Process Analog Output (PAO)

The Process Analog Output (PAO) instruction drives an analog output and checks for alarm conditions. Use the PAO instruction for a channel of an analog output module. Use the PAO instruction with any analog (REAL) signal.

The PAO instruction:

Available Languages

Ladder Diagram

Function Block Diagram

Structured Text

PAO(PAO tag, BusObj);

Operands

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

Data Conversions

.

Configuration Operands

P_ANALOG_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 analog 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 Logix Tag based alarms are defined for the following 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:

PAOTag.@Alarms.AlarmName.AlarmElement

The Program commands for each alarm enable users to Acknowledge, Suppress, Unsupress and Unshelve the Alarm. These commands are propagated to corresponding commands (ProgAck, ProgSuppress, ProgUnsupress, ProgUnshelve) of the tag-based alarm.

There are also Program commands that enable users to Acknowledge, Reset, Suppress and Unsuppress all alarms of the instruction or an alarm set at the same time.

Operation

The following diagram illustrates the functionality of the PAO instruction:

The following diagram explains logic for Interlock trip status:

The following diagram illustrates the functionality of the PAO instruction for a pulsed device:

Virtualization

When Virtualization is active, the output of the analog output holds at zero and I/O faults are ignored. Manipulate the instruction to operate as if a working analog output is present. Do this 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.

Configuration of Strings for HMI

Configure strings for HMI faceplates, as seen in

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.

Command Source

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

The instruction is able to enable/disable the following operations:

The instruction checks for invalid configurations on control forcing the nearest valid configuration.

The core control model deals specifically with the arbitration of the source of the commands and parameters currently being accepted by the receiving function. More specifically, whether the source is:

These sources are known as Prog (Program) and Oper (Operator) control, respectively.

The optional ability to lock into one control source or the other is required to ensure that the other control source cannot acquire privilege when the designer wants to prevent it.

Core Command Source Model

The core control model consists of the following control sources: Oper, OperLocked, Prog, ProgLocked. 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 utilizes the entire base model. The differentiation being that 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 will win 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.

Change 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 is done to maintain 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

Higher priority command sources which operate independently within the model: Override, Maintenance, Out-of-Service, In-Service and Hand.

Monitoring the PAO 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. Therefore, if the instruction is in the control path activated by the logic it will execute.

Example 1: Analog valve

The following example provides a demonstration of using a PAO instruction to control a valve. During normal operation, the operator sets the valve position using PAO through the HMI faceplate. This example also includes interlock conditions low-low level and high-high temperature provided by outputs of PAI instructions. The interlock conditions are used as inputs by PAO to set the valve (Local:0:O.Ch0Data = Out_CVData) to an interlock position (for example, closed). This is done by setting the PAO configuration parameter Cfg_Intlk to 0.

Ladder Diagram

Function Block Diagram

Structured Text

FCV_103.Inp_NBIntlkOK := NOT(TT_103.Sts_HiHi OR LT_103.Sts_LoLo);

FCV_103.Inp_IOFault := Local:0:I.Ch0Fault;

PAO(FCV_103, 0);

Local:1:O.Ch0Data := FCV_103.Out_CVData;

Example 2: Manual loading station

This example uses the PAO instruction to implement a manual loading station for a pressure control valve that is used to regulate gas supply to a process. The control valve in our example has opened and closed limit switches and a position feedback. The desired valve position is provided by the operator through the HMI faceplate.

The field inputs for position feedback, opened limit switch, and closed limit switch are connected to the instruction inputs Inp_PosFdbk, Inp_OpenedFdbkData, and Inp_ClosedFdbkData. The Out_CVData is connected to the field output going to the valve.

The parameters Cfg_HasOpenedFdbk and Cfg_HasClosedFdbk are both set to 1 so the instruction knows the field is providing opened and closed limit switches. The parameters Cfg_UseOpenedFdbk and Cfg_UseClosedFdbk are set to 1 so that these limit switches are used to determine device status.

The analog output card is expecting an output in units of 4…20 mA; however, the faceplate shows the value in terms of 0…100% open. Therefore, the scaling parameters are set as follows. Cfg_CVEUMin=0, Cfg_CVEUMax=100, Cfg_CVRawMin=4, Cfg_CVRawMax=20.

The valve position feedback is provided in CV engineering units, percent in our particular case, via PAI instruction.

Ladder Diagram

Function Block Diagram

Structured Text

PCV_110812.Inp_OpenedFdbkData := I_PSO110812;

PCV_110812.Inp_ClosedFdbkData := I_PSC110812;

PCV_110812.Inp_PosFdbk := I_PZV110812.Val;

PAO(PCV_110812, 0);

O_PV110812 := PCV_103.Out_CVData;

Example 3: Ratchet valve

This example uses the PAO instruction to automate a ratcheting valve that is driven open or closed by using two discrete outputs to control flow. The flow valve in our example has a position feedback. The desired valve position is provided by an output of a control algorithm that is elsewhere in logic.

In this example, the field inputs for position feedback are wired (or connected) to the instruction input Inp_PosFdbk. Out_CVOpenData and Out_CVCloseData are connected to the field outputs going to the valve. The input to the instruction to set valve position is wired to PSet_CV. Cfg_ProgNormal is set to 1 so the instruction defaults to Program mode.

The analog output is not used; however, the faceplate shows the value of Val_CVOut in terms of 0…100% open, CV in engineering units. Therefore, the scaling parameters are set as follows. Cfg_CVEUMin=0, Cfg_CVEUMax=100, Cfg_CVRawMin=0 (default), Cfg_CVRawMax=100 (default)

The feedback signal is provided in CV engineering units via PAI instruction which scales the signal from feedback raw units to CV engineering units.

In this example, the ratcheting control valve is to be adjusted by cycling the open or close valve command for a period of time proportional to the amount the valve is to be moved. Cfg_CycleTime is set to 10, to define the overall period of the cycle to cycle on and off the open or close output. Cfg_OpenRate and Cfg_CloseRate are both set to 1, which means the required valve output is energized 1 second for every 1% difference between the target position and the actual position provided by feedback.

Cfg_MaxOnTime is set to 5 so that the output is energized for no more than 5 seconds of the 10-second cycle time to allow for the valve to move, and the feedback to be verified before the next cycle. Cfg_MinOnTime is set to 1 so that the output does not pulse if the calculated pulse time is less than 1 second.

Ladder Diagram

Function Block Diagram

Structured Text

FCV_110813.PSet_CV := FIC_110813_Out;

FCV_110813.Inp_PosFdbk := I_FVZ110813;

PAO(FCV_110813, 0);

O_FYO110813 := FCV_110813.Out_CVOpenData;

O_FYC110813 := FCV_110813.Out_CVCloseData;