The Modbus protocol uses serial communication lines, Ethernet, or the Internet protocol suite as a transport layer. For higher speeds and longer lines, the termination resistances are necessary on both ends of the line to eliminate reflections.Use 100 Ω resistors on both ends. To avoid reflections on longer cables it is necessary to use appropriate termination resitors. With such an implementation of a RS485 network it is necessary that there is error detection implemented in the higher level protocol to detect the data corruption and resend the information at a later time.There is no need for the senders to explicity turn the RS485 driver on or off. This is, because on the other interfaces the communication channel is shared by multiple receivers and-in the case of RS485-by multiple senders. Default, all the senders on the RS485 bus are in tri-state with high impedance. Therefore it is not needed to have delays between the data packets on the RS485 bus. In most higher level protocols, one of the nodes is defined as a master which sends queries or commands over the RS485 bus. The InitSPI() function provides a convenient way to initialize the SPI as the master at a 2MHz baud rate.

Any required SPI output signals must be configured as outputs, either by calling InitSPI() or by setting the appropriate bits in the Port D data direction register DDRD. The QED-Forth kernel includes pre-coded drivers that configure and control the SPI for maximum speed data transfers. Therefore RS485 can overcome the practical communication speed limit of RS232. Unit Load: The network’s unit load impacts the number of devices that can be attached to it. Modbus is restricted to addressing 247 devices on one data link, which limits the number of field devices that may be connected to a parent station (again, Ethernet TCP/IP is an exception). The maximum slew rate also limits the maximum communication speed on the line.For both other interfaces-RS422 and RS485-the slew rate is indefinite. At small transmission distances speeds up to 35Mbps can be realized with RS485 although the transmission speed will decrease with distance. First of all we see that the speed of the differential interfaces RS422 and RS485 is far superior to the single ended versions RS232 and RS423.We also see that there is a maximum slew rate defined for both RS232 and RS423. Network topology is probably the reason why RS485 is now the favorite of the four mentioned interfaces in data acquisition and control applications.

We also see that the maximum allowed voltage levels for all interfaces are in the same range, but that the signal level is lower for the faster interfaces.Because of this RS485 and the others can be used in situations with a severe ground level shift of several volts, where at the same time high bit rates are possible because the transition between logical 0 and logical 1 is only a few hundred millivolts.Interesting is, that RS232 is the only interface capable of full duplex communication. Electric noise can cause all kinds of problems for electronics. Because of this, the resulting noise current is many factors lower than with an ordinary straight cable.Shielding-which is a common method to prevent noise in RS232 lines-tries to keep hostile magnetic fields away from the signal lines. Chassis and signal grounds are connected together to the digital ground (GND) signal. Currently available high-resistance RS485 inputs allow this number to be expanded to 256.RS485 repeaters are also available which make it possible to increase the number of nodes to several thousands, spanning multiple kilometers.And that with an interface which does not require intelligent network hardware: the implementation on the software side is not much more difficult than with RS232.

The RS485 network must be designed as one line with multiple drops, not as a star. Modbus supports communication to and from multiple devices connected to the same cable or Ethernet network. In the straight cable, all noise current is flowing in the same direction, practically generating a looping current just like in an ordinary transformer.When the cable is twisted, we see that in some parts of the signal lines the direction of the noise current is the oposite from the current in other parts of the cable. If the total Unit Load is too high, the voltage drop across all devices will be too significant, leading to signal degradation. A slave will never transmit any data or perform any action without a request from the master, and slaves cannot communicate with each other. Because there is a chance of data collosion with this implementation, theory tells us that in this case only 37% of the bandwidth will be effectively used. Therefore RS485 will be in use for many years in the future. We start with including the standard library for driving the LCD and declare the D8 pin of the Arduino Nano as an output pin which we will later use to declare the MAX485 Module as a transmitter or Receiver.

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