The PDU of the Modbus response must be prefixed with an MBAP header built from the information stored when receiving the request: Unit Identifier, Protocol Identifier, and Transaction Identifier. Once the request has been processed, the Modbus server builds a response that must be sent to the TCP Management module.ĭepending on the treatment outcome, two types of responses are possible: a positive response or an exceptional response, whose objective is to provide the customer with information concerning the detected errors while processing the request. The server is ready to perform the requested service if this is recognized as a valid field. Then, it is the turn of the PDU field to be analyzed. The Unit Identifier is also contained in the MBAP header.The Modbus transaction ID is contained in the MBAP header. ![]() the identifier of the TCP connection used (provided by the TCP module Management).Otherwise, a transaction is instantiated and initialized with the following information: When the maximum number of simultaneous transactions allowed exceeds, the server constructs an exception response (Exception Code 6: Server Busy). If this corresponds to a Modbus header, a Modbus transaction is instantiated. Upon receiving a client request, the server analyzes the MBAP header of the Modbus ADU. ![]() In the same way, the Function field PDU code indicates to the server the action to take. The client (who initiates the Modbus transaction) builds the ADU. When the server device responds to the client, it uses the function code to indicate either a normal response or an exception response if an error has occurred.Ī normal response simply echoes the original function code of the query, while an exception response returns the original function code with its most significant bit set to ‘1’. Report Slave ID (17) – This command returns the model, serial, and firmware.įor a function to work, one must specify the starting register and quantity of registers to be read/write.Preset Multiple Registers (16) – Presets a block of holding registers (4x reference addresses) to specific values.Preset Single Register (06) – This command will preset a single holding register (4x reference address) to a specific value.įorce Multiple Coils (15) – Simultaneously forces a series of coils (0x reference address) either ON or OFF.Input registers (3x reference addresses) in the slave device.įorce Single Coil (05) – Forces a single coil/output (0x reference address) ON or OFF.Read Input Registers (04) – This command will read the binary contents of Read Holding Registers (03) – Reads the binary contents of holding registers (4x reference addresses) in the slave device.Read Coil Status (01) – This command will read the ON/OFF status of discrete outputs or coils (0xxxx reference addresses) in the slave/server. ![]() The request data typically includes register addresses, offset values, and data to be written. The client (master) provides the server (slave) with additional information required by the slave to complete the operation specified by the function code. The Modbus TCP protocol offers fast communication to many devices in a single network. On a Modbus TCP/IP network, a device can be: ModbusTCP protocol uses a 10 Mbps Ethernet standard to convey the entire structure of Modbus messages. At the same time, several clients can send requests, many servers can respond, and a client can talk to various servers.Ī server can simultaneously respond to several clients.įurthermore, Ethernet directs and ensures data delivery to all the devices at the same time. Therefore, many devices are the same time clients and servers.Ī network can bring together many clients. Like Modbus RTU, Modbus TCP operates on a client/server principle, but in this case, the client (master) initiates both requests and responses from a server (slave).Īny device can be a client or a server. Modbus TCP is basically the Modbus RTU protocol with a TCP interface running on Ethernet.
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