Diagnostic Detect (DDT)

The DDT instruction compares bits in a Source array with bits in a Reference array to find mismatch bit. The mismatch bit location is then recorded and the mismatch Reference bit is changed to match Source bit.

When enabled, the DDT instruction compares the bits in the Source array with the bits in the Reference array, records the bit number of each mismatch in the Result array, and changes the value of the Reference bit to match the value of the corresponding Source bit.

IMPORTANT: The DDT instruction operates on contiguous memory. You must test and confirm that the instruction does not change data that you don’t want it to change.

The difference between the DDT and FBC instructions is that each time the DDT instruction finds a mismatch, the DDT instruction changes the reference bit to match the source bit. The FBC instruction does not change the reference bit.

If the instruction tries to read past the end of an array, the instruction sets the .ER bit and generates a major fault.

Available Languages

Ladder Diagram

Operands

There are data conversion rules for mixed data types within an instruction. See Data Conversion.

Ladder Diagram

Operand	Type	Format	Description
Source	DINT	array tag	Array to compare to the reference do not use CONTROL.POS in the subscript
Reference	DINT	array tag	Array to compare to the source do not use CONTROL.POS in the subscript
Result	DINT	array tag	Array to store the results do not use CONTROL.POS in the subscript
Cmp. Control	CONTROL	structure	Control structure for the compare
Length	DINT	immediate	Number of bits to compare
Position	DINT	immediate	Current position in the source initial value typically 0
Result control	CONTROL	structure	Control structure for the results
Length	DINT	immediate	Number of storage locations in the result
Position	DINT	immediate	Current position in the result initial value typically 0

IMPORTANT: Use different tags for the compare control structure and the result control structure. Using the same tag for both could result in unpredictable operation, possibly causing equipment damage and/or injury to personnel.

COMPARE Structure

Mnemonic	Data Type	Description
.EN	BOOL	The enable bit indicates the DDT instruction is enabled.
.DN	BOOL	The done bit is set when the DDT instruction compares the last bit in the Source and Reference arrays.
.FD	BOOL	The found bit is set each time the DDT instruction records a mismatch (one-at-a-time operation) or after recording all mismatches (all-per-scan operation).
.IN	BOOL	The inhibit bit indicates the DDT search mode. 0 = all mode 1 = one mismatch at a time mode
.ER	BOOL	The error bit is either POS or LEN are invalid.
.LEN	DINT	The length value identifies the number of bits to compare.
.POS	DINT	The position value identifies the current bit.

RESULT Structure

Mnemonic	Data Type	Description
.DN	BOOL	The done bit is set when the Result array is full.
.LEN	DINT	The length value identifies the number of storage locations in the Result array.
.POS	DINT	The position value identifies the current position in the Result array.

Select the search mode

If you want to detect:	Select this mode:
One mismatch at a time	Set the .IN bit in the compare CONTROL structure. Each time the EnableIn goes from false to true, the DDT instruction searches for the next mismatch between the Source and Reference arrays. Upon finding a mismatch, the instruction stops, sets the .FD bit, and records the position of the mismatch.
All mismatches	Clear the .IN bit in the compare CONTROL structure. Each time the EnableIn goes from false to true, the DDT instruction searches for all mismatches between the Source and Reference arrays.