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Modbus Coil/DI array fix, support for Word Swap #2192

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692931e
Update ModbusOptimizer.java
IsmoLeszczynski Apr 9, 2025
07bce89
Merge branch 'develop' of https://github.com/IsmoLeszczynski/plc4x in…
IsmoLeszczynski Apr 9, 2025
0909be4
Update ModbusOptimizer.java
IsmoLeszczynski Apr 16, 2025
ecc2792
Merge branch 'apache:develop' into develop
IsmoLeszczynski Jul 23, 2025
66467ef
fix(modbus): Modbus Coil/DI array fix
IsmoLeszczynski Jul 23, 2025
cdebad6
feat(modbus): Word Swap support
IsmoLeszczynski Jul 23, 2025
09aa0a3
Merge branch 'develop' of https://github.com/IsmoLeszczynski/plc4x in…
IsmoLeszczynski Jul 23, 2025
34d5b04
Merge branch 'apache:develop' into develop
IsmoLeszczynski Nov 5, 2025
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51 changes: 46 additions & 5 deletions ...ers/modbus/src/main/java/org/apache/plc4x/java/modbus/base/optimizer/ModbusOptimizer.java
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Original file line number Diff line number Diff line change
Expand Up @@ -51,6 +51,7 @@
import org.apache.plc4x.java.spi.messages.utils.PlcTagItem;
import org.apache.plc4x.java.spi.optimizer.SingleTagOptimizer;
import org.apache.plc4x.java.spi.values.PlcBOOL;
import org.apache.plc4x.java.spi.values.PlcList;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

Expand Down Expand Up @@ -208,11 +209,25 @@ protected PlcReadResponse processReadResponses(PlcReadRequest readRequest, Map<P

// Calculate the byte that contains the response for this Coil
byte[] responseData = response.getResponseData();
int bitPosition = modbusTag.getAddress() - response.startingAddress;
int bytePosition = bitPosition / 8;
int bitPositionInByte = bitPosition % 8;
boolean isBitSet = (responseData[bytePosition] & (1 << bitPositionInByte)) != 0;
values.put(tagName, new DefaultPlcResponseItem<>(PlcResponseCode.OK, new PlcBOOL(isBitSet)));

// Check if we're dealing with an array of Coils or Discrete Inputs
if (modbusTag.getNumberOfElements() > 1) {
PlcList bitValues = new PlcList();
for (int i = 0; i < modbusTag.getNumberOfElements(); i++) {
int bitPosition = modbusTag.getAddress() - response.startingAddress + i;
int bytePosition = bitPosition / 8;
int bitPositionInByte = bitPosition % 8;
boolean isBitSet = (responseData[bytePosition] & (1 << bitPositionInByte)) != 0;
bitValues.add(new PlcBOOL(isBitSet));
}
values.put(tagName, new DefaultPlcResponseItem<>(PlcResponseCode.OK, bitValues));
} else {
int bitPosition = modbusTag.getAddress() - response.startingAddress;
int bytePosition = bitPosition / 8;
int bitPositionInByte = bitPosition % 8;
boolean isBitSet = (responseData[bytePosition] & (1 << bitPositionInByte)) != 0;
values.put(tagName, new DefaultPlcResponseItem<>(PlcResponseCode.OK, new PlcBOOL(isBitSet)));
}
break;
}
}
Expand Down Expand Up @@ -436,6 +451,32 @@ private ReadBuffer getReadBuffer(byte[] data, ModbusByteOrder byteOrder) {
byte[] reordered = ModbusProtocolLogic.byteSwap(data);
return new ReadBufferByteBased(reordered, ByteOrder.LITTLE_ENDIAN);
}
case BIG_ENDIAN_WORD_SWAP: {
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@chrisdutz chrisdutz Jul 23, 2025

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I'm sort of not entirely happy with the names. I asked chatgpt for help 😉 here's the response:

You're dealing with eight different byte/word permutations for Modbus data interpretation, which is common when mapping PLC registers to types like int32, float, or double. The current enum names are a bit ambiguous and repetitive.

🧠 Goal

Come up with more descriptive, unambiguous names that clearly convey:

Byte order within a word (e.g., big vs. little endian)

Word order (e.g., whether 16-bit words are swapped)

Whether both are swapped (combined swap)

🔢 First, mapping by example

Let's take the base array:
[1, 2, 3, 4, 5, 6, 7, 8]
Split into 16-bit words (2 bytes each):
[1,2], [3,4], [5,6], [7,8]

Then apply the transformations per enum:

EnumResultByte OrderWord OrderByte Swap?Word Swap?BIG_ENDIAN[1,2,3,4,5,6,7,8]BigNormal❌❌LITTLE_ENDIAN[8,7,6,5,4,3,2,1]LittleReversed✅✅BIG_ENDIAN_BYTE_SWAP[2,1,4,3,6,5,8,7]LittleNormal✅❌LITTLE_ENDIAN_BYTE_SWAP[7,8,5,6,3,4,1,2]BigReversed❌✅BIG_ENDIAN_WORD_SWAP[3,4,1,2,7,8,5,6]BigSwapped❌✅LITTLE_ENDIAN_WORD_SWAP[6,5,8,7,2,1,4,3]LittleSwapped✅✅BIG_ENDIAN_WORD_SWAP_BYTE_SWAP[4,3,2,1,8,7,6,5]ReversedSwapped✅✅LITTLE_ENDIAN_WORD_SWAP_BYTE_SWAP[5,6,7,8,1,2,3,4]NormalSwapped❌✅

