Process Discrete Input (PDI)

The Process Discrete Input (PDI) instruction monitors a discrete (true or false) input, and checks for alarm conditions. Use the PDI instruction to process a signal from a channel of a discrete input module. Use the PDI instruction with any discrete (BOOL) signal.

The PDI instruction provides these capabilities:

Available Languages

Ladder Diagram

Function Block Diagram

Structured Text

PDI(PDI tag, BusObj);

Configuration Operands

P_DISCRETE_INPUT Structure

Public members are standard, visible tag members that are programmatically accessible. Private, or hidden, members are used in HMI faceplates and are not programmatically accessible. Private members are listed in separate tables after public members.

BUS_OBJ Structure

The BUS_OBJ structure links the discrete input instruction to other devices and instructions in a complex control strategy, typically into a hierarchy. A Bus Object rolls up status and alarm information from lower level devices to higher level control and fans out commands from higher level control to lower level devices. Items link to the bus by referencing a single member of the BUS_OBJ array associated with the bus.

This parameter links the instruction to an external tag that contains necessary data for the instruction to operate. The external tag must be of the data type shown, or may be NULL. If NULL, the Bus functions of this instruction are not available.

Alarms

Discrete tag-based alarms are defined for these members.

Mark the alarm as used or unused and set standard configuration members of the discrete Logix Tag based alarm. Access alarm elements using this format:

PDITag.@Alarms.AlarmName.AlarmElement

There are Program, Operator, and External commands that enable the Reset and Reset & Acknowledge of all alarms of the instruction (Alarm Set) at the same time. This diagram shows how the commands interact with the instruction.

Operation

The Gate function provides the ability to raise an abnormal condition (alarm condition) when another condition is true. For example, a high vibration switch should only generate an alarm when the associated motor is running long enough to stabilize. The Inp_Gate input must be set to 1 (its default value) and the tag-based alarm for Sts_TgtDisagree enabled for alarm to occur. The alarm will not occur until the Inp_Gate input has been set for the Gate Delay (Cfg_GateDly) time.

The alarm is generated when the Inp_PVData (process variable) input is different from the Inp_Target (target) input. The Target indicates the normal condition. For example, a flow switch should indicate flow when a pump is running and should not indicate flow when a pump is stopped. The pump run status is used as the Target input, and when the switch does not match the target (within the allotted time), the Alarm (Flow Loss / Switch Failure) is generated.

This diagram illustrates the functionality of the PDI instruction:

Debouncing

A bounce is a reversal of state that occurs immediately after a deliberate transition. When filtering bounces you should acknowledge the very first transition as early as possible and ignore subsequent changes until the end of a hold-off period. Use Cfg_Debouncing to specify the hold-off period in seconds.

For example, as soon as the Input PV changes state from 0 to 1, the output changes to 1 and will not then follow the Input PV back to 0 until the Cfg_Debounce time has expired. If Cfg_Debounce is set to 1 second, the output remains at 1 for at least 1 second. After 1 second, if the Input PV was then 0, the output would change to 0 and would remain at 0 for at least 1 second.

This illustration shows the Debounce operation.

Virtualization

Virtualization in PDI provides a virtual 0-state or 1-state input (Set_VirtualPV) that processes like an input. Use virtualization for instruction testing and operator training. Use PCmd_Virtual or MCmd_Virtual to enable virtualization. After finishing virtualization, use PCmd_Physical or MCmd_Physical to return to normal (physical device) operation.

Initialization

The instruction is normally initialized in the instruction first run. Re-initialization can be requested any time by setting Inp_InitializeReq = 1. For proper initialization, when adding the instruction while performing an online edit of the code, make sure that Inp_InitializeReq = 1, the default value.

Configuration of Strings for HMI

Configure strings for HMI faceplates, as seen in

FactoryTalk View

FactoryTalk View

, and for the

Logix Designer

configuration dialog box. The strings are set to extended properties of tag items. Configure the strings in the Logix Designer application only.

Implementation

This illustration shows normal implementation with the input condition mapped to Inp_PVData on a separate branch.

This illustration shows the implementation with the input condition mapped to the PDI instruction using the rung-condition-in.

Monitor the PDI Instruction

Use the operator faceplate from the PlantPAx library of Process objects for monitoring.

Affects Math Status Flags

No.

Major/Minor Faults

None specific to this instruction. See Index Through Arrays for array-indexing faults.

Execution

Ladder Diagram

Function Block Diagram

Structured Text

In Structured Text, EnableIn is always true during normal scan. The instruction executes when it is in the control path activated by the logic.

Example

In this example, tag I_VSH_P50 is the digital process value monitored by the PDI instruction. This tag provides a Boolean indication of High Vibration. The bad quality indication for the value of the process variable (Inp_ChanFault) comes from the connection status indication on the input module.

Inp_Target is defaulted to 1 indicating that the normal condition for I_VSH_P50 is also 1, and tag comments confirm that 1=OK for this process value. Inp_Gate is connected to the Motor Running status tag (P50_Running) that comes from the Sts_Running output of the P_Motor instruction instance for this motor (P50_Motor). The gate delay is configured to give the motor sufficient time after starting to settle into full normal speed run before enabling the high vibration indication (Sts_TgtDisagree) and alarm. The tag-based alarm for Target Disagree status (Sts_TgtDisagree) applies On Delay timing so the alarm will not raise until after delay time has expired.

Finally, P50_HighVibr is the output tag that indicates the status of I_VSH_P50 with appropriate gate delays based on whether the motor is running.

Ladder Diagram

Function Block Diagram

Structured Text

VAH_50.Inp_PV_Data := I_VSH_P50;

VAH_50.Inp_ChanFault := CAF_117[5];

VAH_50.Inp_Gate := P50_Running;

PDI(VAH_50);

P50_HighVibr := VAH_P50.Sts_TgtDisagree;