Both methods ensure that the bus maintains a logical high state, corresponding to a positive differential voltage. RS-485 compliant drivers deliver a minimum differential output of 1.5 V over a 60-Ω load, while compliant receivers can detect a minimum differential input of 200 mV. 4 (DI – the driver’s input pin) is high, the output voltage on pin 6 (The A output) will be positive with respect to pin 7 (the B output). Clearly the termination resistor of 33 Ω is still a tad too high, there is a positive reflection at the far end. The transceiver converts logic level 3.3 or 5.0 voltage Rx/Tx signals to the RS485 A/B current drive signals, “A positive to B” meaning 1 and “B positive to A” meaning 0. Think about it like swapping a battery end for end on a voltmeter. Even though the logic function of the generator and receiver are not defined by RS-485, it makes sense to many engineers to have a binary 1 appear on the RS-485 wires when a binary 1 is being transmitted. How much of each depends on the wires and the insulation. Much like RS232, RS485 doesn’t have an actual standard connector.
Other standards may define the connectors and signals on the pins of the connector. The RS485 page is used to configure the operating parameter of RS485 serial connector. RS485 communication is a widely accepted standard and is supported by a large number of devices, making it easy to integrate various devices from different manufacturers into a single system. RS485 is a half-duplex communication protocol, which means that data can only be transmitted in one direction at a time. But a lot of RS-485 hardware uses only 1 pair of wires (half-duplex). To understand how to terminate a network (the wires) you must first understand transmission lines. The first is that RS-485 denies any control of the logic function of the generator and receiver. This is a good first step if the communications are not working. It is usually a fairly simple matter to reverse the wires and test the communications again. Simple Wiring: RS-485 typically requires only two twisted-pair wires for communication.
Against, RS-232, this standard allows long cabling distances and multidrop communication links where multiple devices are organized on the same bus. The bus still didn’t work. If you are running asynchronous start/stop communications (a UART) across the RS-485 wires, it simply will not work if the polarity is backwards. Of course, many other problems can cause a communications failure and these will be addressed in the troubleshooting section. What features of the transceiver microcircuit help to protect against many problems during installation and operation? This is too complicated a subject to go into depth here but a basic tutorial using oversimplified concepts may help. Termination is a less controversial subject than a misunderstood subject. In applications where the bit time closely aligns with the cable loop time, termination is vital to minimize reflections. This will decrease the amount of time to wait for the signal to stabilize, increasing the possible bit rate. If the data bit is sampled while there is ringing it may be sampled as an incorrect value. While Ethernet is the most commonly used communication protocol among multiple types of devices, ranging from consumer gadgets to industrial devices, TIA/EIA-485 commonly known as RS-485 is still broadly used in industrial devices even if it is older than Ethernet.
POTS telephone cable varies from 600 Ω at audio frequencies (say 1kHz) to 100 Ω at 1MHz, while CAT-5 cable is around 100 Ω over a wider frequency range. Another thing is that different cables have different characteristic impedances at different frequencies. This is why long cables and high frequencies are incompatible. That is why RS485 is currently a widely used communication interface in data acquisition and control applications where multiple nodes communicate with each other. RS-485 is a balanced data transmission standard for serial communication. Receiver Input Sensitivity: RS-485 has a receiver input sensitivity of ±200mV, which means a receiver must see signals above or below 200mV to recognize a mark. This is exactly what is expected from the symbol of RS-485 figure 1, and exactly backwards from what is expected based on the signaling waveform of RS-485 figure 1, because this driver does not invert its input. Since the RS-485 driver is “made passive” (disconnected from the wires) when not transmitting, and the other end of the cable can then drive the network, a termination resistor should be added to both ends. A pair of wires has capacitive coupling and a finite (usually very high) resistance between the wires.
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