✅ Proposed Enum Names

Old NameNew ProposalReasoningBIG_ENDIANBYTE_ORDER_BIG__WORD_ORDER_NORMALExplicitLITTLE_ENDIANBYTE_ORDER_LITTLE__WORD_ORDER_REVERSEDReflects full reversalBIG_ENDIAN_BYTE_SWAPBYTE_ORDER_LITTLE__WORD_ORDER_NORMALOnly byte swap = little endianLITTLE_ENDIAN_BYTE_SWAPBYTE_ORDER_BIG__WORD_ORDER_REVERSEDOnly word swap = word reversalBIG_ENDIAN_WORD_SWAPBYTE_ORDER_BIG__WORD_ORDER_SWAPPEDWords out of orderLITTLE_ENDIAN_WORD_SWAPBYTE_ORDER_LITTLE__WORD_ORDER_SWAPPEDByte+Word swapBIG_ENDIAN_WORD_SWAP_BYTE_SWAPBYTE_ORDER_REVERSED__WORD_ORDER_SWAPPEDBoth swapped, full reverseLITTLE_ENDIAN_WORD_SWAP_BYTE_SWAPBYTE_ORDER_NORMAL__WORD_ORDER_SWAPPEDJust word swap

💡 Optional Shorter Variant

If you want brevity but still clarity:

NORMAL, // [1,2,3,4] REVERSED, // [4,3,2,1] BYTE_SWAP, // [2,1,4,3] BYTE_SWAP_REVERSED, // [7,8,5,6,3,4,1,2] WORD_SWAP, // [3,4,1,2] BYTE_WORD_SWAP, // [6,5,8,7,2,1,4,3] FULL_REVERSED, // [4,3,2,1,8,7,6,5] WORD_SWAP_CONTIGUOUS // [5,6,7,8,1,2,3,4]

Would you prefer long descriptive or shorter but still clear names? I can adjust accordingly.

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@chrisdutz chrisdutz Jul 23, 2025

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Argh.. It fu**ed up the table 🙁

// [3, 4, 1, 2]
// [3, 4, 1, 2, 7, 8, 5, 6]
byte[] reordered = ModbusProtocolLogic.wordSwap(data);
return new ReadBufferByteBased(reordered, ByteOrder.BIG_ENDIAN);
}
case LITTLE_ENDIAN_WORD_SWAP: {
// [2, 1, 4, 3]
// [6, 5, 8, 7, 2, 1, 4, 3]
byte[] reordered = ModbusProtocolLogic.wordSwap(data);
return new ReadBufferByteBased(reordered, ByteOrder.LITTLE_ENDIAN);
}
case BIG_ENDIAN_WORD_SWAP_BYTE_SWAP: {
// [4, 3, 2, 1]
// [4, 3, 2, 1, 8, 7, 6, 5]
byte[] reordered = ModbusProtocolLogic.byteSwap(data);
reordered = ModbusProtocolLogic.wordSwap(reordered);
return new ReadBufferByteBased(reordered, ByteOrder.BIG_ENDIAN);
}
case LITTLE_ENDIAN_WORD_SWAP_BYTE_SWAP: {
// [1, 2, 3, 4]
// [5, 6, 7, 8, 1, 2, 3, 4]
byte[] reordered = ModbusProtocolLogic.byteSwap(data);
reordered = ModbusProtocolLogic.wordSwap(reordered);
return new ReadBufferByteBased(reordered, ByteOrder.LITTLE_ENDIAN);
}
default:
// 16909060
// [1, 2, 3, 4]
Expand Down
14 changes: 14 additions & 0 deletions .../modbus/src/main/java/org/apache/plc4x/java/modbus/base/protocol/ModbusProtocolLogic.java
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Original file line number Diff line number Diff line change
Expand Up @@ -418,4 +418,18 @@ public static byte[] byteSwap(byte[] in) {
return out;
}

public static byte[] wordSwap(byte[] in) {
if (in.length % 2 != 0) {
throw new PlcRuntimeException("Input byte array length must be a multiple of 2 for word swapping.");
}
byte[] out = new byte[in.length];
for (int i = 0; i < in.length; i += 4) {
out[i] = in[i + 2];
out[i + 1] = in[i + 3];
out[i + 2] = in[i];
out[i + 3] = in[i + 1];
}
return out;
}

}
14 changes: 13 additions & 1 deletion plc4j/drivers/modbus/src/main/java/org/apache/plc4x/java/modbus/types/ModbusByteOrder.java
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Original file line number Diff line number Diff line change
Expand Up @@ -30,5 +30,17 @@ public enum ModbusByteOrder {
BIG_ENDIAN_BYTE_SWAP,
// [3, 4, 1, 2]
// [7, 8, 5, 6, 3, 4, 1, 2]
LITTLE_ENDIAN_BYTE_SWAP
LITTLE_ENDIAN_BYTE_SWAP,
// [3, 4, 1, 2]
// [3, 4, 1, 2, 7, 8, 5, 6]
BIG_ENDIAN_WORD_SWAP,
// [2, 1, 4, 3]
// [6, 5, 8, 7, 2, 1, 4, 3]
LITTLE_ENDIAN_WORD_SWAP,
// [4, 3, 2, 1]
// [4, 3, 2, 1, 8, 7, 6, 5]
BIG_ENDIAN_WORD_SWAP_BYTE_SWAP,
// [1, 2, 3, 4]
// [5, 6, 7, 8, 1, 2, 3, 4]
LITTLE_ENDIAN_WORD_SWAP_BYTE_SWAP,
}