Allow CRC mismatch to be ignored

modbus/src/main/java/com/digitalpetri/modbus/client/ModbusRtuClient.java

@@ -3,6 +3,7 @@
 import com.digitalpetri.modbus.Crc16;
 import com.digitalpetri.modbus.ModbusRtuFrame;
 import com.digitalpetri.modbus.TimeoutScheduler.TimeoutHandle;
+import com.digitalpetri.modbus.exceptions.ModbusCrcException;
 import com.digitalpetri.modbus.exceptions.ModbusException;
 import com.digitalpetri.modbus.exceptions.ModbusExecutionException;
 import com.digitalpetri.modbus.exceptions.ModbusResponseException;
@@ -80,7 +81,7 @@ public CompletionStage<ModbusResponsePdu> sendAsync(int unitId, ModbusRequestPdu
         // The frame parser needs to be reset!
         // It could be "stuck" in Accumulating or ParseError states if the timeout was caused by
         // an incomplete or invalid response rather than no response.
-        transport.resetFrameParser();
+        resetFrameParser();
 
         promise.future.completeExceptionally(
             new TimeoutException("request timed out after %sms".formatted(timeoutMillis))
@@ -167,9 +168,9 @@ private void onFrameReceived(ModbusRtuFrame frame) {
       }
 
       if (!verifyCrc16(frame)) {
-        transport.resetFrameParser();
+        resetFrameParser();
 
-        promise.future.completeExceptionally(new ModbusException("CRC mismatch"));
+        promise.future.completeExceptionally(new ModbusCrcException(frame));
         return;
       }
 
@@ -229,13 +230,40 @@ private void onFrameReceived(ModbusRtuFrame frame) {
     }
   }
 
+  /**
+   * Reset the transport's frame parser.
+   */
+  protected void resetFrameParser() {
+    transport.resetFrameParser();
+  }
+
+  /**
+   * Calculate the CRC-16 for the given frame (unit ID and PDU).
+   *
+   * @param unitId the unit ID.
+   * @param pdu the PDU.
+   * @return a {@link ByteBuffer} containing the calculated CRC-16.
+   */
+  protected ByteBuffer calculateCrc16(int unitId, ByteBuffer pdu) {
+    var crc16 = new Crc16();
+    crc16.update(unitId);
+    crc16.update(pdu);
+
+    ByteBuffer crc = ByteBuffer.allocate(2);
+    // write crc in little-endian order
+    crc.put((byte) (crc16.getValue() & 0xFF));
+    crc.put((byte) ((crc16.getValue() >> 8) & 0xFF));
+
+    return crc.flip();
+  }
+
   /**
    * Verify the reported CRC-16 matches the calculated CRC-16.
    *
    * @param frame the frame to verify.
    * @return {@code true} if the CRC-16 matches, {@code false} otherwise.
    */
-  private static boolean verifyCrc16(ModbusRtuFrame frame) {
+  protected boolean verifyCrc16(ModbusRtuFrame frame) {
     var crc16 = new Crc16();
     crc16.update(frame.unitId());
     crc16.update(frame.pdu());
@@ -249,19 +277,6 @@ private static boolean verifyCrc16(ModbusRtuFrame frame) {
     return expected == reported;
   }
 
-  private ByteBuffer calculateCrc16(int unitId, ByteBuffer pdu) {
-    var crc16 = new Crc16();
-    crc16.update(unitId);
-    crc16.update(pdu);
-
-    ByteBuffer crc = ByteBuffer.allocate(2);
-    // write crc in little-endian order
-    crc.put((byte) (crc16.getValue() & 0xFF));
-    crc.put((byte) ((crc16.getValue() >> 8) & 0xFF));
-
-    return crc.flip();
-  }
-
   /**
    * Create a new {@link ModbusRtuClient} using the given {@link ModbusRtuClientTransport} and a
    * {@link ModbusClientConfig} with the default values.

modbus/src/main/java/com/digitalpetri/modbus/exceptions/ModbusCrcException.java

@@ -0,0 +1,28 @@
+package com.digitalpetri.modbus.exceptions;
+
+import com.digitalpetri.modbus.ModbusRtuFrame;
+import java.io.Serial;
+
+public class ModbusCrcException extends ModbusException {
+
+  @Serial
+  private static final long serialVersionUID = -5350159787088895451L;
+
+  private final ModbusRtuFrame frame;
+
+  public ModbusCrcException(ModbusRtuFrame frame) {
+    super("CRC mismatch");
+
+    this.frame = frame;
+  }
+
+  /**
+   * Get the frame that caused the exception.
+   *
+   * @return the frame that caused the exception.
+   */
+  public ModbusRtuFrame getFrame() {
+    return frame;
+  }
+
+}