How to handle Modbus TCP exception response in PHP

How to handle Modbus TCP exception response in PHP

When using the Modbus TCP communication protocol for data transmission, exception responses are often encountered. In order to ensure communication stability and data accuracy, we need to correctly handle Modbus TCP exception responses in PHP. This article will introduce how to handle Modbus TCP exception response in PHP and give code examples.

Modbus TCP is a commonly used industrial communication protocol. It is based on the TCP/IP protocol and is mainly used to exchange data between computers and PLC and other equipment. In Modbus TCP communication, common abnormal responses include timeout, invalid function code, invalid data address, etc. These abnormal responses are very important for application scenarios such as data collection and monitoring.

To handle Modbus TCP exception responses in PHP, you first need to connect to the Modbus device through TCP/IP. We can use the socket function provided by PHP for network communication. The following is a sample code for connecting to a Modbus device using PHP:

$ip = "192.168.0.1";
$port = 502;
$timeout = 5;

$socket = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);
socket_set_option($socket, SOL_SOCKET, SO_RCVTIMEO, array('sec' => $timeout, 'usec' => 0));
$result = socket_connect($socket, $ip, $port);

Among them, $ip and $port need to be modified according to the actual situation. $timeout represents the connection timeout, in seconds.

After establishing a connection with the Modbus device, we can use the function code of Modbus TCP to read and write data. When the Modbus device returns an abnormal response, we can determine the exception type by parsing the error code in the Modbus response data. The following is a sample code for handling Modbus TCP exception response:

$functionCode = 0x03;
$address = 0;
$length = 10;

$request = pack("nnnn", 0, $functionCode, $address, $length);
socket_write($socket, $request, strlen($request));

$response = socket_read($socket, 1024);
if ($response !== false) {
    $data = unpack("ntransactionId/nprotocol/nlength/CunitCode/CfunctionCode/data", $response);
    $errorCode = ord($data['data'][0]);
    if ($errorCode != 0) {
        echo "Modbus TCP exception: " . $errorCode;
    } else {
        // 处理正常响应
        $values = array_slice($data['data'], 1);
        // ...
    }
} else {
    echo "Failed to read Modbus TCP response";
}

In the above code, we send a request to read data through the socket_write function, and then use the socket_read function to read the response returned by the Modbus device. After parsing the response data, determine whether the errorCode in the data field is 0. If it is not 0, it means an exception response was returned.

Through the above example code, we can flexibly handle Modbus TCP exception responses and handle them according to specific needs. You can make logical judgments on error codes or perform operations such as data resend under abnormal circumstances to ensure the stability of Modbus TCP communication and the accuracy of data.

In summary, handling Modbus TCP exception responses requires correctly using the socket function in PHP for network communication, and judging the exception type by parsing the error code in the Modbus response data. The specific processing method can be determined according to actual application requirements to ensure the reliability and accuracy of Modbus TCP communication.

Reference materials:

1. PHP official documentation (https://www.php.net/)
2. Modbus protocol documentation (https://modbus.org /specs.php)

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