The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. 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. Be sure to use a wire pair such as the blue and blue/white pair for the data lines and a third wire or pair twisted together for the common connection. This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. It’s reasonable to question why one wouldn’t simply transmit signals at the quickest possible speed to hasten network updates.The answer lies in the degradation of communication signals due to transmission losses on the cable when the product of the communication trunk length and the baud rate exceeds the recommended limit.

This robustness is the main reason why RS-485 is well suited for long-distance networking in noisy environment. RS-422 and RS-485 Standards Overview and System Configurations, Application Report (pdf). Unlike Ethernet standards like TCP that repeatedly transmit packets until acknowledged (error correction), RS-485 signals transmit without requiring confirmation. It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. RS-485 does not define a communication protocol; merely an electrical interface. RS-485 communication involves a driver that produces signals and a receiver that detects them. 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. 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. Under some conditions it can be used up to data transmission speeds of 64 Mbit/s. As a rule of thumb, the speed in bit/s multiplied by the length in metres should not exceed 108. Thus a 50-meter cable should not signal faster than 2 Mbit/s.

The total length of your network is not strictly governed by the baud rate. These rates won’t degrade signal quality over the maximum network cable length of 1200 meters (4000 ft). Biasing: Biasing is beneficial for maintaining signal quality over long distances. 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. The issue also exists in programmable logic controller applications. RS-485 is a robust and versatile communication standard that is widely used in industrial applications. The data rates, often termed baud, of RS-485 can vary from a rapid 10 Mbps (megabits per second) to a sluggish 9.6 kbps (kilobits per second). 24 AWG solid wires, which are typically used in Cat 5 cables, can break easily when connected to a screw terminal block, which does not provide any strain relief. For an off, mark or logic 1 state, the driver's A terminal is negative relative to the B terminal. Because a mark (logic 1) condition is traditionally represented (e.g. in RS-232) with a negative voltage and space (logic 0) represented with a positive one, A may be considered the non-inverting signal and B as inverting.

If A is negative with respect to B, the state is binary 1. The reversed polarity (A positive with respect to B) is binary 0. The standard does not assign any logic function to the two states. Idle State: When no device is transmitting, the line is in an idle state. DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units. It is also used in Digital Command Control (DCC) for model railways. Terminators and bias circuit resistors also place a load on the communication link. Termination resistors also reduce electrical noise sensitivity due to the lower impedance. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). Ideally, the two ends of the cable will have a termination resistor connected across the two wires. For effective interference mitigation, the shield should provide a current path that doesn’t impact the communication wires. A(-), and REF terminals for multiple devices along a single communication link.

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