Fixed Modbus write single coil function

Author: dc42

src/Platform/Platform.cpp

@@ -2286,7 +2286,7 @@ GCodeResult Platform::SendI2cOrModbus(GCodeBuffer& gb, const StringRef &reply) T
 					reply.copy("Invalid Modbus data");
 					return GCodeResult::error;
 				}
-				registersToSend[0] = (values[0] == 0) ? 0 : 0x00FF;
+				registersToSend[0] = (values[0] == 0) ? 0 : 0xFF00;
 				break;
 
 			case (uint8_t)ModbusFunction::writeSingleRegister:

src/modbus-rtu.txt

@@ -39,7 +39,7 @@ rcv: slave addr, function, byte count, (data hi, data lo)*, crc
 Read input registers: as read holding registers
 
 Write coil:
-send: slave addr, function, coil addr hi, coil addr lo, write data hi, write data lo, CRC. Write data hi:lo must be 00:00 or 00:FF. Other values are illegal and do not write the coil.
+send: slave addr, function, coil addr hi, coil addr lo, write data hi, write data lo, CRC. Write data hi:lo must be 00:00 or FF:00. Other values are illegal and do not write the coil.
 receive: same, returning state of the coil after it has been written.
 
 Write single register: