Modbus application and solution

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Modbus TCP / IP Ethernet has a very high speed performance: the response time is less than 10ms, which can currently meet 90% of the application requirements in industrial control, and its next goal is: the response time is less than 1ms!
Because Modbus is a true open protocol in the manufacturing and infrastructure environment, it has been widely supported by industry and is the de facto industry standard. Because of its simple protocol, easy implementation and high cost performance, it is supported by more than 400 manufacturers worldwide, using more than 7 million device nodes, and as many as 250 hardware manufacturers providing Modbus compatible products: such as Ebtron ’s air flow Monitoring station; Acrison's weight feed controller; DVT-Cognex's image recognition sensor; Flow-serve's flow controller; Honeywell-Eurotherm's recorder; Ormec's motion controller; GE Fanuc's PLC product. Up to 150 other software products support Modbus, such as Matrikon's OPC products. There are also many development tools, such as Ethereal-Cimetrics's protocol analyzer; Ergotec's Java Beans ().

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This article introduces Modbus-based applications and solutions.

The remote terminal equipment (RTU) provided by Schneider Electric is a complete solution for industrial applications. It is mainly used for the status monitoring and data collection of remote equipment. It is based on Schneider Electric PLC based on user needs (including hardware and Software), complete all precise control and data transmission. This solution can be used in a variety of harsh industrial environments, as shown in Figure 1.
The communication protocol uses Modbus. RTU can play the role of master device or slave device in the Modbus network. It can also exchange the master / slave role at any time as required. The speed is 1200 ～ 19200 baud rate, 8 or 7 data bits / No parity bit, one stop bit.
There are various communication interfaces and media. The interface has RS-232 / 485/422 multi-station mode, and the wired mode includes direct connection, leased telephone line, public telephone line PSTN, local area network, wide area ISDN, ATM, FR and XDSL, etc., wireless methods include radio frequency (RF), infrared, microwave, spread spectrum and satellite (VSAT). Because Modbus is a message-oriented protocol, it has nothing to do with the interface and the medium. Almost all media can be used for transmission. The application in China has also covered the above methods.
The exception report can be realized through the standard function XMIT, which completes the conversion from the device and the timely sending of exception information. The entire RTU device can be compactly installed in a NEMA12 anti-rust steel cabinet or a NEMA4X fiberglass cabinet. Optional power supply: 85 ～ 264VAC; 24V DC; 105 ～ 370VDC.

Two man-machine interface + programmable controller /> + frequency conversion speed governor application

This is a solution often used by equipment manufacturers (OEMs) and traditional equipment retrofits. The typical configuration is shown in Figure 2. The human-machine interface (HMI) in the figure can be graphic or text, and the general text HMI is cheaper than the graphic HMI. The frequency converter (VSD) in the picture only draws one, actually uses RS-485 multi-station bus, and can connect up to 31 units. PLC can control the speed of the inverter according to the process requirements.
The key to completing the above program is to know the command words and register numbers of each parameter in the inverter. The following uses Schneider ATV58 inverter as an example to explain how the PLC operates the inverter:
Command word CMD (inverter address W400)
Bit 15: 1: Simple control mode Bit 1: = 0 Serial connection is invalid = 1 Serial connection is valid Bit 7: = 0
= 1 Fault reset bit 8: = 0 Activate serial connection control = 1 Cancel serial connection control Bit 11: = 0 Forward rotation = 1 Reverse bit 12: = 0 Run = 1 Stop (deceleration stop)
Bit 13: = 0
= 1 DC injection into parking space 14: = 0
= 1 Fast stop string 2
Through the Modbus write command of the PLC, the code is sent to the W400 register to realize the control of the inverter, for example:
Forward running CMD = H'8002 '
Frequency setting W401 LFR
Speed ​​setting W603 LFRD
Reverse run CMD = H'8802 '
Frequency setting W401LFR
Speed ​​setting W603 LFRD
Deceleration stop CMD = H'9002 '
DC injection stop CMD = H'A002 '
Quick stop CMD = H'C002 '
Free parking CMD = H'8000 '
Because equipment manufacturers are sensitive to price, there are also simplified versions of the above standard configuration. As shown in Figure 3, the control and operation principles are basically the same.

The computer communication structure has transitioned from the oldest host / terminal (Mainframe-Terminal), to the client / server (C / S), and then to the current Web server / Browser browser. The Web service in this article refers to the latter.
Web services actually define three standards:
(1) Web server uses HTML language programming.
(2) Browser browser uses an Internet browser: such as Microsoft Internet Explorer, Netscape Navigator, etc.
(3) The network transmission protocol is: HTTP protocol based on TCP / IP.
In this way, the graphic screen can be used to dynamically interact with the server from the client.
The emergence of the Web on the Internet is comparable to the appearance of Windows on the computer. Users can obtain various resources on the Internet through the graphical interface. The access threshold is greatly reduced, and the number of Internet users is greatly increased. It is also the reason why Microsoft and Netscape are fighting for the market share of browsers.
Now that Web services are very mature and successful in the IT field, can it be introduced into industrial networks to add value to user networks? The answer is yes, Schneider has conducted beneficial explorations on this, and many users have also benefited from it, and achieved many results.

