Response String is Null

[Ramasundaram62]

We are using the above code to read data from a PLC .

1. We are able Initialize & Open port and send the query but we are getting NULL response..

2. Finally, and in desperation are directly passing the following query where we are trying to read Holding Register 4237

$modbus->sendQuery(1,3,"108D",1);

( Is the query correct ? )
Still the response is Null.

3. We came to the following breakpoint which we found was not executing :

// Read serial port buffer (with three seconds timeout)
$responseString='';
while( ($byte = $this->serial->ReadPort()) && ((microtime(true)-$startTime)<3.0))
{
$byte = $this->serial->ReadPort();
$responseString=$responseString.$byte;
sleep(3);
}

$responseString is NULL .

[LordRegentTB]

A significant issue is that the sleep in the cited code is too long (3 seconds) so the loop would terminate with a maximum opportunity for one byte of data. The repo code as of 2025 Feb 03 uses usleep(50) in line 195 which is much too short (50 microseconds), so perhaps no data would have had a chance to arrive on the wire and the loop would terminate prematurely. Cite: Answer from Olivier at StackOverflow question 78891203. Olivier makes the astute suggestion to use usleep(50000) for a 50 milliseconds delay, which should be suitable for both old and new devices and for longer cable runs; the original poster of the question indicates that the longer value was successful.

It is possible that the device is not actually receiving the command; common causes are wiring issues and Baud Rate mismatch. Insert a terminal or tap at the device end of the wire and validate what is being seen at that end.

Another issue is that the Register address may be off by one, due to a difference in how the ModBus Protocol and the physical device number the registers. The ModBus protocol defines the first Register address as 0. Check the device documentation to see if the first Register address is referenced as 0 or 1, and adjust accordingly. Further, it is not unusual for a hardened ModBus client--one which allows access to only certain addresses--to refuse to respond to requests to read restricted addresses; although the protocol states that an error should be returned at that point, reality wins.