PLC Communication

The I/O block defined in this document will link a PLC to many mobile devices. The application can be configured to talk to several of these data blocks on multiple PLC's. Each block will control a group of mobile devices with all devices in that group receiving a inspection request (trigger) when the PLC sends the command. Each mobile device can only be configured to connect to a single I/O block.

plc overview

Communication between MVS Server and a PLC takes place via OPC. For more information see KepwareConfiguration

Block Tag Overview

  • Outputs: Data written by the Application Server (these values should be considered read only by the PLC, as the MVS application will only write these values)

    • Heartbeat: Heartbeat of the application server (toggled every X seconds)
      • DataType: Boolean (1bit)
    • InspectionStatus: A byte describing the state of the inspection (see Inspection Status)
      • DataType: Int (16bit)
    • Inspection_PASS: true if inspection has passed.
      • DataType: Boolean (1bit)
    • Inspection_FAIL: true if inspection has failed.
      • DataType: Boolean (1bit)
    • ErrorCode: If the InspectionStatus is in Error this error code will indicate the specific state. (see Error Codes)
      • DataType: Int (16bit)
    • Inspection_STRING: additional field if the inspection is reading any OCR/Barcode data.
      • DataType: String (recommended length: 32 character)
    • PartSerial_ECHO: Echo of the PartSerial.
      • DataType: String (controls/kepware defined length, same as PartSerial)
    • InspectionPointID_ECHO: Echo of the InspectionPointID.
      • DataType: String (16 char)
    • TriggerInspection_ECHO: Echo of the TriggerInspection bit.
      • DataType: Boolean (1bit)
    • ClearResults_ECHO: Echo of the ClearResults bit.
      • DataType: Boolean (1bit)
    • CancelInspection_ECHO: Echo of the CancelInspection bit.
      • DataType: Boolean (1bit)
    • SendMessage_ECHO: Echo of the SendMessage bit.
      • DataType: Boolean (1bit)
    • MessageID_ECHO: Echo of the MessageID int.
      • DataType: Word (16bit)
    • MessageStatus: The status of the last message.
      • DataType: Word (16bit)

  • Inputs: Data written by the PLC and read by the app server (these values are considered read only by the MVS application)

    • InspectionPointID: A unique key of the InspectionPointDto the PLC is requesting to run. (user configurable field)
      • DataType: String (16 char)
    • TriggerInspection: The app server will watch for the rising edge of this bit to trigger a inspection (ie. send the InspectionRequest via the camera status hub). The PartSerial and InspectionPointID fields should be echoed prior to raising this bit.
      • DataType: Boolean (1bit)
    • ClearResults: A signal to tell the application server to clear the results it has written to the PLC.
      • DataType: Boolean (1bit)
    • CancelInspection: A signal to tell the application server to cancel any pending inspections triggered by this plc.-
      • DataType: Boolean (1bit)
    • PartSerial: The serial number /unique identifier of the part. The value in this field does not effect logic, it is saved into the database with the image and the inspection result. It is purely for traceability.
      • DataType: String (controls/kepware defined length)
    • MessageID: The Id of the message (defined in MVS Server) to send to all connected mobile devices.
    • DataType: Word (16bit)
    • SendMessage: A command bit to send the MessageID to the connected clients. This command will override any previous messages that the user has not responded to.

Inspection Status (look up table)


        /// Initialzing state, default until image processing is ready
        WaitingForIPM = 0,

        /// System is ready for a trigger
        Ready=1,

        /// Inspection processing is complete and results can be read
        Complete=2,

        /// Requesting a image from the remote device/camera
        Requesting=3,

        /// Image has been received and is being processed
        Processing = 4,      

        /// There was a error. see <see cref="PlcErrorCodes"/> for details
        Error=5,

        /// the inspection was canceled by the PLC
        Canceled = 6,

Error Codes (look up table)

        NoError = 0,

        /// Remote camera is not connected, can't be located, or isn't ready to take images
        DeviceNotFound = 1,