1. Web service scalability At present, Schneider's automation products (such as NOE, ETY Ethernet templates) have built-in many communication and Web service functions, users can achieve different levels of service according to their needs. String 7
(1) Level A: No Web function, users can implement Modbus information exchange and FDR (Faulty Device Replacement) through TCP / IP Modbus, device monitoring and I / O services.
(2) Level B: Basic Web functions. Through the built-in web server, the device can be configured, configured, maintained, monitored, diagnosed and documented, and the network can be managed.
(3) Level C: Configurable Web function, users can make dynamic web pages by themselves to realize the thin client's human-machine interface function, in addition, it can also realize information publishing and subscription (also called global data function), bandwidth management, The I / O scanner function, completing a higher step of the so-called "collaborative automation" concept, is a value-added service.
(4) Level D: more effective Web services, through further expanded services, such as future distributed control, more advanced human-machine interfaces, and a closer connection to the IT network; such as through e-mail, mobile phone SMS, various A database interface to achieve seamless integration of management network and control network.

2. The advantages that Web automation /> brings to users (1) For the operator: easy to use, because the same browsing tool is used, and the browser is currently free, it can also be downloaded from the Internet for free.
No prior knowledge and training is required to use the operation interface. It can be connected to servers such as operation, maintenance, process flow, and product information through different hyperlinks to realize online learning, operation, and maintenance, as well as LAN, local network, and remote network cross-regional remote The monitoring function of field equipment.
(2) For enterprise managers: low investment, scalability, and "thin client" -based human-machine interface, so that the information can be accessed at any network level within the enterprise, and keep abreast of the company's product rate, finished product rate, and sales and financial status Through interconnection with SCM, ERP, CRM, etc., you can keep abreast of market changes, logistics supply and enterprise resources and other information to ensure the normal operation of the system and create a virtuous circle of enterprises.
(3) For IT managers: Because of the use of standardized general-purpose networks, the information network and control have been combined into one, reducing multiple databases, reducing the difficulty of system integration, and reducing overall costs.
Due to the use of common components, IT procurement costs are reduced from the network and computers to the application software (such as browsers, network computers, thin clients, TCP / IP Ethernet, etc.), and the maintenance and spare parts costs are also very low.
Through the existing TCP / IP Ethernet, it can also provide various value-added services, such as IP telephone, IP video and so on.
If users are interested, IT personnel can also connect the LAN to the Internet, plus several security mechanisms, such as firewalls, VPNs, and other technologies. Users can obtain enterprise information through the Internet in various ways at any time and anywhere.

Four examples of partial applications of Modbus and Modbus TCP (1) Semiconductor manufacturing equipment: SCADA / PLC; functions: pure water production, chemical liquid leak detection, clean plant air conditioning and facilities.
(2) Textile machinery and equipment: PLC / motion control; function: automatic adding machine for T-shirt pockets. String 9
(3) Public facilities, such as supermarkets, warehouses, business office buildings and other equipment: PLC / PC; functions: power distribution, air conditioning system, heating and cooling.
(4) Mining industry equipment: SCADA / PLC / DCS / Ethernet; function: send remote related information to the main station, such as train and truck transportation status.
(5) Plastic machinery and equipment: PLC / PC / network; function: monitoring and control through network weight system.
(6) Microelectronics equipment: PLC / PC; function: clean air conditioner, air treatment and vacuum, waste treatment, water treatment, raw material distribution, environmental monitoring, power monitoring of the workshop.
(7) Equipment for the food and beverage industry: PLC / PC / MODEM; function: reliable, repeatable and detailed testing and reporting of beverages.
(8) Water treatment industry equipment: RTU / Ethernet / Wireless network; Function: Power meter combined with remote control can obtain real-time detection of the US dollar / gallon value of the pump.
(9) Cement manufacturing equipment: PLC / PC / network; function: automation of cement production process.
(10) Nuclear technology equipment: MCC / PLC / network; function: control the production of tritium (radioactive isotope of hydrogen).
(11) Lifting equipment: PLC / DC Drives / AC Drives; function: operation control of port lifting machinery.
(12) Metallurgical equipment: PLC / motion control / DC Drives; function: precise cutting of steel plate length.
(13) Water supply system string 7
Equipment: PLC / PC / network; function: complete monitoring and control of water supply system.

Five future development of Modbus In order to better popularize and promote the distributed application of Modbus on the Ethernet, Schneider has transferred the ownership of the Modbus protocol to the IDA (Interface for Distributed AutomaTIon, distributed automation interface) organization (www. Modbus-IDA.org), and established the Modbus-IDA organization, which laid the foundation for the future development of Modbus. As a non-profit, independent, Web-based community, Modbus-IDA's mission is to encourage independent users and manufacturers of automation equipment to adopt Modbus communication protocol and use distributed automation architecture for multiple markets Provide corresponding products. Modbus-IDA will provide a basic platform to enable users to obtain and share protocol, application and authentication information, and ultimately reduce the cost of use. Modbus-IDA has been awarded by the IEC as a liaison for the "Category D" of the world's leading electrical and service standards organization. It is currently involved in the definition of digital communications in manufacturing and process automation environments, and has submitted Modbus based on TCP / IP And RTPS (Real-TIme Publish / Subscribe, real-time data publishing and subscription) agreement, as the PAS (Publicly Available SpecificaTIon, publicly available specifications) of the IEC.