For an on, space or logic 0 state, the driver's A terminal is positive relative to the B terminal. 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. If the total Unit Load is too high, the voltage drop across all devices will be too significant, leading to signal degradation. Its use of differential signaling makes it resistant to noise, and its support for up to 32 devices on a single network makes it suitable for a wide range of applications. The issue also exists in programmable logic controller applications. These characteristics make RS-485 useful in industrial control systems and similar applications.

In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the DMX512 protocol. Care must be taken when using A/B naming. To ensure the appropriate device responds to a command, the signal includes an identification or address for the device to follow the command while others ignore it.Each device must have a unique address to avoid accidental command reactions. Only end devices will have a single pair of wires terminated into the communication connector, except for necessary End of Line (EOL) terminators or bias circuits. Ideally, the two ends of the cable will have a termination resistor connected across the two wires. The equipment located along a set of RS-485 wires are interchangeably called nodes, stations or devices. Let’s consider a simple example of an RS-485 network with one master device and two slave devices. Other elements like wire gauge, biasing circuit characteristics (impedance or capacitance), the existence of stubs from the main trunk, and the physical arrangement of your network cable can all affect your network’s maximum length.Take into account all these factors and maintain a buffer to stay within the advised limit for the overall max network cable length. These rates won’t degrade signal quality over the maximum network cable length of 1200 meters (4000 ft).

For isolated gadgets, the reference voltage can significantly impact communication within an RS-485 network. The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. The termination also includes pull up and pull down resistors to establish bias for each data wire for the case when the lines are not being driven by any device. PC environment. 8P8C modular connectors are used in this case. This robustness is the main reason why RS-485 is well suited for long-distance networking in noisy environment. That’s why common communication protocols like BACnet MS/TP and Modbus typically recommend baud rates between 9.6 - 115.2 kbps. 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. Grasping this concept is crucial when creating and implementing an RS-485 network. If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans.

Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. TSB-89A, Application Guidelines for TIA/EIA-485-A does not recommend using star topology. However this contradicts the facts that an idle mark state is a logical one and the termination polarization puts B at a higher voltage in Profibus guidelines. Idle State: When no device is transmitting, the line is in an idle state. Slaves Receive: All devices on the network, including the slave devices, constantly monitor the voltage difference between lines A and B. When they detect a change, they interpret this as a bit of data.Slave Responds: If the master device sends a command that requires a response from a slave device, that slave device will wait until the master has finished transmitting, then change the voltage difference between lines A and B to send its response. It may be used to control video surveillance systems or to interconnect security control panels and devices such as access control card readers. With one sing coaxial cable, both the video data transmission and PTZ Control are achievable. It is also used in Digital Command Control (DCC) for model railways. As an instance, a baud rate of 76,800 bps on 1200-meter cable results in a value of 92,160,000, less than the 100,000,000 limits.

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