        /// The specified inspection ID does not exist
        InspectionNotFound = 2,

        /// There was a error during processing, see IPM Logs for details
        ProcessingError = 3,

        /// There was a internal error handling the PLC request. See application server logs for details
        InternalError = 4,

        /// The inspection is not properly configured. This usually means its missing a IPM Workspace.
        InspectionConfigurationError = 5,

        /// IPM client is not available to process the image
        ProcessingNotAvailable = 6,

MessageStatus Codes (Look Up Table)


        /// Ready to send a message
        Ready = 0,
        ///  Waiting for the user to respond to the message.
        Waiting = 1,
        /// Message Not Found. The messageKey does not exist on the server.
        MessageNotFound = 2,
        /// Message Complete, no clients were available to receive the message
        NoAvailableClients = 3,
        /// Message Complete, unhandled exception caught in the process.
        ApplicationException = 4,
        /// The user responded in the Affirmative (`Yes`,`Accept`,`Ok`)
        AffirmativeResponse = 5,
        /// The user responded in the Negative (`No`,`Reject`,`Cancel`)
        NegativeResponse = 6,

Allen Bradley Tag Structure

MVSCAMERA.CANCELINSPECTION Boolean
MVSCAMERA.CANCELINSPECTION_ECHO Boolean
MVSCAMERA.CLEARRESULTS Boolean
MVSCAMERA.CLEARRESULTS_ECHO Boolean
MVSCAMERA.EMPLOYEEOVERRIDE_ECHO.DATA/32 String
MVSCAMERA.ERRORCODE Short
MVSCAMERA.HEARTBEAT Boolean
MVSCAMERA.INSPECTION_FAIL Boolean
MVSCAMERA.INSPECTION_PASS Boolean
MVSCAMERA.INSPECTION_STRING.DATA/32 String
MVSCAMERA.INSPECTIONPOINTID.DATA/16 String
MVSCAMERA.INSPECTIONPOINTID_ECHO.DATA/16 String
MVSCAMERA.INSPECTIONSTATUS Short
MVSCAMERA.LOGOVERRIDE Boolean
MVSCAMERA.LOGOVERRIDE_ECHO Boolean
MVSCAMERA.OVERRIDEEMPLOYEEID.DATA/32 String
MVSCAMERA.PARTSERIAL.DATA/32 String
MVSCAMERA.PARTSERIAL_ECHO.DATA/32 String
MVSCAMERA.TRIGGERINSPECTION Boolean
MVSCAMERA.TRIGGERINSPECTION_ECHO Boolean
MVSCAMERA.MESSAGEID Short
MVSCAMERA.MESSAGEID_ECHO Short
MVSCAMERA.SENDMESSAGE Boolean
MVSCAMERA.SENDMESSAGE_ECHO Boolean
MVSCAMERA.MESSAGERESULT.DATA/32 String
MVSCAMERA.MESSAGESTATUS Short

Siemens S7 Block Structure (UDT):

Total Size 348 bytes (depending on string length)

Name DataType Offset Notes
INPUTS
Heartbeat Bool 0.0
TriggerInspection_ECHO Bool 0.1
ClearResults_ECHO Bool 0.2
CancelInspection_ECHO Bool 0.3
LogOveride_ECHO Bool 0.4
Inspection_PASS Bool 0.5
Inspection_FAIL Bool 0.6
SendMessage_ECHO Bool 0.7
InspectionStatus Int 2.0
ErrorCode Int 4.0
InspectionPointID_ECHO String 6.0 Fixed Length: 16
PartSerial_ECHO String 24.0 Custom Length: 48
EmployeeOverride_ECHO String 74.0 Custom Length: 48
Inspection_STRING String 124.0 Custom Length: 48
MessageId_ECHO Int 296.0
MessageStatus Int 298.0
MessageResult String 300.0 Custom Length: 48
OUTPUTS
TriggerInspection Bool 174.0
ClearResults Bool 174.1
CancelInspection Bool 174.2
LogOveride Bool 174.3
SendMessage Bool 174.4
InspectionPointID String 176.0 Custom Length: 16
PartSerial String 194.0 Custom Length: 48
OverrideEmployeeID String 244.0 Custom Length: 48
MessageId Int 294.0

