Mapping Modbus input registers and holding registers

Modbus input registers and holding registers are both 16-bit values read from or written to the Modbus device. The Serial Card treats both register types the same with regard to data mapping.

Table: Mapping Input and Holding Register Data to DeltaV Data Types
DeltaV Data Type	Mapping Method
Boolean	The card sets the Boolean value to 0 if the Modbus register has a value of 0 and to a value of 1 if the Modbus register has a value other than 0.
Discrete, 8-bit signed integer and 8-bit unsigned integer	The card maps the lower eight bits of the Modbus register to the DeltaV value with no additional conversion.
16-bit signed integer and 16-bit unsigned integer	The card maps all 16 bits of the Modbus register directly into the DeltaV integer value.
32-bit signed integer and 32-bit unsigned integer	The card maps two consecutive Modbus registers to the DeltaV value. The first register is the least significant word, and the second register is the most significant word.
Floating point	Floating point values are stored on Modbus PLCs in IEEE format in two consecutive registers where the first is the least significant word and the second is the most significant word. The Serial Card maps the two consecutive Modbus registers to the DeltaV floating point value using the first register as the least significant word and the second as the most significant word.
String	The Number of values field indicates the size of the string. The Serial Card maps the specified registers to a single DeltaV string with a length equal to number of values × 2. The first register specified contains the first two characters of the string where the high order byte of the register contains the first character and the low order byte contains the second character. The remaining registers contain the rest of the string characters in the same order.