Qt modbus serial port flow control handling

Question

I'm writing a small program using QModbusDevice over the serial port (using the QModbusRtuSerialMaster class) and have some problems.

One of the problems seems to be that the flow control of the serial port is incorrect. Checking in a serial port sniffer I see that a working client sets RTS on when it sends requests, and then RTS off to receive replies. When I use QModbusRtuSerialMaster to send messages that doesn't happen.

The message is sent correctly (sometimes, subject for another question) compared to the working client. It's just the control flow that doesn't work and which causes the servers to be unable to reply.

I have set the Windows port settings for the COM-port in question to hardware flow control but it doesn't matter, the sniffer still reports no flow control.

Is there a way to get QModbusRtuSerialMaster to set the flow control as I would like? Or is there a way to manually handle the flow control (which is what the working client does)? Or is the only solution to skip the Qt modbus classes and make up my own using the serial port directly?

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A short summary of what I'm doing...

First the initialization of the QModbusRtuSerialMaster object:

QModbusDevice* modbusDevice = new QModbusRtuSerialMaster(myMainWindow);

modbusDevice->setConnectionParameter(QModbusDevice::SerialPortNameParameter, "COM3");
modbusDevice->setConnectionParameter(QModbusDevice::SerialParityParameter, QSerialPort::NoParity);
modbusDevice->setConnectionParameter(QModbusDevice::SerialBaudRateParameter, QSerialPort::Baud115200);
modbusDevice->setConnectionParameter(QModbusDevice::SerialDataBitsParameter, QSerialPort::Data8);
modbusDevice->setConnectionParameter(QModbusDevice::SerialStopBitsParameter, QSerialPort::OneStop);
modbusDevice->setTimeout(100);
modbusDevice->setNumberOfRetries(3);

modbusDevice->connectDevice();

Then how I send a request:

auto response = modbusDevice->sendReadRequest(QModbusDataUnit(QModbusDataUnit::Coils, 0, 1), 1);

Answer 1

QtModbus does not implement an automatic toggling for the RTS line because it expects your hardware to do it on its own (with a dedicated line instead).

This should be the case for most RS485 converters (even cheap ones). You would only need the RTS line if you have a separate transceiver like this one with a DE/~RE input.

If you were on Linux and had some specific hardware you could try to use the RS485 mode to toggle the RTS line for you automatically. But you don't seem to be on Linux and the supported hardware is certainly very limited.

You can also toggle the line manually with port.setRequestToSend(true) , see here . But note that depending on the timing needs of the device you are talking too, this software solution might not be very reliable. This particular problem has been discussed at length here . Take a look at the links on my answer too, I made some benchmarks with libmodbus that show good results.

Enabling or disabling flow control on the driver won't have any effect on this issue because this is not actually a flow control problem but a direction control one. Modbus runs on two-wire half-duplex links very often, and that means you need a way to indicate which device is allowed to talk on the bus at all times. The RTS (flow control) from an RS232 port can be used for this purpose as a software workaround.

In the end, it would be much less of a headache if you just replace your transceiver with one that supports hardware direction control. If you have a serial port with an FTDI engine you should be able to use the TXEN line for this purpose. Sometimes this hardware line is not directly routed and available on a pin but you can reroute it with MProg .

I would like to highlight that you did not mention if you are running your Modbus on RS485. I guess it's fair to assume you are, but if you have only a couple of devices next to each other you might use RS232 (even on TTL levels) and forget about direction control (you would be running full-duplex with three wires: TX, RX and GND).