The RS-232 driver (Maxim Part No. MAX242) can be tri-stated under software control. In contrast to RS-422, which has a driver circuit which cannot be switched off, RS-485 drivers use three-state logic allowing individual transmitters to be deactivated. RS-485, like RS-422, can be made full-duplex by using four wires. We have been using Microcontroller Development Boards like Arduino, Raspberry Pi, NodeMCU, ESP8266, MSP430, etc. for a long time now in our small projects where most of the times distance between the sensors and board is not more than few centimeters at max and at these distances, the communication between the different sensor modules, relays, actuators, and controllers can be easily done over simple jumper wires without us being worried about the signal distortion in the medium and the Electrical noises creeping into it. Most of the low-cost sensors and other modules like GPS, Bluetooth, RFID, ESP8266, etc. which are commonly used with Arduino, Raspberry Pi in the market uses UART TTL based communication because it only requires 2 wires TX(Transmitter) and RX (Receiver). The below image shows the transmitter and receiver circuit diagram for Arduino's long-distance wired communication. Shown below is the example of how a character is a transmitter over a UART data line.

The most common cable type is Cat 5e-UTP (unshielded twisted pair) which may work over shorter distances in less demanding applications with low EMI noise levels. But if you are building a control system with these development boards over a distance greater than 10 to 15 meters, then you should take the noise and signal power into consideration because if you want your system to work reliably, then you cannot afford to lose the data while transferring. The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible. The maximum number of devices that can be connected to a main cable is 32, what is rs485 cable including the Master. But there are some limitations to it as it cannot support multiple slaves and multiple masters and the maximum data frame is limited to 9 bits. Typical line voltage levels from the line drivers are a minimum of ±1.5 V to a maximum of about ±6 V. Receiver input sensitivity is ±200 mV. This allows the processor that is master to control the input /SS pins of other CPU’s, for example.

The QScreen Controller controls the RS485 transceiver with bit 5 of Port D of the processor. We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in Table 11 5. The transmit and receive data signals carry the messages being communicated between the QVGA Controller and the PC or terminal. Each device has a communication port with two terminals, which are indicated for the sake of convenience as A and B. In these two terminals the communication cable is connected so that all the devices that take part in the communication are connected in parallel. A Modbus RS485 connects a Master device to one or more Slave devices. Henceforth, we shall consider Slave devices to be measuring instruments with serial communication, even if the cabling is similar for all Modbus devices. In order to increase the extent of the Modbus network, repeaters can be used; and signal amplifying and regenerating devices provided with two communication ports that transfer to each what they receive from the other. Using a repeater, the main cable is divided into different segments, each of which can be up to 700 m in length and connect 32 devices (this number includes the repeaters).

If the total length of the main cable is less than 50 m termination resistances can be avoided at the ends of the main cable. For example, Belden 3105A cable can be specified, but different types of cable with equivalent characteristics can be used. In this example, the QVGA Controller selects the serial A/D by outputting a LOW signal on /SS. In this case, cable connections may be made to Serial 1 on either the 10-pin Serial Communications Header or the Serial 1 Connector. The shielding may be braided (be formed by a mesh of thin conducting wires) or be a foil (consisting of a sheet of metal wound around the conductors): the two types are equivalent. In some circumstances a one-way data flow may suffice. The diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line during transmission of one byte (0xD3, least significant bit first) of data using an asynchronous start-stop method. Twisted pair also allows the transmission speeds to be much higher than what is possible with straight cables. This allows standard point-to-point full duplex communications, as well as a multi-drop configuration with one master (a single QVGA Controller or a desktop computer) and multiple QVGA Controller slaves.