Valid ranges allowed in the current specification are 0 to 65,535 (0x0000–0xFFFF). Valid address ranges as originally defined for Modbus were 0 to 9999 (0x0000–0x270F) for each of the above register types. Note that this defacto standard use a 1-based register numbering while addresses use a 0-based index. This defacto standard uses the first digit of a register number to identify the register type. This is to simplify documentation, that an old defacto standard widely recognized as Modbus (sometimes referred to as Modicon convention) is still used. However, the register, coil or input ranges are also sometime given with their numbers, in decimal. The addressing space of each of the four tables is comprised in the range 0–65535 (0x0000–0xFFFF in hexadecimal). ![]() Input Register: Analog input, application defined read only parameter, mapping of the above discrete inputs.Holding Register: Analog output, application defined read/write parameter, mapping of the above coils.Coil (Discrete Output): Relay, Open collector, trigger.It is very common that the four tables are associated with the following data/physical types: MODBUS logical reference number, which are used in MODBUS functions (commands), are unsigned integer (0 to 65535). The only requirement is to link data reference with physical address. It’s obvious that all the data handled via MODBUS (bits, registers) must be located in device application memory.īut physical address in memory should not be confused with data reference. It is perfectly acceptable, and very common, to regard all four tables as overlaying one another, if this is the most natural interpretation on the target machine in question.įor each of the primary tables, the protocol allows individual selection of 65536 data items. The distinctions between inputs and outputs, and between bit-addressable and word-addressable data items, do not imply any application behavior. ![]() MODBUS bases its data model on a series of tables that have distinguishing characteristics. MODBUS is a request/reply protocol and offers services specified by function codes. The Internet community can access MODBUS at a reserved system port 502 on the TCP/IP stack. Today, support for the simple and elegant structure of MODBUS continues to grow. The industry’s serial de facto standard since 1979, Modbus continues to enable millions of automation devices to communicate. MODBUS is an application layer messaging protocol, positioned at level 7 of the OSI model, that provides client/server communication between devices connected on different types of buses or networks.
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