Pymodbus Read/Write 浮动（真实）

Question

我在我的 AB Micro820 PLC 中设置了 modbus 映射。我在 40001 中有一个数组用于写入，在 42001 中有一个数组用于读取。两者都是 200 个元素和 REAL 类型（32 位浮点数）。我目前可以读写，所以我知道代码可以工作，只是不正确。这些值是 read/wrote 作为非常小的值（即 4.58577478E-19）。谁能指出我正确的方向？

#!/usr/bin/env python

from pymodbus.constants import Endian
from pymodbus.payload import BinaryPayloadDecoder
from pymodbus.payload import BinaryPayloadBuilder
from pymodbus.client.sync import ModbusTcpClient

import logging
logging.basicConfig()
log = logging.getLogger()
log.setLevel(logging.INFO)

ip_address = "192.168.2.101"

client = ModbusTcpClient(ip_address)
if client.connect():    # connection is OK
    # write float
    builder = BinaryPayloadBuilder(endian=Endian.Little)
    builder.add_32bit_float(77.77)
    payload = builder.build()
    result  = client.write_registers(1, payload, skip_encode=True)
    # read floats
    result  = client.read_holding_registers(2001, 4)
    decoder = BinaryPayloadDecoder.fromRegisters(result.registers, endian=Endian.Little)
    print "read_holding_registers: " + str(decoder.decode_32bit_float())

    client.close()

Answer 1

我让它在 C++ 中与 libmodbus 一起工作：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>

#include <vector>
#include <iostream>

using namespace std;

extern "C"  // needed to integrate C code into C++
{
    #include "libmodbus/modbus.h"
}

int main (void)
{
    modbus_t *ctx;
    uint16_t read_reg[64];
    uint16_t write_float_holder[2];
    uint16_t read_float_holder[2];
    vector <float> write_reg = {77.77, 15.69, 42.78, 50153.33, -56.23};
    int rc, start_element = 0, j = 0;
    float rc_f[10];

    ctx = modbus_new_tcp("192.168.2.101", 502); // PLC IP address and default Modbus Port (502)
    if (modbus_connect(ctx) == -1) {
        fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
        modbus_free(ctx);
        return -1;
    }

    ///// Reading 402000
    /// REAL array in PLC
    // array fills up right to left with hex: 0x[1][0] >> 0x4174 0x28f6 >> read_from_plc[0] = 15.26
    if (1) 
    {
        rc = modbus_read_registers(ctx, 2000, 8, read_reg); // (connection, start position, how many to read, what read values go into)
        if (rc == -1) {
            fprintf(stderr, "%s\n", modbus_strerror(errno));
            return -1;
        }
        for (int i = 0; i < rc; i++) {
            printf("read_reg[%d]=%d (0x%x)\n", i, read_reg[i], read_reg[i]);
        }
        for (int i = 0; i < rc; i += 2)
        {
            read_float_holder[0] = read_reg[i];
            read_float_holder[1] = read_reg[i + 1];
            rc_f[j] = modbus_get_float(read_float_holder);      
            printf("rc_f[%d]: %f\n", j, rc_f[j]);
            j++;    
        }
    }

    ///// Writing 40000
    /// REAL array in PLC
    if (1)
    {
        for (size_t i = 0; i < write_reg.size(); i++)   // write two registers (32-bit float) at a time
        {
            modbus_set_float(write_reg[i], write_float_holder); // set float to hexadecimal
            rc = modbus_write_registers(ctx, start_element, 2, write_float_holder);
            start_element += 2;
            cout << "modbus_set_float: writing write_reg[" << i << "] = " << write_reg[i] << "\trc: " << rc << endl;
        }       
    }

    // close connection and free memory
    modbus_close(ctx);
    modbus_free(ctx);   

    return 0;
}

Answer 2

TLDR：

使用：

decoder = BinaryPayloadDecoder.fromRegisters(result.registers, Endian.Big, wordorder=Endian.Little)

而不是：

decoder = BinaryPayloadDecoder.fromRegisters(result.registers, endian=Endian.Little)

解释尝试：

文档中描述了此包 BinaryPayloadDecoder.fromRegisters 的参数。

这种情况下要注意：

byteorder – The Byte order of each word

wordorder – The endianess of the word (when wordcount is >= 2)

我相信适合 modbus 寄存器（2 字节）字节顺序的值始终必须是 Endian.Big

wordorder 类似于 byteorder 但用于 modbus 寄存器。

对于 Modbus TCP 始终必须是 byteorder=Endian.Big，wordorder=Endian.Little 因为由超过 2 个字节组成的值的字节顺序在协议规范 OPEN MODBUS/TCP SPECIFICATION(Appendix B. Data Encoding for non-word data).

对于值的 Modbus RTU 字节顺序，由超过 2 个字节组成的协议规范中未明确指定 Somehow described here。

大多数实施采用 Modbus TCP 方法并将浮点数传输为 [2, 1, 4, 3] 字节。

不过，还有其他的可能性：

[4, 3, 2, 1] - 字节序=Endian.Big，字序=Endian.Big
[3, 4, 1, 2] - 字节序=Endian.Little，字序=Endian.Big
[1, 2, 3, 4] - 字节序=Endian.Little，字序=Endian.Little

Pymodbus Read/Write 浮动（真实）

Pymodbus Read/Write Floats (REAL)

python

ethernet

modbus

plc

TLDR：

解释尝试：