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The mapping of MODBUS protocol on specific buses or network requires some additional fields, which are defined as application data unit (ADU). Modbus defines a protocol data unit (PDU) independently to its lower layer protocols in its protocol stack. Modbus is often used to connect a plant/system supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. RS485 is also used in performance venues to control lights and systems such as the one shown below. In addition, it has strong anti-interference performance. And that with an interface which does not require intelligent network hardware: the implementation on the software side is not much more difficult than with RS232. If more than one slave tried to drive the transmit line simultaneously, their serial drivers would fight with each other for control of the bus. Since it uses a differential line over twisted pair (like EIA-422), it can span relatively large distances (up to 4000 feet or just over 1200 metres). In the straight cable, all noise current is flowing in the same direction, practically generating a looping current just like in an ordinary transformer.When the cable is twisted, we see that in some parts of the signal lines the direction of the noise current is the oposite from the current in other parts of the cable.

Because of this, the resulting noise current is many factors lower than with an ordinary straight cable.Shielding-which is a common method to prevent noise in RS232 lines-tries to keep hostile magnetic fields away from the signal lines. For data encoding, Modbus uses a big-endian representation for addresses and data fields. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. The Modbus protocol uses serial communication lines, Ethernet, or the Internet protocol suite as a transport layer. It does not specify or recommend any data protocol. The data field of the PDU has the address from 0 to 65535 (not to be confused with the address of the Additional address field of ADU). 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.

Ideally, the two ends of the cable will have a termination resistor connected across the two wires and two powered resistors to bias the lines apart when the lines are not being driven. The pinout of the Communications Connector (H14) is shown in Table 11 2, and the following diagrams show how these signals map to the two serial communications connectors. Modbus TCP nomenclature is the same as for the Modbus over Serial line protocol, as any device which send out a Modbus command, is the 'client' and the response comes from a 'server'. Step 3: Client receives the response and ends the transaction. 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. 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. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption.

TIA/EIA-232-E (RS232) can also be used but is limited to point-to-point short-range communication. The recommended arrangement of the wires is as a connected series of point-to-point nodes, a line or bus. For higher speeds and longer lines, the termination resistances are necessary on both ends of the line to eliminate reflections.Use 100 Ω resistors on both ends. Depending of the information in the sent data, zero or more nodes on the line respond to the master. In former terminology, the client was named master and the server named slave. The DWOM bit determines whether Port D needs pull-up resistors; it should be set to 0. The MSTR bit determines whether the device is a master or slave. Function code 01 (read coils) allow reading the state from 1 to 2000 coil of a remote device. 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.

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