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Reserved Function Codes are function codes used by some companies for legacy product and are not available for public use. Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices. Modbus RTU (Remote Terminal Unit), which is the most common implementation available for Modbus, makes use of a compact, binary representation of the data for protocol communication. The mapping of PDU of Modbus to the serial bus of Modbus over Serial Line protocol results in Modbus Serial Line PDU. 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. ADU is formed by a client inside a Modbus network when the client initiates a transaction. The mapping of MODBUS protocol on specific buses or network requires some additional fields, which are defined as application data unit (ADU).

A serial bus for Modbus over Serial Line can have a maximum of 247 slaves communicating with 1 master. Modbus Serial Line protocol is a master-slave protocol which supports one master and multiple slaves in the serial bus. A slave will never transmit any data or perform any action without a request from the master, and slaves cannot communicate with each other. In this mode, a Modbus transaction includes two messages: one request from the master and one reply from the slave. For data encoding, Modbus uses a big-endian representation for addresses and data fields. Modbus defines a protocol data unit (PDU) independently to its lower layer protocols in its protocol stack. For example, when a Microcontroller unit (MCU) connects to a sensor to read its data by Modbus on a wired network, e.g RS485 bus, the MCU in this context is the client and the sensor is the server.

For example, there can be a device that measures temperature and another device to measure humidity connected to the same cable, both communicating measurements to the same computer, via Modbus. If there is no error during the execution process, the data field of the ADU response from server to client will include the data requested, i.e. the data the client previously received. 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. Modbus defines three types of function codes: Public, User-Defined and Reserved. With Modbus protocol on the application layer, client/server model is used for the devices on the communication channel. This makes it more immune to common-mode noise and interference, ensuring reliable communication in electrically noisy environments, such as industrial settings. To ensure frame integrity during the transmission, the time interval between two frames must be at least the transmission time of 3.5 characters, and the time interval between two consecutive characters must be no more than the transmission time of 1.5 characters.

Here at Seeed, we believe that the Internet-of-Things (IoT) will be a new class of digital and physical infrastructures that will allow a more intelligent society. Upon receiving and finishing the request, the slave will respond with a message to the master. Function Code: Function code's MSB bit of Exception is 1. This will make this function code 0x80 higher than then request message function code. If server cannot receive the request as having communication channel error, server will not response anything to the client. Noise can also be reduced by using common-mode chokes and filters in the communication line. While RS422 is comparable to RS485, it is limited to unidirectional data traffic, and is terminated only on the end of the line opposite the transmitter. You aren’t limited to fifty feet of cable like with RS232. In this setup both the TX and RX share a single pair of wires, what is rs485 cable therefore it is limited to half-duplex data transmission as the data cannot be transmitted simultaneously to and fro each device. RS485 at its core with 2 wires allows half-duplex data transmission. What is rs485 - It is a serial communication protocol that allows multiple devices to communicate over long distances using a single twisted pair of wires.