[PLC4X-341] PLC4J GIP/CIP Read STRING tag returns null w/error

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 0.9.1
• Component/s: Driver-Ethernet/IP, PLC4J

Language

Java

• Assignee: Unassigned
• Reporter: Andy Grebe
• Created: 06/May/22 12:50
• Updated: 09/Nov/22 10:21

Description

STRING tag reads in plc4j using eip are not returning the string.  The error returned is:

java.lang.NullPointerException: Cannot invoke "org.apache.plc4x.java.api.value.PlcValue.getString()" because the return value of "org.apache.plc4x.java.api.messages.PlcReadResponse.getPlcValue(String)" is null

It looks like plc4j is considering it a structure or array.  If I read with 

%MyString[{index}]:STRING:1 

I can get the individual characters in the string by placing the actual index in {index}.  So it knows where to start reading the data from. 

When running a wireshark cap, I can see the returned string in the response from the PLC.  Attached is a pcap of the transaction.

 

Code Snippet:

try (PlcConnection plcConnection = new PlcDriverManager().getConnection("eip://127.0.0.1")) {
    if (!plcConnection.getMetadata().canRead()) {
        logger.error("PLC connection doesn't support reading.");
        return;
    }
    logger.info("PLC connector connected");    // Create a new read request:
    PlcReadRequest.Builder builder = plcConnection.readRequestBuilder();
    logger.info("Created Builder");
    builder.addItem("value-1", "%MyString:STRING:1");
    PlcReadRequest readRequest = builder.build();    // Register a callback executed as soon as a response arrives.
    logger.info("Make sync request for PLC data");
    PlcReadResponse readResponse = readRequest.execute().get(5000, TimeUnit.MILLISECONDS);
    if (readResponse != null) {
        printPlcResponse(readResponse);
    } else {
        logger.error("An error reading PLC, response is NULL");
    }
    for (String field : readResponse.getFieldNames()){
        rtnArray.add(new PlcReadResponseData(field,
            readResponse.getPlcValue(field).getString()) );  // Exception is thrown here
    }
} catch (PlcConnectionException e) {
    e.printStackTrace();    
}