[Bug]: Modbus: Reading single discrete-input throws ArrayIndexOutOfBoundsException

[IsmoLeszczynski]

What happened?

I'm trying out the Modbus driver with a simulator and things were going well while reading, for example, INT from holding registers, but when I tried to read a single BOOL from discrete inputs, the ModbusOptimizer throws an exception when handling the response. With a quick glance at the decompiled sources, I'm guessing this happens because BOOL is defined with a dataTypeLength of 2:

public enum ModbusDataType { BOOL((short)1, (short)2), ... }

Is this correct? I'd expect BOOL to be defined with a length of 1, but I'm not sure if there is some technical reason for this deeper down.

The tag address I tested: discrete-input:1:BOOL

2025-04-08 20:52:05,056 ERROR [nioEventLoopGroup-3-1] org.apache.plc4x.java.modbus.base.optimizer.ModbusOptimizer - Error processing response: java.lang.ArrayIndexOutOfBoundsException: arraycopy: last source index 2 out of bounds for byte[1] at java.base/java.lang.System.arraycopy(Native Method) at org.apache.plc4x.java.modbus.base.optimizer.ModbusOptimizer$Response.getResponseDataForTag(ModbusOptimizer.java:410) at org.apache.plc4x.java.modbus.base.optimizer.ModbusOptimizer.processReadResponses(ModbusOptimizer.java:232) at org.apache.plc4x.java.spi.optimizer.BaseOptimizer.lambda$1(BaseOptimizer.java:112) at org.apache.plc4x.java.spi.optimizer.BaseOptimizer.lambda$8(BaseOptimizer.java:205) at java.base/java.util.concurrent.CompletableFuture.uniHandle(CompletableFuture.java:934) at java.base/java.util.concurrent.CompletableFuture$UniHandle.tryFire(CompletableFuture.java:911) at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510) at java.base/java.util.concurrent.CompletableFuture.complete(CompletableFuture.java:2179) at org.apache.plc4x.java.modbus.tcp.protocol.ModbusTcpProtocolLogic.lambda$11(ModbusTcpProtocolLogic.java:163) at java.base/java.util.function.Consumer.lambda$andThen$0(Consumer.java:65) at org.apache.plc4x.java.spi.Plc4xNettyWrapper.decode(Plc4xNettyWrapper.java:194) at io.netty.handler.codec.MessageToMessageCodec$2.decode(MessageToMessageCodec.java:81) at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:88) at io.netty.handler.codec.MessageToMessageCodec.channelRead(MessageToMessageCodec.java:111) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:442) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103) at io.netty.handler.codec.MessageToMessageCodec.channelRead(MessageToMessageCodec.java:111) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:442) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:346) at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:318) at io.netty.handler.codec.ByteToMessageCodec.channelRead(ByteToMessageCodec.java:103) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:442) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1407) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:440) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:918) at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:166) at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:788) at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:724) at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:650) at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:562) at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:994) at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) at java.base/java.lang.Thread.run(Thread.java:1583) 2025-04-08 20:52:05,068 ERROR [nioEventLoopGroup-3-1] org.apache.plc4x.java.modbus.base.optimizer.ModbusOptimizer - Original Request: <PlcReadRequest> <PlcTagRequest> <tags isList="true"> <ModbusTagDiscreteInput {address=0, quantity=1, dataType=BOOL, unitId=null }> <PlcTagItem> <tag> <ModbusTagDiscreteInput> <address dataType="uint" bitLength="16">0</address> <numberOfElements dataType="uint" bitLength="16">1</numberOfElements> <dataType dataType="string" bitLength="32" encoding="UTF-8">BOOL</dataType> </ModbusTagDiscreteInput> </tag> </PlcTagItem> </ModbusTagDiscreteInput {address=0, quantity=1, dataType=BOOL, unitId=null }> </tags> </PlcTagRequest> </PlcReadRequest> java.lang.ArrayIndexOutOfBoundsException: arraycopy: last source index 2 out of bounds for byte[1] at java.base/java.lang.System.arraycopy(Native Method) at org.apache.plc4x.java.modbus.base.optimizer.ModbusOptimizer$Response.getResponseDataForTag(ModbusOptimizer.java:410) at org.apache.plc4x.java.modbus.base.optimizer.ModbusOptimizer.processReadResponses(ModbusOptimizer.java:232) at org.apache.plc4x.java.spi.optimizer.BaseOptimizer.lambda$1(BaseOptimizer.java:112) at org.apache.plc4x.java.spi.optimizer.BaseOptimizer.lambda$8(BaseOptimizer.java:205) at java.base/java.util.concurrent.CompletableFuture.uniHandle(CompletableFuture.java:934) at java.base/java.util.concurrent.CompletableFuture$UniHandle.tryFire(CompletableFuture.java:911) at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510) at java.base/java.util.concurrent.CompletableFuture.complete(CompletableFuture.java:2179) at org.apache.plc4x.java.modbus.tcp.protocol.ModbusTcpProtocolLogic.lambda$11(ModbusTcpProtocolLogic.java:163) at java.base/java.util.function.Consumer.lambda$andThen$0(Consumer.java:65) at org.apache.plc4x.java.spi.Plc4xNettyWrapper.decode(Plc4xNettyWrapper.java:194) at io.netty.handler.codec.MessageToMessageCodec$2.decode(MessageToMessageCodec.java:81) at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:88) at io.netty.handler.codec.MessageToMessageCodec.channelRead(MessageToMessageCodec.java:111) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:442) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103) at io.netty.handler.codec.MessageToMessageCodec.channelRead(MessageToMessageCodec.java:111) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:442) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:346) at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:318) at io.netty.handler.codec.ByteToMessageCodec.channelRead(ByteToMessageCodec.java:103) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:442) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1407) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:440) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:918) at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:166) at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:788) at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:724) at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:650) at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:562) at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:994) at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) at java.base/java.lang.Thread.run(Thread.java:1583) 2025-04-08 20:52:05,078 ERROR [nioEventLoopGroup-3-1] org.apache.plc4x.java.modbus.base.optimizer.ModbusOptimizer - Sub Request: <PlcReadRequest> <PlcTagRequest> <tags isList="true"> <registers0> <PlcTagItem> <tag> <ModbusTagDiscreteInput> <address dataType="uint" bitLength="16">0</address> <numberOfElements dataType="uint" bitLength="16">1</numberOfElements> <dataType dataType="string" bitLength="32" encoding="UTF-8">WORD</dataType> </ModbusTagDiscreteInput> </tag> </PlcTagItem> </registers0> </tags> </PlcTagRequest> </PlcReadRequest> 2025-04-08 20:52:05,086 ERROR [nioEventLoopGroup-3-1] org.apache.plc4x.java.modbus.base.optimizer.ModbusOptimizer - Sub Response (Success): <PlcReadResponse> <request> <PlcReadRequest> <PlcTagRequest> <tags isList="true"> <registers0> <PlcTagItem> <tag> <ModbusTagDiscreteInput> <address dataType="uint" bitLength="16">0</address> <numberOfElements dataType="uint" bitLength="16">1</numberOfElements> <dataType dataType="string" bitLength="32" encoding="UTF-8">WORD</dataType> </ModbusTagDiscreteInput> </tag> </PlcTagItem> </registers0> </tags> </PlcTagRequest> </PlcReadRequest> </request> <values isList="true"> <registers0> <PlcResponseItem> <code dataType="string" bitLength="16" encoding="UTF-8">OK</code> <value> <PlcRawByteArray dataType="byte" bitLength="8">0x00</PlcRawByteArray> </value> </PlcResponseItem> </registers0> </values> </PlcReadResponse>

