MODBUS :CASE STUDY AND IMPLEMENTATON

DEVELOPMENT OF MODBUS BASED DIGITAL I/O CARD WITH RS-485 PORT FOR INDUSTRIAL APPLICATIONS

Abstract

To receive the Temperature values from the far distance where human beings cannot go frequently, because the Temperature is very high .so we fix the temperature sensor there and we receive the temperature from the receiver and display.

            Development of MODBUS based digital I/O cards are very easy to deploy the network and it is royalty free for distribution and it is widely used in industries and industrial application.

            This application is very easy to deploy the network and it is very easy to access the data from the large distance about 4000 meters with using wire or wire less also.

Objective

A data logger is any device that can be used to store data. This includes many data acquisition devices such as plug-in boards or serial communication systems which use a computer as a real time data recording system. However, we can consider a data logger a stand alone device that can read various types of electrical signals and store the data in internal memory for later download to a computer.

The advantage of data loggers is that they can operate independently of a computer, unlike many other types of data acquisition devices. Data loggers are available in various shapes and sizes.

1.2 Block Diagram

 

                                                               

FIGURE 1.1: BLOCK DIAGRAM

MODBUS FOR COMMUNICATON

MODBUS is a serial communication protocol published by Modicon in 1979 for the use with its programmable logic controllers (PLC’s).It has become a defacto standard commucations protocol in industry and now the most commonly means of connecting industrial electronic devices. The main reasons for the extensive use of Modbus over other communications protocols are

1.      It is openly published and royalty-free.

2.      Relatively easy industrial network to deploy.

3.      It moves raw bits or words without placing many restrictions on vendors.

It is the communication protocol used for client-server communication

• the Master is a Client
• the Slave is a Server

FIGURE 5.2 MODBUS COMMUNICATION

MODBUS FRAME FORMAT

The MODBUS protocol defines a simple protocol data unit (PDU) independent of the underlying communication layers. The mapping of MODBUS protocol on specific buses or network can introduce some additional fields on the application data unit (ADU).

                                            FIGURE 5.3 General frame format

The MODBUS application data unit is built by the client that initiates a MODBUS transaction. The function indicates to the server what kind of action to perform. The MODBUS application protocol establishes the format of a request initiated by a client.

The function code field of a MODBUS data unit is coded in one byte. Valid codes are in the range of 1 ... 255 decimal (the range 128 – 255 is reserved and used for exception responses). When a message is sent from a Client to a Server device the function code field tells the server what kind of action to perform. Function code "0" is not valid.

                                                                            

                             

FIGURE 2.1MODBUS transaction (error free)

If an error is occur than the following trasaction will be done

                          

FIGURE 2.2: MODBUS transaction (exception response)

The size of the MODBUS PDU is limited by the size constraint inherited from the first MODBUS implementation on Serial Line network (max. RS485 ADU = 256 bytes). Therefore: MODBUS PDU for serial line communication = 256 - Server address (1byte) - CRC (2 bytes) = 253 bytes. Consequently:

RS232 / RS485 ADU = 253 bytes + Server address (1 byte) + CRC (2 bytes) = 256bytes.
 TCP MODBUS ADU = 253 bytes + MBAP (7 bytes) = 260 bytes.
2.1 RS-485

Rs-485 is formally called as EIA-485(Electronic industry association) only specifies electrical characteristics of the drivers and the receiver.It does not specify or recommend any data protocol.EIA-485 enables the configuration of inexperience local networks and multidrop commumications links.it offers high data transmission speeds(35 Mbit/s up to 10 m and 100 Kbit/s at 1200 m).

Applications

EIA-485 signals are used in a wide range of computer and automation systems. In a computer system, SCSI -2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive. EIA-485 is used for low-speed data communications in commercial aircraft cabins vehicle bus.It requires minimal wiring, and can share the wiring among several seats, reducing weight.

EIA-485 is used as the physical layer underlyingmany standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and profibus. These are used in programmable logic controllers and on factory floors. Since it is differential, it resists electromagnetic interference from motors and welding equipment.

