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Unit identifier is used with Modbus TCP devices that are composites of several Modbus devices, e.g. Modbus TCP to Modbus RTU gateways. In such a case, the unit identifier is the Server Address of the device behind the gateway. A MODBUS TCP/IP ADU/Modbus TCP frame format then will be: [31] [30]
A telegram (message frame) consists of 4-bit user data. The master communicates with the participants with a serial transmission protocol. Each subscriber is assigned an address by an addressing device or via the master. A maximum of 31 (standard devices) or 62 (advanced devices) nodes can be connected.
Modbus PEMEX; Modbus Plus; Modbus RTU or ASCII or TCP; MPI – Multi Point Interface; OSGP – The Open Smart Grid Protocol, a widely use protocol for smart grid devices built on ISO/IEC 14908.1; OpenADR – Open Automated Demand Response; protocol to manage electricity consuming/controlling devices
While IEEE 488.2 provided a device-independent syntax, there was still no standard for instrument-specific commands. Commands to control the same class of instrument, e.g., multimeters, would vary between manufacturers and even models. The United States Air Force, [9] and later Hewlett-Packard, recognized this problem.
A Remote Terminal Unit for the DNP3 protocol can be a small, simple embedded device, or it can be a large, complex rack filled with equipment. The DNP User Group has established four levels of subsets of the protocol for RTU compliance. The DNP Users Group has published test procedures for Levels 1 and 2, the simplest implementations.
ASDU Address Denotes separate segments and its address inside a device: Information Object: Function Type Provides function type of the protection equipment used: Information Number Defines the information number within a given function type: Information Elements (n) Contains details of the information element depending on the type: Stop Frame ...
bit 7, if high use Unique (5 byte) address, else use Polling (1 Byte) addresses. bit 6 and 5, Number of Expansion bytes normally it set if Expansion field is used, normally 0. bit 4 and 3, Physical layer type 0=Asynchronous, 1=Synchronous bit 2, 1 and 0, Frame type 1=BACK Burst Acknowledge send by Burst-mode Device 2=STX Master to Field Devices.
Before a register access, PHY devices generally require a preamble of 32 ones to be sent by the MAC on the MDIO line. The access consists of 16 control bits, followed by 16 data bits. The control bits consist of 2 start bits, 2 access type bits (read or write), the PHY address (5 bits), the register address (5 bits), and 2 "turnaround" bits.