For example, Belden 3105A cable can be specified, but different types of cable with equivalent characteristics can be used. The main cable must be no longer than 700 m! 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. End of line termination resistor (120 ohm 1/4 watt) to be installed at both ends of network. The end resistance must be used only at the ends of the main cable. 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). The maximum number of devices that can be connected to a main cable is 32, including the Master. In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable.
Bias termination - Using resistive termination decreases noise immunity, particularly if the cable is loaded with many devices. If your application requires RS485, you can use the secondary serial port (serial2) to program and debug your application code using the RS232 protocol, and use the primary serial port (Serial1) for RS485 communications. 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). 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. This method is one of the simple and easy to implement methods to transfer the data over long distances.
The local and remote must share a common ground, so all serial cables include at least one ground conductor. Shield on communication cable is to remain continuous and ground only at the controller. The RS232 protocol specifies the use of two separate grounds, a signal ground and a protective (or chassis) ground. There are many different types of serial communication protocols like I2C and SPI which can be easily implemented with Arduino and today we are going to look at another most commonly used protocol called RS485 which is very commonly used in high noise industrial environments to transfer the data over a long distance. In this tutorial, we are going to learn about the RS485 communication protocol and how to implement it with the two Arduino Nano we have with us and how to use the MAX485 RS485 to UART conversion Module. DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units. 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. Figure 1 shows an example of a correct Bus connection. A 2-wire synchronous IIC (Inter-IC) bus provides multi-drop signaling at rates up to 100 Kbaud. You may use nonstandard baud rates if both devices support them. 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. Polarity of the wires must be observed. The recommended arrangement of the wires is as a connected series of point-to-point (multidropped) nodes, i.e. a line or bus, not a star, ring, or multiply connected network. The 735 series are desk top terminals and the companion 720 wall mount units. In this case it is possible to connect the main inlet and outlet terminal directly to the terminals of an instrument without creating a branch. If on the other hand each terminal can accept only a single cable, rs485 cable a proper branch must be created using three auxiliary terminals for each instrument to be connected.