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. The primary serial port, Serial1, is supported by the 68HC11's on-chip hardware UART (sometimes called a USART), and does not require interrupts to work properly. UART is an Asynchronous transmission device hence there is no clock signal to sync the data between the two devices instead it uses start and stop bits at the start and end of each data packet respectively to mark the extremities of the data being transferred. The DWOM bit (port D wired-or mode) should always be set to 0. Setting DWOM to 1 takes away the processor’s ability to pull the Port D signals high unless there is a pull-up resistor on each bit of the port.

It provides a convenient means of connecting the QScreen Controller to a variety of peripheral devices, including analog to digital and digital to analog converters, real time clocks, and other computers which use high speed communication. This UART-RS485 Converter module has an on-board MAX485 chip which is a low-power and slew-rate-limited transceiver used for RS-485 communication. Previously we have also performed MAX485 communication with Arduino and also MAX485 Communication with Raspberry pi, you can also check them out if interested. You need a special Ethernet cable for realizing this communication protocol. This with WiFi transmission for the Standard version, and in the RF version also with radio transmission at 433MHz (covering longer distances and with less battery consumption thanks to special optimizations). At small transmission distances speeds up to 35Mbps can be realized with RS485 although the transmission speed will decrease with distance. Under some conditions it can be used up to data transmission speeds of 64 Mbit/s. Shown below is the example of how a character is a transmitter over a UART data line. 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).

Each packet contains 1 start bit, 5 to 9 data bits (depending on the UART), an optional parity bit, and 1 or 2 stop bits. The two lowest order bits in the SPCR control register, named SPR1 and SPR0, rs485 cable determine the data exchange frequency expressed in bits per second; this frequency is also known as the baud rate. The master and slave could even exchange ascii QED-Forth commands. This ability to exchange messages means that the SPI is capable of full duplex communication. The below image shows the transmitter and receiver circuit diagram for Arduino's long-distance wired communication. The connection diagram for the above circuit is also given below. 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. It operates on a differential signaling method of measurement rather than voltage measurement wrt GND pin. The WattNode® BACnet and Modbus® meters communicate using RS-485 (TIA/EIA-485-A) half-duplex, differential signaling at 9600, 19200, 38400, and 76,800 baud. A single master can broadcast commands to all the slaves, and can direct commands to an individual slave using its unique address.

For the transfer of data, the baud rates of both Master and Slave must be between 10% of each other. Polarity of the wires must be observed. There are many categories of Ethernet cables we can use like CAT-4, CAT-5, CAT-5E, CAT-6, CAT-6A, etc. In our tutorial, we are going to use CAT-6E cable which has 4 twisted pairs of 24AWG wires and can support up to 600MHz. It is terminated at both ends by an RJ45 connector. In practice, Cat 5 cables have been used successfully in many installations, but there are some concerns. Because they are produced in such large quantities, Cat 5 cables are relatively inexpensive, often less than half the price of specialty RS-485 cabling. Each of the two UARTs on the wildcard is capable of full-duplex communications, meaning that both transmission and reception can occur simultaneously (although the RS485 protocol is half duplex as explained below). Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485.