Byte ordering

Modbus is a big-endian protocol.  The more significant byte of a 16-bit value is sent before the less significant byte So, for bytes on the bus a UInt16 is sent like this: Bits15:8 | Bits7:0 Converting the modbus 16 bit words UInt32 conversion UInt16[0] | UInt16[1] UInt64 conversion UInt16[0] | UInt16[1] | UInt16[2] | UInt16[3] Float conversion UInt16[0] | UInt16[1] //Converting big endian modbus […]

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Modbus Apps

Free sourceforge.net/projects/qmodmaster/  Nice interface but only works with single register values (no float, ASCII strings etc support) Paid simplymodbus.ca/RTUmaster.htm Poor operation in Windows 7 but does actually work and does support multi register values so actually a very handy app and well priced for what it is at $60.   We use it for development work as once you […]

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Function 16 Write Multiple Registers

Modbus RTU Function 16 'Preset Multiple Registers' To write 3 words (48 bits) to 3 consecutive registers you send the following: Byte 0 Modbus Device Address (1 to 247) Byte 1 Function Code / Message Type (0x10) Byte 2 Start Register Number (high byte) Byte 3 Start Register Number (low byte) Byte 4 Number of registers to write(high byte) […]

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Function 06 Write Single Register

Modbus RTU Function 06 'Preset Single Register' To write a word (16 bits) to a register you send the following: Byte 0 Modbus Device Address (1 to 247) Byte 1 Function Code / Message Type (0x06) Byte 2 Register Number (high byte) Byte 3 Register Number (low byte) Byte 4 Data0 Byte 5 Data1 Byte […]

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Function 03 Read Holding Registers

Modbus RTU Function 03 'Read Holding Registers' To read a word (16 bits) from a register you send the following: Byte 0 Modbus Device Address (1 to 247) Byte 1 Function Code / Message Type (0x03) Byte 2 Register Number (high byte) Byte 3 Register Number (low byte) Byte 4 Data Length (high byte, specified […]

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Modbus Comms General

Resources modbus.org Modbus Application Protocol Describes function codes modbus.org Modbus over Serial Line Specification and Implementation Guide Register Addressing The Modbus register number should really be thought of as an offset rather than an address, as different vendors can use different register addressing schemes.  Firstly registers may start from 0 for some devices and 1 for others. The register addressing […]

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Modbus TCP/IP

Resources Acromag Introduction To Modbus TCP/IP Has a good explanation of how a RTU serial packet is transferred over TCP/IP modbus.org​ Modbus Messaging On TCP/IP Implementation Guide Simply Modbus TCP Page Open Modbus/TCP Specification        

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Modbus General

Good Resources http://www.rtaautomation.com/modbusrtu/ http://www.rtaautomation.com/modbustcp/ Serial Modbus Serial MODBUS connections can be ASCII and RTU (binary coded). Modbus RTU: Messages are a smaller size and therefore faster, Uses timing of bus silence Modbus ASCII messages are easily human readable. All nodes within one MODBUS network must use the same transmission mode. UART Setup No specific baud […]

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