fixed produce coil value method. Added a basic test

plc4j/protocols/modbus/src/main/java/org/apache/plc4x/java/modbus/netty/Plc4XModbusProtocol.java

@@ -249,7 +249,7 @@ private void checkSupportedDataType(List<?> values) {
     private byte[] produceCoilValue(List<?> values) throws PlcProtocolException {
         List<Byte> coils = new LinkedList<>();
         Byte actualCoil = 0;
-        int i = 8;
+        int i = 7;
         for (Object value : values) {
             boolean coilSet = false;
             if (value.getClass() == Boolean.class) {
@@ -282,7 +282,7 @@ private byte[] produceCoilValue(List<?> values) throws PlcProtocolException {
             }
             byte coilToSet = (coilSet ? (byte) 1 : (byte) 0);
             actualCoil = (byte) (actualCoil | coilToSet << i);
-            i++;
+            i--;
             if (i < 0) {
                 coils.add(actualCoil);
                 actualCoil = 0;

plc4j/protocols/modbus/src/test/java/org/apache/plc4x/java/modbus/netty/Plc4XModbusProtocolTest.java

@@ -0,0 +1,49 @@
+/*
+ Licensed to the Apache Software Foundation (ASF) under one
+ or more contributor license agreements.  See the NOTICE file
+ distributed with this work for additional information
+ regarding copyright ownership.  The ASF licenses this file
+ to you under the Apache License, Version 2.0 (the
+ "License"); you may not use this file except in compliance
+ with the License.  You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing,
+ software distributed under the License is distributed on an
+ "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ KIND, either express or implied.  See the License for the
+ specific language governing permissions and limitations
+ under the License.
+ */
+package org.apache.plc4x.java.modbus.netty;
+
+import org.apache.commons.lang3.reflect.MethodUtils;
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.BitSet;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertSame;
+
+public class Plc4XModbusProtocolTest {
+
+    @Test
+    public void coilEncoding() throws Exception {
+        byte[] bytes = (byte[]) MethodUtils.invokeMethod(new Plc4XModbusProtocol(), true, "produceCoilValue", Arrays.asList(false, false, true, false, false, false, true, false));
+        assertSame(1, bytes.length);
+        String s1 = String.format("%8s", Integer.toBinaryString(bytes[0] & 0xFF)).replace(' ', '0');
+        assertEquals("00100010", s1);
+
+        // TODO: just for reference we could use a bitset too
+        BitSet bitSet = new BitSet(8);
+        bitSet.set(1);
+        bitSet.set(5);
+        byte[] bytes1 = bitSet.toByteArray();
+        assertSame(1, bytes1.length);
+
+        String s2 = String.format("%8s", Integer.toBinaryString(bytes1[0] & 0xFF)).replace(' ', '0');
+        assertEquals("00100010", s2);
+    }
+}