Deadtime (DEDT)

The Deadtime (DEDT) instruction performs a delay of a single input. You select the amount of deadtime delay.

Available Languages

Ladder Diagram

This instruction is not available in ladder diagram logic.

Function Block

Structured Text

DEDT(DEDT_tag,storage);

Operands

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See

Structured Text Syntax

for more information on the syntax of expressions within structured text.

Function Block

DEADTIME Structure

Description

The DEDT instruction uses a data buffer to store delayed data, thereby allowing any length deadtime desired. The DEDT instruction is designed to execute in a task where the scan rate remains constant.

To use the DEDT instruction, create a storage array to store the deadtime buffer to hold the samples of (In x Gain) + Bias. The storage array should be large enough to hold the largest desired deadtime, using this formula:

StorageArray Size Needed = Maximum Deadtime (secs) / DeltaT (secs)

Servicing the Deadtime Buffer

During runtime, the instruction checks for a valid Deadtime. Deadtime must be between 0.0 and (StorageArray Size x DeltaT).

If the Deadtime is invalid, the instruction sets an appropriate Status bit and sets Out = (In x Gain) + Bias.

The deadtime buffer functions as a first-in, first-out buffer. Every time the deadtime algorithm executes, the oldest value in the deadtime buffer is moved into Out. The remaining values in the buffer shift downward and the value ((In x Gain) + Bias) is moved to the beginning of the deadtime buffer. A new value that is placed in the deadtime buffer appears in the Out after Deadtime seconds.

The number of array elements required to perform the programmed delay is calculated by dividing Deadtime by DeltaT. If Deadtime is not evenly divisible by DeltaT, then the number of array elements and the programmed delay are rounded to the nearest increment of DeltaT. For example, to find the number of array elements required to perform the programmed delay given Deadtime = 4.25s and DeltaT = 0.50s:

4.25s / 0.50s = 8.5

The actual delay applied to the input in this example is:

number of array elements x DeltaT = programmed delay or

Runtime changes to either Deadtime or DeltaT change the point in which values are moved out of the buffer. The number of elements required to perform the programmed delay can either increase or decrease. Prior to servicing the deadtime buffer, the following updates occur:

If the number of required elements needs to increase, the new buffer elements are populated with the oldest value in the current deadtime buffer.

If the number of required elements needs to decrease, the oldest elements of the current deadtime buffer are discarded.

Instruction Behavior on InFault Transition

When InFault is true (bad), the instruction suspends execution, holds the last output, and sets the appropriate bit in Status.

When InFault transitions from true to false, the instruction sets all values in the deadtime buffer equal to In x Gain + Bias.

Affects Math Status Flags

No

Major/Minor Faults

None specific to this instruction. See

Common Attributes

for operand-related faults.

Execution

Function Block

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Example

In this example, the DEDT instruction simulates a deadtime delay in a simulated process. The output of the PIDE instruction is passed through a deadtime delay and a first-order lag to simulate the process. The array DEDT_01array is a REAL array with 100 elements to support a deadtime of up to 100 samples. For example, if this routine executes every 100 msec, the array would support a deadtime of up to 10 seconds.

Function Block