Discrete 3-State Device (D3SD)

The Discrete 3-State Device (D3SD) instruction controls a discrete device having three possible states, such as fast/slow/off or forward/stop/reverse.

Available Languages

Ladder Diagram

This instruction is not available in ladder diagram logic.

Function Block

Structured Text

D3SD(D3SD_tag)

Operands

Structured Text

See

Structured Text Syntax

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

Function Block

DISCRETE_3STATE Structure

Description

The D3SD instruction controls a discrete device having three possible states, such as fast/slow/off or forward/stop/reverse. Typical discrete devices of this nature include feeder systems, and reversible motors.

Affects Math Status Flags

No

Major/Minor Faults

None specific to this instruction. See

Common Attributes

for operand-related faults.

Execution

Function Block

Structured Text

In Structured Text, EnableIn is always true during normal scan. Therefore, if the instruction is in the control path activated by the logic it will execute.

Examples

The D3SD instruction is typically used to control 3-state devices such as high/low/off feed systems. In this example, the D3SD instruction controls a feed system consisting of a pair of solenoid valves adding vegetable oil to a batch tank. One of the valves is on a large diameter feed pipe into the batch tank, and the other valve is plumbed in parallel on a small diameter feed pipe. When oil is first added, the D3SD instruction is commanded to the fast feed state (state 2) where both valves are opened. When the oil added approaches the target amount, the D3SD instruction is commanded to the slow feed state (state 1) where the "large valve" is closed and the "small valve" is kept open. When the target is reached, the D3SD instruction is commanded to go to the off state (state 0) and both valves are closed. As long as the D3SD instruction is in Program control, the valves open according to the CloseOilFeed, SlowOilFeed, and FastOilFeed inputs. The operator can also take Operator control of the feed system if necessary. The solenoid valves in this example have limit switches which indicate when the valves are fully closed or opened. These switches are wired into the FB0, FB1, FB2, and FB3 feedback inputs. This allows the D3SD instruction to generate a FaultAlarm if the solenoid valves do not reach their commanded states within the configured FaultTime.

Function Block

Structured Text

OilFeedController.Prog0Command := ClosedOilFeed;

OilFeedController.Prog1Command := SlowOilFeed;

OilFeedController.Prog2Command := FastOilFeed;

OilFeedController.FB0 := SmallOilValveClosed;

OilFeedController.FB1 := SmallOilValveOpened;

OilFeedController.FB2 := LargeOilValveClosed;

OilFeedController.FB3 := LargeOilValveOpened;

D3SD(OilFeedController);

SmallOilValve := OilFeedController.Out0;

LargeOilValve := OilFeedController.Out1;

Switch Between Program Control and Operator Control

The following diagram shows how the D3SD instruction changes between Program control and Operator control.

(1) The instruction remains in Operator control mode when ProgOperReq is true.

Commanded State in Program Control

The following table shows how the D3SD instruction operates when in Program control.

If more than one program command input is true:

Commanded State in Operator Control

The following table shows how the D3SD instruction operates when in Operator control.

If more than one operator command input is true:

After every instruction execution, the instruction:

Hand Mode or Override Mode

The following table describes how the D3SD instruction determines whether to operate in Hand or Override mode.

When Override is set, it takes precedence over Program and Operator control. The following table describes how the Override mode affects the commanded state.

If OverrideState is invalid, the instruction sets the appropriate bit in Status and does not enter the override state.

When Hand is true, it takes precedence over Program and Operator control. The following table describes how the Hand mode affects the commanded state.

If more than one HandFB input is true, the instruction sets the appropriate bit in Status and, if Hand is true, the instruction holds the previous state.

Output State

The D3SD output state is based on the state of the command status.

Fault Alarm Conditions

The D3SD instruction checks for these fault alarm conditions.

If there is no fault present, FaultAlarm is cleared to false if one of the following conditions is met:

FaultAlarm cannot be cleared to false when FaultAlarmLatch is true, unless FaultAlmUnlatch is true and no fault is present.

Mode Alarm Conditions

The mode alarm reminds an operator that a device has been left in Operator control. The mode alarm only turns on when, in Operator control mode, the program tries to change the state of the device from the operator’s commanded state. The alarm does not turn on if an operator places a device in Operator control mode and changes the state. The D3SD instruction checks for mode alarm conditions, using these rules.