node-red：modbus协议数据格式转换


node-red：MODBUS协议数据格式转换


  本文将描述通过node-red采集工业数据时，采集到的数据需要进行转换的操作方法。
  对于运行在RTU或ASCII传输模式下的modbus协议，它接收主设备发送的指令报文，并根据窗口配置的数据返回主设备需要的指令报文。根据Modbus协议的规定，所有的数值都是以16位二进制形式进行存储和运算的。

  • 注意：node-red通过modbus协议采集时，端口不能在0-100以内，node-red会出问题

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一、32word转换

  Modbus协议中，32位整数的存储格式会因具体的数据类型而有所不同。Modbus协议对32位整数的存储方式是将该整数分解为两个16位的寄存器来处理，第一个元素是高16位，第二个元素是低16位，具体的字节顺序可以有4种选择：大端序、小端序、大端字节交换和小端字节交换。例如，一个的32位整数0x12345678在Modbus中的存储形式可能会是：0x12 0x34 0x56 0x78，具体取决于所选择的字节顺序。其中：

  • 大端序Big-Endian（BE）：将最高位字节存储在最低的地址位置；
  • 小端序（LE）：则是将最高位字节存储在最高的地址位置；
  • 大端字节交换和小端字节交换（BIG_ENDIAN和LITTLE_ENDIAN）：指将每个字节的顺序颠倒，但不考虑符号位。

1.1 和1.2 涉及的所有js转换代码，有符号、无符号的32位整数都适用

1.1 写操作

  1. 将32位整数转换为16位整数数组
  • 使用大端序（BE）的转换方式，js代码如下： - modScan32的modbus默认转换方式是这个
/** 使用大端序（BE）的转换方式，将一个32位整数，转换为一个包含两个元素的数组，
 * 其中第一个元素是高16位，第二个元素是低16位 */
function int32ToArrayBE(num) {
    const high = (num >> 16) & 0xFFFF;
    const low = num & 0xFFFF;
    return [high, low];
}
  • 使用小端序（LE）的转换方式，js代码如下：
/** 使用小端序LE的方式的转换方式，将一个32位整数，转换为一个包含两个元素的数组,其中第一个元素是高16位，第二个元素是低16位 */
function int32ToArrayLE(num) {
  const high = (num >> 16) & 0xFFFF;
  const low = num & 0xFFFF;
  return [high, low];
}
  1. 通过modbus节点写入，示例如下
    2
    数据已成功写入：
    2

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1.2 读操作

实现步骤：

  1. 将一个包含两个元素的数组转换为一个32位整数
  • 使用大端序（BE）的转换方式，js代码如下：
//将一个包含两个元素的数组转换为一个32位整数，其中第一个元素是高16位，第二个元素是低16位。
//转换方式是先将第一个元素左移16位，然后与第二个元素进行按位或操作。
function arrayToInt32BE(data) {
    return (data[0] << 16) | data[1];
}
  • 使用小端序（LE）的转换方式，js代码如下：
/** 
将一个包含两个元素的数组转换为一个32位整数，其中第一个元素是高16位，第二个元素是低16位。
 */
function arrayToInt32LE(arr) {
  return (arr[0] << 16) | arr[1];
}
  1. 通过modbus节点读取，示例如下：
    2

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二、字符串转换

  modbus支持对字符串类型的数据进行读写操作，进行字符串类型数据的操作时，是基于ASCII码格式进行的，一个字符占一个字节，支持字母、中文已经符号。

2.1 字符串写操作

实现步骤：

  1. 将字符串转换成字节数组，函数如下：
/**将一个字符串，获取每个字符的Unicode编码，转换为字节数组 */
function stringToByteArray(str) {
    let byteArray = [];
    for (let i = 0; i < str.length; i++) {
        byteArray.push(str.charCodeAt(i));
    }
    return byteArray;
}
  1. 将字节数组批量写入modbus中 ，示例如下：
    1
    执行后，可以看到对应地址的数据：
    2

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2.2 字符串读操作

实现步骤：

  1. modbus节点读取起始地址为n，直至字符串字节数组长度的连续地址的数据数组
  2. 将得到到的字节数组转换为字符串，转换函数如下：
//将Unicode编码字节数组转换为对应的字符
msg.payload = String.fromCharCode(...msg.payload.slice(0, 8));
//有需要的话可以去掉字符串中的null和结尾的空字符
msg.payload = msg.payload.trim().replace(/\u0000/g, '');
return msg;

示例如下：
1

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三、有符号整数转换

  1. 正整数就是正常的读\写，不需要转换。
  2. 对于负数，在计算机和数字系统中，负数通常使用二进制补码形式来表示。这种方法是将负数的绝对值取反后再加一得到的。如’-80’的二进制补码就是’65456’。

如我们在modsim32中输入-55，实际显示在计算机上的是65481
1

3.1 有符号16word转换

3.1.1 负数 读 操作

  有符号16word转换成补码是占一位地址，通过modbus读取过来是十进制整数，我们需要将其通过node-red的modbus协议读取对应地址下的数据，该数据为补码，需要将其转换为实际的有符号16位整数。有两种转换方式，如下示例所示：
1

  1. 通过js函数转换：
/** 有符号整数补码转换转成16位有符号整数：
 * 如-80的补码为65456，读取出来是65456，需要通过该函数转换成-80
 */
function Signed16ToInt16Be(num){
    //32767是16位带符号证书能表示的最大正数，当数值从32767再增加1时，就会上溢变成负数。
    //即mod通信中所有的负数都是用补码形式存储（如‘-80’实际读\写出来是‘65456’）
    //需要检测是否发生了从正数到负数的转换，即是否发生了上溢或下溢,如果有，进行转换
    return num >= 32768 ? num - 65536 : num;
}
  1. 使用parse节点工具转换：
    2

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3.1.2 负数 写 操作

   直接将负数写入对应地址时，不生效。需要先将其转换成补码，然后再将补码写入地址。js将负数转换成补码的函数如下：

/** 
 * 负数转成对应的补码
 * 在计算机中，负数通常使用补码表示，如'-80'用补码表示为'65456'.
 */
function Int16BeToSigned16(num) {
    //如果数字小于0，将其转换成十进制补码：65536是2的16次幂，也就是2^16
    if(num < 0){
        num += 65536
    }
    return num;
}

实战示例如下：
1

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源码

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        "outputs": 1,
        "noerr": 0,
        "initialize": "",
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        "libs": [],
        "x": 360,
        "y": 80,
        "wires": [
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                "11e1bfef49c08c4d",
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        "tcpType": "DEFAULT",
        "serialPort": "/dev/ttyUSB",
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        "reconnectOnTimeout": false,
        "reconnectTimeout": "2000",
        "parallelUnitIdsAllowed": true,
        "showWarnings": true,
        "showLogs": true
    }
]

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