With Modbus, BACnet and Profibus, A/B labeling refers A as the negative green wire and B as the positive red wire, in the definition of the D-sub connector and M12 circular connector, as can be seen in Profibus guides. 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. The RS485 receiver compares the voltage difference between both lines, instead of the absolute voltage level on a signal line. The receiver input has a fail-safe feature that guarantees logic high output if the input is open circuit. The truth tables of most popular devices, starting with the SN75176, show the output signals inverted. The Serial1 and Serial2 ports have identical communications capabilities, although more of the Serial1 signals (both RS232 and RS485) are made available on the Docking Panels headers and connectors.

RS-485 signals are used in a wide range of computer and automation systems. RS485 extends the common mode range for both drivers and receivers in the "tri-state" mode and with power off. RS-485 standard conformant drivers provide a differential output of a minimum 1.5 V across a 54-Ω load, whereas standard conformant receivers detect a differential input down to 200 mV. RS-485 serves as a physical layer for the AES3 digital audio interconnect. In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive. RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus. DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units. Line A voltage, implying A, the green wire, is indeed connected to the driver inverting signal, as seen in a whitepaper. Done that establishes the RS485 receive mode is coded such that it waits until all queued (pending) characters have been transmitted before the driver chip is taken out of transmit mode. 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.

RS485 multi-drop networks are daisy-chained networks with a single cable connecting multiple devices. If on the other hand each terminal can accept only a single cable, a proper branch must be created using three auxiliary terminals for each instrument to be connected. RS423 is another single ended specification with enhanced operation over RS232; however, it has not been widely used in the industry. Care must be taken that an SC connection, especially over long cable runs, does not result in an attempt to connect disparate grounds together - it is wise to add some current limiting to the SC connection. This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. In this project, we have only used a baud rate of 9600 which is well under the maximum transfer speed we can achieve with the MAX-485 Module but this speed is suitable for most of the sensor modules out there and we don’t really need all the maximum speeds while working with Arduino and other development boards unless you are using the cable as an ethernet connection and require all the bandwidth and transfer speed you can get.

This robustness is the main reason why RS-485 is well suited for long-distance networking in noisy environment. The main reason behind using these Ethernet Cables over normal wires is that they provide much better protection against noise creeping in and distortion of the signal over high distances. Shown below is the example of how a character is a transmitter over a UART data line. The GROUND line serves as a common voltage reference for the master and slave. The Silence() routine searches the incoming serial characters for a pre-determined keyword (for example, the ascii name of this particular slave). For example, at 4800 baud (bits per second), each bit lasts about 200 microseconds (µs), and if communications are full duplex (e.g., if the QScreen Controller echoes each incoming character), then there is a serial interrupt every 100 µs or so. Thus, as a rough approximation, operating at 4800 baud full duplex requires about 40 to 50% of the 6811's CPU time (that is, an average of approximately 40 to 50 µs service time every 100 µs).

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