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Both methods ensure that the bus maintains a logical high state, corresponding to a positive differential voltage. Typically, an RS-485 transceiver has a differential input threshold voltage of ±200 mV. Typically, this occurs between the transmission of messages by different nodes. I connected the ground at both nodes of interest straight to the relevant earth (I didn't install a "separation resistor") - so the grounds actually formed a hard ground loop (closed across the power supplies and mains earth), which traditionally is another big thing to avoid. Another thing to note is that biasing may not be required. The timing constant corresponds to about 1 character at 9600, as can be seen on the overview scope screenshot for CAT7 - note the upward slope. The standard 120-Ω value for termination resistors corresponds to the differential-mode characteristic impedance of the twisted-pair bus wires. How much of each depends on the wires and the insulation.

This impedance is determined by factors such as wire gauge, insulation type and thickness, and the number of twists per unit length, which collectively influence the impedance detected by high-speed data signals. If the outside serial device is RS485 standard, you can attach this unit onto M5stack, therefore, to implement the communication with RS485 device by TTL protocol. The RS232 Serial Interface is designed to connect two devices. 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. One advantage of this solution is that it doesn’t impact the loading of transceivers on the network. These high voltages need attenuation down to voltages that 3.3 V or 5 V transceivers can handle. This means that at certain times, all RS-485 transceivers on the bus may present a high impedance state, resulting in no actively driven logical state.

In RS-485, the receiver's internal biasing will be such that the "receiver will remain in the intended binary state when a differential voltage (VR3) of ±0.40 V is applied through matched resistors equal to 1500/nUL 1/2 to each input terminal, as shown in figure 13, with the input voltages VR1 and VR2 (and resulting VR3) to achieve any allowed input condition.". When a binary 1 is transmitted, the signal is spacing, and the voltage on the wire must be more negative than -3V. Another major advantage is that there can be more than one slave in the connection. 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. RS-485 voltages are referenced differentially from one wire to the other, but are also referenced to circuit common. For existing installations or setups with previously designed modules that are challenging to update, implementing this method may not be feasible. One method involves selecting receivers with built-in fail-safe input thresholds. Detecting the faults listed above is one reason, but biasing by itself cannot not detect all of them. Attenuating the signal and biasing it toward VCC/2 prevents the inputs of the comparator from getting saturated; thus enabling the comparator to properly evaluate the differential voltage between the A and B terminals.

This fail-safe biasing helps ensure that the Receiver output will be high in the presence of bus idle or bus short-circuit conditions. Another approach is to use additional external resistors to establish an external bias on the idle bus. The second important function of the resistor-divider network is to bias the bus voltages toward VCC/2. Matching the cabling’s characteristic impedance to the termination network allows the receiver to maximize signal power detection. But, the RS-232 standard allows for synchronous communications, which is not compatible with the IBM PC serial port, as well as asynchronous communications, which can be. RS-485 allows for multiple devices on the two wires. This will reduce the reflections on a RS-422 network to some extent, but RS-422 also recommends a termination resistor at the receiver's end of the wires (well it sort of recommends, depending on the "data rate" or the "signal rise time at the load end of the cable"). It is usually a fairly simple matter to reverse the wires and test the communications again. It can be burned to the master through simple steps, and a series of function verification can be performed. 1.EasyLoader is a simple and fast program burner.