In theatre and performance venues, EIA-485 signals are used to control sound systems and lighting. The EIA-485 link is typically implemented over standard cables more usually used for microphones, and so can be run between stage and control desk without laying special cables. Theatrical and disco lighting is controlled with the DMX protocol.

EIA-485 also is used in Building automation as the simple bus wiring and long cable length is ideal for joining remote devices. It may be used to control video surveillance systems or to interconnect security control panels and devices such as access control card readers.

Although many applications use EIA 485 signal levels, the speed, format, and protocol of the data transmission is not specified by EIA 485. Interoperation even of similar devices from different manufacturers is not assured by compliance with the signal levels alone.

Full duplex operation

EIA-485, like EIA-422 can be made full-duplex by using four wires. Since EIA-485 is a multi-point specification, however, this is not necessary in many cases. EIA-485 and EIA-422 can interoperate with certain restrictions.

Converters between EIA-485 and other formats are available to allow a personal computer to communicate with remote devices. By using "Repeaters" and "Multi-Repeaters" very large RS485 networks can be formed. The Application Guidelines for TIA/EIA-485-A has one diagram called "Star Configuration. Not recommended." Using an RS485 "Multi-Repeater" can allow for "Star Configurations" with "Home Runs" (or multi-drop) connections similar to Ethernet Hub/Star implementations (with greater distances). Hub/Star systems (with "Multi-Repeaters") allow for very maintainable systems, without violating any of the RS485 specifications. Repeaters can also be used to extend the distance or number of nodes on a network.

2.2 Configuring Minicom (Linux Host)

• Minicom is a sophisticated tool for dialing, but it emulates a fancy ANSI and VT102 terminal
• Which is quite enough for us. Minicom is installed by default with most of the Linux distribution.
• The default configuration of minicom for using with TinyBOOT is as follows:
• Bits per second 57600
• Data bits 8
• Parity None
• Stop bits 1
• Flow control none
• To configure minicom:
• Run the program by typing “minicom” on the console (Terminal).
•  Press CTRL-A and then O to open the configuration dialog box.
•  Use the arrow keys to move down to Serial port setup and press Enter.
•  Press “A” to select the serial port that is connecting to the developing board,   

       Such as

• “/dev/ttyS0”. Press “Enter” to make it accepted.
• Type E to open the “Comm Parameters” dialogue box.
• Type “H” and “Q” to select 57600 8N1 as the Bps/Par/Bits.
• Ensure that Hardware Flow Control and Software Flow Control are both set to No.
• Press ESC three times to exit the configuration dialog boxes.
• Reset the developing board to see the banner of TinyBOOT, and then the command
• Prompt, in minicom.
• Press CTRL-A and then X to exit minicom at any time.

3. RESULTS

TB550 with MAX3232 for UART COMMUNICATION

4. CONCLUSION

In this study, The data logger is very useful in broad range of applications. We chose a data logger system among many themes for graduation thesis. As prerequisite for accomplishment of the logger system project, we must have integrated knowledge related with microcontroller, C programming skill, digital circuit design, digital communication and so on.

Because it is hard to implement, it is required for them to possess capabilities of hardware design, programming of microcontroller and GUI. We selected carefully a microcontroller so that it could effectively support sensors currently available as well as any others that may be added in the future.

Thus, first of all we must select a general low cost and high performance microcontroller. After looking into microcontroller products, we have decided to use the EP9302 microcontroller with ARM920T core.

The data logger system using EP9302 microcontroller with ARM920T core was implemented during one semester. Proposed data logger will be used in many different applications to monitor and collect specific types of information.

 Here are a few examples:

 1) Assess operating temperatures of monitors.

 2) Monitor equipment operating temperatures

3) Monitor temperature of rooms in building remotely and so on.

Our designed schematic, circuit board layouts, parts list, EP9302 microcontroller with ARM920T core and host software, and other associated items will be helpful for others as sufficient information to develop their own data logger.