ModbusClient.requestFrom() crashes Finder Opta if id > 247

[gareddustephane]

When implementing Modbus RTU on an Opta Finder, with the classical Arduino IDE, in order to read coils on a remote slave (server), we must use the following method :

ArduinoModbus/src/ModbusClient.h

Line 172 in cdae56d

int requestFrom(int id, int type, int address,int nb);

However, the device crashes if the id is greater than 247 as in the following piece of code :

ModbusRTUClient.requestFrom(248, COILS, 0x00, 10)

the RESET LED blinks red and no code is executed. Moreover, we can't upload a sketch anymore onto the device. The RESET button has to be pressed twice in order to make the Opta Finder be recognized again.

The only place in the requestFrom() method where the id is used here :

ArduinoModbus/src/ModbusClient.cpp

Line 335 in cdae56d

modbus_set_slave(_mb, id);

Calling this function:

ArduinoModbus/src/libmodbus/modbus-rtu.cpp

Lines 131 to 144 in cdae56d

	/* Define the slave ID of the remote device to talk in master mode or set the
	* internal slave ID in slave mode */
	static int _modbus_set_slave(modbus_t *ctx, int slave)
	{
	/* Broadcast address is 0 (MODBUS_BROADCAST_ADDRESS) */
	if (slave >= 0 && slave <= 247) {
	ctx->slave = slave;
	} else {
	errno = EINVAL;
	return -1;
	}
	return 0;
	}

Hence, here, a test is performed in order to verify the id of the targeted slave. But as the line modbus_set_slave(_mb, id); in the requestFrom() method doesn't, this leads to a crash. In other words, the error raised by modbus-rtu.cpp is not propagated to the requestFrom() method.

This could be easily fixed by making a simple check.