Some basics for the Modbus Beginners.

MODBUS:

• Modbus is an open protocol, so manufacturers do not need to pay royalties.
• It has become a standard communications protocol in industry, and is now the most commonly available means of connecting industrial electronic devices.
• Modbus is typically used to transmit signals from instrumentation and control devices back to a main controller or data gathering system, for example a system that measures temperature and humidity and communicates the results to a computer.
• Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Versions of the Modbus protocol exist for serial lines (Modbus RTU and Modbus ASCII) and for Ethernet (Modbus TCP).
• The original Modbus specification included two possible transmission modes: ASCII and RTU. Modbus RTU mode is the most common implementation, using binary coding and CRC error-checking. Modbus ASCII messages (though somewhat more readable because they use ASCII characters) is less efficient and uses less effective LRC error checking. ASCII mode uses ASCII characters to begin and end messages whereas RTU uses time gaps (3.5 character times) of silence for framing. The two modes are incompatible so a device configured for ASCII mode cannot communicate with one using RTU.  Modbus ASCII messages require twice as many bytes to transmit the same content as a Modbus RTU message.

How does it work?

• Modbus is transmitted over serial lines between devices. The simplest setup would be a single serial cable connecting the serial ports on two devices, a Master and a Slave.

• The data is sent as series of ones and zeroes called bits. Each bit is sent as a voltage. Zeroes are sent as positive voltages and a ones as negative. The bits are sent very quickly. A typical transmission speed is 9600 baud (bits per second).

• Information is stored in the Slave device in four different tables. Two tables store on/off discrete values (coils) and two store numerical values (registers). The coils and registers each have a read-only table and read-write table. Each table has 9999 values. Each coil or contact is 1 bit and assigned a data address between 0000 and 270E. Each register is 1 word = 16 bits = 2 bytes and also has data address between 0000 and 270E.

• Coil/Register Numbers can be thought of as location names since they do not appear in the actual messages. The Data Addresses are used in the messages. For example, the first Holding Register, number 40001, has the Data Address 0000. The difference between these two values is the offset. Each table has a different offset. 1, 10001, 30001 and 40001.

What is the Slave ID?

• Each slave in a network is assigned a unique unit address from 1 to 247. When the master requests data, the first byte it sends is the Slave address.

What is a function code?

• The second byte sent by the Master is the Function code. This number tells the slave which table to access and whether to read from or write to the table.

What is a CRC?

• CRC stands for Cyclic Redundancy check. It is two bytes added to the end of every modbus message for error detection. Every byte in the message is used to calculate the CRC. The receiving device also calculates the CRC and compares it to the CRC from the sending device. If even one bit in the message is received incorrectly, the CRCs will be different and an error will result. .

What is the differnce between Modbus and RS-485?

• Some might think this as a baseless question posted,but i had the difficulty when in began working.For those who don”t know .
• Rs-485 is a physical connection (wired connection) and the Modbus is a communication protocol that defines the content of the message.