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A serial bus for Modbus over Serial Line can have a maximum of 247 slaves communicating with 1 master. The master doesn't need to have an address. Those slaves have a unique address ranging from 1 to 247 (decimal value). Each slave must have a unique address (from 1 to 247) to be addressed independently for the communication. In such a case, the unit identifier is the Server Address of the device behind the gateway. If server receives the request and is unable to execute it (e.g client requests to read a non-existent register), server will return an exception response to client to indicate the nature of the error. 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'. Modbus TCP frame by other parties. With Modbus over Serial Line, client's role is implemented by master, and the server's role is implemented by slave.

In Modbus over Serial Line, the master initiates requests to the slaves in unicast or broadcast modes. Configured as a master device, the 68HC11 transmits bytes via the "master out/slave in" pin, MOSI. Transmissions are always initiated by the master device, and consist of an exchange of bytes. The GROUND line serves as a common voltage reference for the master and slave. Its simplest implementation requires only three wires: one to transmit serial data, a second to receive serial data, and a third to provide a common ground reference. The organization's naming convention inverts the common usage of having multiple clients and only one server. Data: Server returns the exception code inside the Data field. To avoid this confusion, the RS-485 transport layer uses the terms "node" or "device" instead of "server", and the "client" is not a "node". Modbus standard also defines Modbus over Serial Line, a protocol over the data link layer of the OSI model for the Modbus application layer protocol to be communicated over a serial bus.

The Error check field with CRC/LRC: The error check methods depend on the protocol versions of the MODBUS over Serial Line, what is rs485 cable whether it is Modbus RTU or Modbus ASCII. 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 protocol 2006, p. There are several protocols that govern the format of exchanged data, with the RS232 protocol used primarily by personal computers, and the RS485 protocol used in industrial control systems. The RTU format follows the commands/data with a cyclic redundancy check checksum as an error check mechanism to ensure the reliability of data. The ASCII format uses a longitudinal redundancy check checksum. For example, a value of 122 (7A16) is encoded as two ASCII characters, "7" and "A", and transmitted as two bytes, 55 (3716, ASCII value for "7") and 65 (4116, ASCII value for "A").

LRC is specified for use only as a checksum: because it is calculated on the encoded data rather than the transmitted characters, its 'longitudinal' characteristic is not available for use with parity bits to locate single-bit errors. Modbus ASCII makes use of ASCII characters for protocol communication. Modbus Serial Line protocol is a master-slave protocol which supports one master and multiple slaves in the serial bus. The QVGA Controller makes it easy to configure a multi-drop network of QVGA Controllers controlled by a single master desktop computer. In a multi-drop configuration, the TxD of the master is connected to the RxD of each slave, and the RxD of the master is connected to the TxD of each slave. Upon receiving and finishing the request, the slave will respond with a message to the master. In this mode, a Modbus transaction includes two messages: one request from the master and one reply from the slave.