Setting a new RX pin not possible

[umutcanuguz]

RS485Class::setPins has a txPin parameter, but no rxPin parameter.

Additional context

Additional reports

• https://forum.arduino.cc/t/uno-spe-shield-libraries/1401686

[PeeJay]

Actually it is:

HardwareSerial Serial2(mbrxPin, mbtxPin);
RS485Class Rs485(Serial2, mbtxPin, dePin, rePin);

[SunboX]

Short question, why does the

void RS485Class::setPins(int txPin, int dePin, int rePin)

function only accept TX (D pin), DE and RE pins? Shouldn't there also be a RX (R pin)? And why is it named "TX" and not "D"? All RS485 transceiver I know are specifying D for"Driver data input" and R for "Receive data output".

Could someone please clearify this for me? It somehow confuses me.

[SunboX]

And second question, if I have a circuit like this:

Do I have to set both, DE pin and RE pin, to GPIO2 ?

[per1234]

The "TX" pin is used in two distinct ways:

• serial communication
• set LOW when sending "break" signals

The serial communication is done using a HardwareSerial object. For example:

ArduinoRS485/src/RS485.cpp

Line 129 in af9604e

return _serial->write(b);

Setting the pin low while sending the "break" signals can not be done using the serial interface functions of the Arduino core API. This must be done by manipulating the TX pin as a normal GPIO pin via digitalWrite:

ArduinoRS485/src/RS485.cpp

Lines 173 to 191 in af9604e

	void RS485Class::sendBreak(unsigned int duration)
	{
	_serial->flush();
	_serial->end();
	pinMode(_txPin, OUTPUT);
	digitalWrite(_txPin, LOW);
	delay(duration);
	_serial->begin(_baudrate, _config);
	}
	void RS485Class::sendBreakMicroseconds(unsigned int duration)
	{
	_serial->flush();
	_serial->end();
	pinMode(_txPin, OUTPUT);
	digitalWrite(_txPin, LOW);
	delayMicroseconds(duration);
	_serial->begin(_baudrate, _config);
	}

So the Arduino pin number of the TX pin must be known by the library in addition to the HardwareSerial object of the serial interface. The Arduino core API doesn't provide a way for the TX pin to be determined from the HardwareSerial object.

For this reason, in order for the user to configure the serial interface used by the library, the user must provide the library with two separate, but intrinsically linked pieces of information:

• The HardwareSerial object of the serial interface
• The Arduino pin number of the TX pin of the serial interface

The interface used for serial communication is configured via the hwSerial parameter of the library's constructor:

ArduinoRS485/src/RS485.h

Line 69 in af9604e

RS485Class(HardwareSerial& hwSerial, int txPin, int dePin, int rePin);

The TX pin of that same serial interface is configured via the txPin parameter of the constructor.

Unless I miss some use case, it really doesn't make sense to allow configuration of the TX pin via the library's setPins function because you will always be configuring the serial interface object at the same time you are configuring the TX pin and thus must use the constructor. The inclusion of that parameter in the setPins function only misleads the user into thinking the serial interface can be configured via that function. It does make sense to provide a function for configuring the "DE" and "RE" pins because the user will typically be using the default serial interface (with some boards, there is only the one anyway), but may well want to choose whichever arbitrary pins are most convenient for "DE" and "RE".

---

So the defect here is the fact that the documentation does not explain the information I provided above (and in fact the constructor is not even documented). The resolution would be to properly explain in the documentation how the user can configure the serial interface used by the library.