintenance.
In addition, there is a certain degree of insecurity due to the Internet network. In the implementation of this plan, it is necessary to properly consider measures to ensure the security of data transmission, especially the transmission of some key and important data. The security of data security through VPN (Virtual Private Network) and IPsec (Internet Protocol Security) protocols is currently an effective solution. VPN technology creates a temporary, secure connection between a PC and a PLC. It is also understood as a secure, stable tunnel through a chaotic public network. IPSec provides two security mechanisms for this "tunneling technology": authentication and encryption. The authentication mechanism enables the data receiver of the IP communication to confirm the authenticity of the data sender and whether the data has been tampered with during transmission. The encryption mechanism guarantees the confidentiality of data by encoding the data to prevent data from being eavesdropped during the transmission process, thus ensuring the encryption and authentication of data at the network layer. Current Industrial Routers (eg Phoenix Electric FL
MGuard) has built-in VPN and IPsec functions. The configuration is simple and easy to operate. This article will not repeat the configuration of the router.
Obviously, in the above two schemes, the PC terminal of the engineering station and the remote PLC station need to lay telephone lines or network cables in order to access the telephone network or the Internet. However, in some applications, it may be inconvenient not to have a telephone line or some mobile or rotating equipment to lay cables. At the same time, reciprocating movements after laying lines can easily cause the cables to break. Therefore, it is suitable for wireless. Next, this article will introduce a wireless remote access scheme based on the Internet and GPRS network.
Solution 3: The specific configuration topology for establishing a remote connection based on the mobile network and the Internet through the GPRS Modem is shown in FIG. 3 . Compared with Scenario 2, the PC is still connected to a router, and the PLC is connected to the GPRS via a serial cable or network cable.
Modem connection. For network support, on the PLC remote station side, the user needs to apply to the operator China Mobile to apply for a GPRS/EDGE service SIM card (there are monthly charges or data flow fees) inserted into the GPRS.
Modem for Internet use. Its implementation principle is the same as that of scheme 2, and it passes router and GPRS.
Establish a VPN between modems to establish connections between PCs and PLC devices in different LANs. When implementing the solution, it is also necessary to consider the issue of fixed IP addresses. In data transmission, GPRSModem
The router is usually connected as the initiator of the visit. So Control Engineering Network Copyright, GPRS
Modem needs to obtain the public IP address of the router to access the router. The public IP here is the fixed IP of the engineering station. If the method of obtaining the IP address described in scheme 2 is adopted, it is obviously GPRS.
Modem needs to have DDNS support. 
At present, except for South Korea and Japan, GPRS networks have been widely covered in the world. GPRS is based on packet switching technology and uses the TCP/IP communication protocol to connect mobile communications with the Internet. With the acceleration of operator network construction, the 2.75G developed on the basis of GPRS network
EDGE network, transmission rate up to 210kbit/s. At the same time, GPRS is permanently online. Therefore, the GPRS network has the advantages of universality, real-time performance, high-speed transmission, and low operating cost in data communications. At present, more networks are used in the field of wireless communications. Especially with the development of 3G technology, the data transmission performance based on mobile networks will be greatly improved. The remote diagnosis and maintenance program of PLC based on GPRS network is more convenient and flexible than the wired communication method. At the same time, the permanent online and high communication rate meet the requirements of engineers' rapid diagnosis and maintenance of PLC.
At present, more and more leading equipment manufacturers in the industry begin to provide or sell equipment online services, which can increase brand competitiveness while greatly reducing the cost of after-sales services. The three PLC remote maintenance solutions proposed in this paper are applicable to the needs of different applications. But with the development of Ethernet technology in industrial automation, we can foresee that the future of remote access based on Ethernet and the Internet will certainly become a trend.
Abstract: With the rapid development of network communication technology, from the early use of public switched telephone network (hereinafter referred to as PSTN network) to today based on the Internet, the current solution for remote diagnosis and maintenance has gradually improved and matured. This article will be based on the specific application of the situation, put forward several solutions based on hardware devices to achieve PLC remote diagnosis and maintenance solutions.
Fig. 1 establishes the remote connection between PC and PLC equipment through Modem direct dialing.
Figure 2 Through the VPN router, establish a remote connection between the PC and the PLC device.
Figure 3 establishes a remote connection between the PC and the PLC device via the GPRS Modem.