Siemens S7 Configuration

To communicate via Kepware the PLC must allow access from an external systems. Enable in TIA Portal: Device Configuration > Protection & Security > Connection mechanisms > Permit access with PUT/GET communication from remote partner.

enable communication

Description of Logic

Echo Tags

All Outputs from the plc have a corresponding "TAGNAME_ECHO" field. This field will be written with the value of the corresponding tag as soon as possible by the software. Its purpose is to confirm to the PLC that the software has read the values prior to executing any logic. This will prevent any race conditions (and unintended behavior) if the PLC updates several values on the same scan.

If the PLC sets the InspectionPointID field and TriggerInspection boolean on the same scan, the software might read and execute its trigger logic before receiving the updated InspectionPointID value - running the wrong inspection. Therefore its best practice to confirm that the ECHO fields match prior to triggering any logic.

This should also be used to confirm that the trigger command was read by the MVS Application. Since the scan rate of the PLC is likely much higher than the polling rate of the OPC server, a one-shot pulse will likely be missed by the application.

Heartbeat Logic

The application will toggle the heartbeat boolean every 5 seconds. If this value does not change within some timeframe, say 10-20 seconds, the PLC can assume that a fault has occurred and the software is unresponsive or shutdown. Once the heartbeat resumes the PLC can assume the software is functional again.

Command Logic

The application can receive several commands from the PLC. Each command takes the form of a boolean value and the application will attempt to execute the command on detecting a rising edge (0 to 1) transition of the value. They are as follows:

  • TriggerInspection: Trigger a inspection using the values in the InspectionPointID and PartSerial addresses.
  • ClearResults: Clear any results/errors in the vision system. Specifically the application will set the following values:
    • Inspection_PASS : 0
    • Inspection_FAIL : 0
    • InspectionStatus : Ready=1 (if initialized) or Initializing = 0 if the system is loading
    • ErrorCode : NoError = 0
  • CancelInspection: Abort and cancel any trigger signals that have not received a response from a mobile device. The intention behind this command it to allow the PLC to timeout if a operator takes too long to respond. If a inspection was canceled the InspectionStatus will be set to Canceled=6

Trigger Command Logic

Triggering is how the plc will request a image from a mobile camera. The logic is as follows:

  1. The plc needs to wait for the InspectionStatus to be Ready=1, Complete=2, or Canceled=6 for the application to accept a trigger command. Otherwise the command will be ignored. Note: If the InspectionPointID has no IPM workspace assigned the InspectionStatus can be WaitingForIPM=0 when the trigger is sent.

  2. The plc will populate the InspectionPointID field with a unique value (defined in the MVS software) that identifies the requested inspection.

  3. [Optional] The PLC will populate the PartSerial field with the serial number of the part being inspected. The value entered in this field will be added to the application's database along with the image and results of the inspection.

  4. The PLC will wait for the InspectionPointID_ECHO (and possibly for the PartSerial_ECHO) values to match the InspectionPointID value (also PartSerial if set).

  5. The PLC will set the TriggerInspection Boolean from 0 to 1.

  6. The application will do the following:

    • Echo the TriggerInspection value in TriggerInspection_ECHO.
    • Update the InspectionStatus to Requesting=3
    • Validate the InspectionPointID value exists, if not it will set the InspectionStatus to Error=5, the ErrorCode to InspectionNotFound = 2, and abort the trigger.
    • Validate at least one mobile device is connected to the server that is configured to process the InspectionPointID, if not it will set the InspectionStatus to Error=5, the ErrorCode to DeviceNotFound = 1, and abort the trigger.
    • If the above conditions are met, it will send the request to the mobile device(s) to acquire a image.

