Process Discrete Output (PDO)

The Process Discrete Output (PDO) instruction drives a discrete (true / false) output, monitors discrete inputs serving as feedbacks from a device driven by the discrete output, and checks for alarm conditions. Use the PDO instruction for a channel of a discrete output module. Use the PDO instruction with any discrete (BOOL) signal.

The PDO instruction:

Available Languages

Ladder Diagram

Function Block Diagram

Structured Text

PDO(PDO tag, BusObj);

Operands

There are data conversion rules for mixing numeric data types within an instruction. See Data Conversions.

Configuration Operands

P_DISCRETE_OUTPUT 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 output 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 tag-based alarm. Access alarm elements using this format:

PDOTag.@Alarms.AlarmName.AlarmElement

The PDO instruction handles alarm conditions described by these four use cases and conditions:

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 PDO instruction.

Operation

The following diagram illustrates functionality of the PDO instruction:

Operator command request confirmation

The PDO instruction enables operator command requests OCmd_Off, OCmd_On, OCmd_OffPulse, OCmd_OnPulse and OCmd_ContPulse. Enforced security might require the request to be confirmed or canceled before the selected command executes. The instruction checks the security rules inspecting Cfg_CnfrmReqd. If Cfg_CnfrmReqd=0 no confirmation is required and the request executes immediately. If Cfg_CnfrmReqd=1 the instruction waits for confirmation before executing. For Cfg_CnfrmReqd=2 or 3 eSignature is needed before the confirmation and cancellation is enabled.

Virtualization

Use virtualization for instruction testing and operator training. Set the Inp_Virtual operand to 1 to enable virtualization. After finishing virtualization, set the Inp_Virtual operand to 0 to return to normal operation.

When Virtualization is active, the output of the discrete output holds at 0, virtual feedback of a device is provided and I/O faults are ignored. Setting of Cfg_VirtualFdbkTime operand delays the echo of the On/Off status of the device. Manipulate the instruction to operate as if a working discrete output is present.

Initialization

The instruction is normally initialized in the instruction first run. Request re-initialization 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

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

Command Source

The instruction uses these command sources. The command sources are prioritized in order from highest to lowest in this table.

The instruction enables or disables these operations:

The instruction checks for invalid configurations of control and forces the nearest valid configuration.

The core control model arbitrates the source of the commands and parameters that the receiving function accepts. The core control model determines if the source is:

Locking a control source prevents the other control source from acquiring privilege.

Core Command Source Model

The core control model consists of these control sources:

The control model defaults to this configuration. Other control sources may be present in the model but act as overriding control sources, acting independent of the base Operator/Program state machine.

Enabling control sources as Configuration

The individual control sources may be enabled or disabled by the user. The default configuration uses the entire base model; upon power-up of the processing environment the control source will be the designated default. Some combinations of enabled control sources are disallowed as they are either unnecessary or could create unintended changes.

Prog Power Up

Configuration allows the user to specify whether Operator or Program will be the power-up default.

Prog Priority

Configuration allows the user to specify whether Operator or Program commands take priority when simultaneously asserted.

Automatic reset of commands

All commands are treated as one-shot-latched. This means that all commands are automatically cleared when the instruction executes and processes them.

Changing Destination States

Under certain configurations the destination command source for some commands may change. This is in keeping with the intent of the command. Example: If the Prog is disabled, the destination of the OCmd_Prog command is directed to the ProgLocked state instead of the Prog state. This maintains the intent of the OCmd_Prog command: the operator entity wishes to place the function in control of the program. If the command was eliminated then there would be no way to accomplish this. This is only done in configurations where it would cause no conflict or race condition, but serves to preserve as much user functionality as is practical.

Higher Priority Command Sources

These Higher priority command sources operate independently within the model:

Monitor the PDO 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

Use the PDO instruction to control a heating jacket on a vent filter. The heating jacket is being used in this case to keep the vent filter dry when there is potential for condensate buildup. The vent filter heater jacket does not provide the feedback on its status. In normal operating conditions, the vent filter heater jacket is being commanded on or off by the control sequence configured in the controller. Always command the vent filter off on interlock associated with the status of the controlling sequence of the heating jacket.

The controlling sequence issues a single bit for the desired state of the vent filter heater. The operand PCmd_On is connected to this bit to command the vent filter heater on and off. The operand Cfg_PCmdOnAsLevel is set to 1, indicating that the instruction acts upon PCmd_On based on value (level) instead of acting only on transition to true (edge) so that PCmd_On can be used to command both the On and Off states. The controlling sequence could be written to set the command bits PCmd_On and PCmd_Off directly (for example, by using structured text within an SFC), in which case Cfg_PCmdOnAsLevel could be left at its default of 0 to cause the instruction to clear the commands once they have been acted upon. The operand Cfg_ProgNormal is set to 1 to indicate that the normal operating state of the controller is Program, meaning it is normally commanded by the control sequence. The status of the sequence is connected to the Inp_IntlkOK operand so that the output to the vent filter heater jacket is always off when the skid is not operating properly, even if the instruction is not in Program mode. The operands Cfg_HasOnFdbk and Cfg_HasOffFdbk are both set to 0 to indicate that the vent filter heater jacket does not provide feedback on its status.

The example is shown in all three languages.

Ladder Diagram

Function Block Diagram

Structured Text

JV9010.Inp_IntlkOK := Status_OK;

JV9010.PCmd_On := YV9010;

PDO(JV9010, 0);

O_JV9010 := JV9010.Out_CVData;