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Server will then read/parse the function code, get the address of the data field of the PDU, then get this data field value and finally perform the action based on the function code. The data field of the PDU can be empty, and then has a size of 0. In this case, the server will not request any information and the function code defines the function to be executed. If there is any error, the server will respond with an exception code. The organization's naming convention inverts the common usage of having multiple clients and only one server. Modbus Serial Line protocol is a master-slave protocol which supports one master and multiple slaves in the serial bus. The communication process is initiated by the master, as only it can initiate a Modbus transaction. Transmissions are always initiated by the master device, and consist of an exchange of bytes. As 19 coils (20-38) are required, what is rs485 cable 3 bytes is used to indicate the coil's state.

Function code 01 (read coils) allow reading the state from 1 to 2000 coil of a remote device. 2 bytes to indicate the number of coils to read. Sometimes you can read from some register that has a known value. A typical RS485 network can operate properly in the presence of reasonable ground differential voltages, withstand driver contentious situations, provide reliable communications in electrically noisy environments (good common mode rejection using twisted pair cable, shielding provides additional protection), and support thirty-two or more (many IC manufacturers have 1/2, 1/4, 1/8 unit load devices) drivers and receivers on the line. To provide a convenient means of attaching two grounds to the serial cable, there are several pins (labeled GND) on the communications connector that are connected to the controller’s ground plane. Function codes are integer values, ranging from 1 to 255, and the range from 128 to 255 is for exception responses.

Reserved Function Codes are function codes used by some companies for legacy product and are not available for public use. MODBUS over Serial Line has two transmission modes RTU and ASCII which are corresponded to two versions of the protocol, known as Modbus RTU and Modbus ASCII. The Error check field with CRC/LRC: The error check methods depend on the protocol versions of the MODBUS over Serial Line, whether it is Modbus RTU or Modbus ASCII. Address, Function, Data, and LRC are ASCII hexadecimal encoded values, whereby 8-bit values (0-255) are encoded as two human-readable ASCII characters from the ranges 0-9 and A-F. PTZ Cameras are commonly used in applications such as surveillance, video conferencing, live production, lecture capture and distance learning. Modbus gives two range of values for user-defined function codes: 65 to 72 and 100 to 110. Obviously, user-defined function codes are not unique. With Modbus over Serial Line, client's role is implemented by master, and the server's role is implemented by slave. As long as there is no error during those steps, the server's responding function code will also be the function code sent from the client. The resistance of the twisted pair cable will drop the voltage difference as it travels down the wire.

The main cable must be no longer than 700 m! Normally, this connection is made at one end of the main cable. It may be that only the byte sent from the master to the slave is meaningful; nevertheless, each device simultaneously transmits and receives one byte. In this mode, a Modbus transaction includes two messages: one request from the master and one reply from the slave. Modbus defines three types of function codes: Public, User-Defined and Reserved. User-Defined Function Codes are function codes defined by users. In practice, Cat 5 cables have been used successfully in many installations, but there are some concerns. A serial bus for Modbus over Serial Line can have a maximum of 247 slaves communicating with 1 master. In Modbus over Serial Line, the master initiates requests to the slaves in unicast or broadcast modes. In former terminology, the client was named master and the server named slave. If server cannot receive the request as having communication channel error, server will not response anything to the client. If server receives the request and execute successfully, server will return a normal response. Step 3: Client receives the response and ends the transaction.