Version

v0.13.0-SNAPSHOT

Programming Languages

• plc4c
• plc4go
• plc4j
• plc4net
• plc4py

Protocols

• AB-Ethernet
• ADS /AMS
• BACnet/IP
• C-Bus
• CANopen
• EtherNet/IP
• Firmata
• IEC-69870
• KNXnet/IP
• Modbus
• OPC-UA
• Profinet
• S7
• S7-light

[chrisdutz]

Discrete inputs are two byte values, right?

[IsmoLeszczynski]

Have to admit I'm not too savvy with the specifics, but the Coils and Discrete Inputs are single bits, and if I understand correctly, the result data is the resulting bits padded to the next full byte, (InputCount + 7) / 8 bytes. In my case, the complete result would then be 3 bytes; the function code byte, the number of inputs byte, and the input data byte - with 1 bit result padded to a full byte.

The Input/Holding Registers are always two bytes per address.

[hutcheb]

Yeah these get packed and padded into bytes, if you request 3 bits you'll receive one byte, 14 bits and you'll get 2 bytes.

As the mspec specifies the number of bytes for a boolean, it doesn't really make sense, booleans (Discrete coils/inputs) are handled with custom logic in each implementation to unpack them.

[glcj]

Hi,
You can try this branch; I think it solves the problem.
I'll keep an eye out.

https://github.com/apache/plc4x/tree/fix/modbus

[IsmoLeszczynski]

As far as I could tell, that solution didn't handle the Discrete Inputs nor Coils, when the response was a single byte - it was still trying to read too many bytes. To me it seems that the only required change is to handle Discrete Inputs just like the Coils, I made a pr for it #2055. This version, at least, worked for me with my modbus simulator (MOD_RSsim).