T_FIX16

Variables of this type represent a signed 16 bit fixed-point number. This data type is often used by systems that have no FPU unit (e.g.: micro controllers or devices from the area of telecontrol). If for example data in fixed point number format has to be transferred via serial interface, then these data have to be converted to a suitable format.

The number of decimal places is chosen as per the required numerical range and resolution. For 15 decimal places it is possible to display fixed-point numbers in the range: -1..1-2^15. This corresponds approximately to the floating-point number range: -1..0.999969482421875.

Unlike the floating-point numbers the resolution of the fixed-point numbers is constant over the complete numerical range. Unfortunately the fixed-point numbers have a smaller number range for display. Care is to be taken with mathematical operations that can generate a positive or negative overflow.

TYPE T_FIX16 :
STRUCT
    value  : INT := 0;
    n      : WORD(0..15);
    status : DWORD := 0;
END_STRUCT
END_TYPE

value: This member variable contains the actual value of the fixed-point number (16 bits before and after the decimal point).

n: Number of decimal places. Permitted range: 0..15. The highest value bit is reserved for the sign bit.

status: Status flags (reserved, not used at the moment).

PROGRAM FIX_TO_FLOAT
VAR
    adc_0        : WORD := 2#1010000000000000;(* = -0.75 (Q0.15) *)
    adc_1        : WORD := 2#0111000000000000;(* = +0.875 (Q0.15) *)
    fix_0, fix_1 : T_FIX16;
    dbl_0, dbl_1 : LREAL;
END_VAR

fix_0 := WORD_TO_FIX16( adc_0, 15 );
fix_1 := WORD_TO_FIX16( adc_1, 15 );
dbl_0 := FIX16_TO_LREAL( fix_0 );
dbl_1 := FIX16_TO_LREAL( fix_1 );

PROGRAM FLOAT_TO_FIX
VAR
    dbl_0          : LREAL := +3.5;
    dbl_1          : LREAL := -3.5;
    fix_0, fix_1   : T_FIX16;
    ctrl_0, ctrl_1 : WORD;
END_VAR

fix_0  := LREAL_TO_FIX16( dbl_0, 8 );
fix_1  := LREAL_TO_FIX16( dbl_1, 8 );
ctrl_0 := FIX16_TO_WORD( fix_0 );
ctrl_1 := FIX16_TO_WORD( fix_1 );