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9.4 M
ODBUS FUNCTIONS
To simplify the communications the S200 Position Node is set up with almost all data defined as
32bit values comprised of two 16-bit registers. The data type of any one of these register sets can
be defined as Long, Text, or Float data types. With a few exceptions, all communications to the
S200 Position Node card will be Read Holding Registers (FC=03) or Write Multiple Registers (FC
= 16).
The register address of each parameter is aligned to an even register address values. Variable
address in the GUI software is set up as 32-bit address space and is called the PDID (using
variable browser utility). The Modbus registers, being 16 bit address space, are addressed at 2 *
PDID value. Please see address table at the end of this chapter.
The first register (even address) is the high word of the 32 bit value, the second register (odd
address) the low word.
Important: Although data transfer should be done using FC 16 (Write Multiple Registers,
quantity 2) the Position Node does support write of a single register:
1. To synchronize the write access when using single register access both registers
of a parameter need to be written to starting with the high word (even address). If
the low word is accessed first, an error exception occurs.
2. The firmware supports only a block read/write access of up to 2 registers. Both
registers must belong to the same parameter.
Each parameter has a set of attributes like minimum value, maximum value, default value, read
only, data type, etc., which can be uploaded from the option card. See tables at the end of this
section for details.
9.4.1 Reading Variables
To read a variable use the Modbus Function 'Read Holding Registers'. As found in the Modbus
documents the serial communication model for a variable read is presented as follows:
UA=xx FC = 3 Address (16bit) Number of registers (16bit) CRC
Where:
UA is the unit address of the drive as set by rotary switches S11 and S12.
FC is the Read Multiple Holding Register Function Code (03).
Address is the variable register address (even number).
Number of registers to read should be set to 2.
As defined by Modbus, the S200 Position Node product will respond with the value set in those
registers:
UA=xx
Where:
UA is the unit address echoed back.
FC is the function code echoed back (03)
Data length will be 2 (Data count)
Data will be the 32 bit value of the variable read. Variable type may be Long or Float.
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FC=3
data length
data
CRC
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