Environments that exhibit high levels of electrical noise and variable ground potential impact RS232’s ability to transfer data efficiently and can lead to data loss or corruption. A byproduct of the differential voltage system is the extended data transfer distance, increased transmission speed, and lower voltage use seen with RS485. Differential data transmission refers to balanced differential signals, which can help minimize induced noise signals and ground shifts. Gaining an understanding of different serial communication protocols is important for industrial, commercial, and domestic users to mitigate challenges arising from increasing data transmission requirements. These specifications offer a considerable margin, ensuring dependable data transmission even in the face of significant signal degradation across cables and connectors. The longer the wires are, the less signal there will be at the end. This is necessary because simply attenuating a negative signal will not bring the voltage between the local ground of the receiver and VCC. Another approach to managing idle bus conditions involves using two external resistors: one connected from the A terminal to VCC and the other from the B terminal to the ground. Another approach is to use additional external resistors to establish an external bias on the idle bus. These resistors, depicted in the figure below, are common circuits utilized to provide an external hard-wired failsafe for an RS485 network.

RS485 is however most popularly used in programmable logic controllers and factory floors where there are lots of electrical noise. Balanced interface: RS-485 uses a balanced transmission line, meaning it has two signal wires carrying equal and opposite signals to reduce noise and improve signal integrity. In the provided circuit, RT represents the resistors used for impedance matching of the transmission line, and they contribute to a voltage divider setup that establishes the steady-state bias voltage. The RS485 standard defines a balanced two-wire transmission line, which may be shared as a bus line by up to 32 driver/receiver pairs. Typically, this occurs between the transmission of messages by different nodes. In RS-485 networks, multiple nodes are interconnected in parallel to form a bus, known as the bus topology. This is a good first step if the communications are not working. The first function is to attenuate large signals that are beyond the range of the supply voltage of the receiver. The differential input threshold voltage (VTH) of a receiver refers to the voltage level on the receiver input that ensures a transition (from low to high or high to low) of the receiver output. These high voltages need attenuation down to voltages that 3.3 V or 5 V transceivers can handle.

Although this voltage may be insufficient to switch the receiver state directly, an internal bias current within the attenuation network creates a voltage difference at the receiver comparator input. A typical attenuation factor should be on the order of 10 to 1, effectively reducing the magnitude of the voltages present internally at the comparator. The receiver input circuit includes electro-static discharge (ESD) protection, a resistor-divider network, and biasing current, influencing the magnitude and common-mode voltage reaching the differential comparator. The objective of each circuit is to uphold a voltage at the receiver inputs above the minimum input threshold and ensure a known logic state under one or more of three fault conditions. This creates a fail-safe bias voltage that causes the negative terminal to have a lower voltage than the positive terminal and the output of the comparator to be in a known high state when applying a 0-V differential voltage to the A and B pins. Both methods ensure that the bus maintains a logical high state, corresponding to a positive differential voltage. The exact voltage level that a logic device considers ON or OFF varies by logic type, but when the voltage is high (usually but not always approaching the IC's supply voltage), the output is on and a binary 1 is on the wire, and when the voltage is approaching 0 the output is off and, a binary 0 is on the wire.

RS485 drivers automatically return to their high impedance tri-state within a few microseconds after the data has been sent. The drivers and receivers, called as transceivers link to a primary cable trunk through short network stubs in this arrangement. Many cable manufacturers can recommend a 120 Ω cable intended to work with RS-422 or RS-485. It demonstrates that the driver’s fundamental operation can be understood in terms of its high- and low-output logic states. RS-485 functions by defining the transmission of ‘0’s and ‘1’s via the positive or negative states of the transceiver terminals, A and B. To perceive the same logical state, all devices on the communication link must generate and detect signals of the same polarity.A reversed device will read and generate incorrect signals, leading to a communication breakdown. A final note on transmission lines is that the termination resistor should match the characteristic impedance of the cable. 120 Ω cable should provide the best performance, but the 100 Ω CAT-X cable may you have laying around may also work. The annex says this connection can be wired directly or made through a 100 Ω resistor.