  7. Once a mobile device receives the request the I/O state will remain unchanged until the PLC sends a CancelInspection command or a image is sent from a mobile device to the application server. If the PLC sets CancelInspection from 0 to 1 at this point the application will abort the trigger and InspectionStatus will be set to Canceled=6

  8. Once a image is received from the mobile device the server will update the InspectionStatus to Processing = 4.

  9. Once processing the image is complete, the application will write the Inspection_PASS and Inspection_FAIL, Inspection_STRING values with the results generated by the image processing. It will then set the InspectionStatus to Complete=2

PLC Override Logic

Overrides are when a supervisor on the line will override an inspection, forcing the logic to move to the next step of the process regardless of the vision result. This logic is handled on the PLC, but for traceability reasons its best to also log the override in the database. This is initiated from the PLC via the LogOverride command, and will log the override along with the value from OverrideEmployeeID to the database.

  1. The plc needs to wait for the InspectionStatus to be Ready=1, Complete=2, or Canceled=6 for the application to accept a override command. Otherwise the command will be ignored.

  2. The PLC will populate the following fields and wait for their corresponding ECHO to be written by the software:

    • OverrideEmployeeID: Identification of the employee who is performing the override (no validation on this value, it will just save to the database)
    • InspectionPointID: The key value of the inspection point that this override applies to.
    • PartSerial: The serial number of the part that the override applies to.

  3. Once the ECHO fields match, the PLC can set the LogOverride Boolean from 0 to 1.

  4. The application will do the following on detecting a rising edge of LogOverride:

    • Echo the LogOverride bit to LogOverride_ECHO.
    • Update the InspectionStatus to Processing=4
    • Lookup the inspection results for the supplied InspectionPointID and PartSerial
      • If the results are located it will Log the override to the database with the supplied OverrideEmployeeId value and set the InspectionStatus to Complete=2
      • If the results can't be located it will set InspectionStatus to Error=5, the ErrorCode to InspectionNotFound=2

Its expected that the above process will complete very quickly, possibly on the same scan, so the transition of InspectionStatus from Ready=1 to Processing=4 and again to Complete=2 might not be observed on the plc.

Send Message Logic

Plc Push Messages are a way for the PLC to push a message dialog to the remote cameras assigned to the PLC. To invoke the message logic, the PLC needs to specify the MessageID (MessageKey) and set the SendMessage bit high. The MessageStatus bit will relay the state and result of the message back to the PLC. If there the message requests a custom text/number/pin the value will be written to the MessageResult field. If any unhandled exceptions are raised by the server during this sequence, the MessageStatus will update to ApplicationException(4), the actual exception will be written to the MVS Server logs.

  1. The PLC sets the desired MessageID value, which corresponds to a MessageKey in the PLC Push Message configuration and confirms the MVS Server has read the value by waiting for MessageID_ECHO to properly echo the value.

  2. The PLC sets SendMessage bit high. This signal can be set low once the SendMessage_ECHO value echos correctly.

  3. The MVS Software will validate the MessageID and connected camera clients.

  4. If there are no connected camera clients the MessageStatus will update to NoAvailableClients(3) and abort the sequence.
  5. If there the MessageID is not configured in the system, the MessageStatus will update to MessageNotFound(2) and abort the sequence.
  6. If the MessageID is valid and there is at least one camera client connected, the MessageStatus will update to Waiting(1) and the sequence will continue. If at any point prior to returning a result all clients registered to this PLC Instance disconnect from the server the MessageStatus will update to NoAvailableClients(3) and abort the sequence.
  7. The MVS Software will send the message to the camera client and remain in the Waiting(1) status until a user submits a response.
  8. The response will be written to the PLC:
  9. If the user pressed a affirmative response option (Yes,Ok,Accept,Continue, etc) the MessageStatus will update to AffirmativeResponse(5).
  10. If the user pressed a negative response option (Cancel,No,Reject,Abort,etc) the MessageStatus will update to NegativeResponse(6).
  11. If the message requested a custom input (text/password/number), the result of the entry will be written to the MessageResult field and the MessageStatus will be set to AffirmativeResponse(5).