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The first time is in the total load limit stating that the total load limit of the network including fail-safe provisions, should be no less than 54Ω. This implies that the fail-safe provisions are resistance external to the receiver. Unfortunately RS-485 section 4.5.3 states that the total load between "A" and "B" should be no less than 54 Ω. RS-485 figure 2 shows a generator (driver) with two interface connection points labeled "A" and "B". Figure 1 shows an example of a correct Bus connection. In the following example the desired result is to get 0.2V of bias across Rterm, and Rterm is 120Ω. It should also be noted that this example ignores any current from or to the drivers and receivers. If an RS-232 to RS-485 converter is used, things start to get even more murky. You can design the system to accomplish this if you pay careful attention to logic function of the driver being used, as well as the labeling of the terminal on the device.

For example, Belden 3105A cable can be specified, but different types of cable with equivalent characteristics can be used. In short, RS485 is a standard defining the electrical characteristics of serial lines for use in serial communications systems. What is the "half-duplex" and "full-duplex" mode in RS485 and RS422? Common mode noise is the noise that is induced equally on both lines. They will usually add some hysteresis to the receiver to reduce its sensitivity to noise, but biasing will decrease the receivers noise sensitivity. This will cause reflections both back to the driver and forward to other receivers on the cable. This resistor will also make any length of cable, even a short length, look like the characteristic impedance of the cable (120 Ω in the model) to the driver. When the transmitter was empty it would set the RTS pin inactive to disable or "make passive" the RS-485 transmitter. Many other IC manufacturers make a drop-in replacement for this IC, and many of them (such as the MAX483) use the same labeling. But some device manufacturers match their A and B labels to the symbol (no inversion), and some match to the signaling waveform (inverted). 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, what is rs485 cable a binary 0 is on the wire.

In RS422 and RS485, a mark is a positive differential voltage and a space is a negative differential voltage. A decent oscilloscope that is isolated from earth ground (battery operated or powered by an unplugged UPS (if you're good enough with a scope you can also subtract channel 1 from channel 2 to see the differential waveform, or use a differential probe, etc.)) can be a very useful troubleshooting tool. As a general rule when the voltage measured (with respect to circuit common) is "high" the binary state is considered to be a 1 and when the voltage is low the state is 0. There are exceptions to this rule (such as differential logic and negative logic), but for the purpose of this discussion we'll talk about normal logic. In RS232, the idle state is a "mark" (negative voltage). Challenges of using RS232, RS422, and RS485 in a networked environment can include managing device addressing, handling contention if multiple devices try to transmit at the same time, extending the range to cover the network area, and ensuring data integrity in the presence of noise or other interference. How is RS485 different from RS232?

RS422 is differential, like RS485, and supports longer distances than RS232. Therefore the longer the cable length is the more the termination resistor decreases signal amplitude. Therefore the longer the cable length, the more important the termination resistor is to improve signal quality. The bias resistance needs to be taken into account when selecting the termination resistor. So why do you need to bias the network? This would ensure that the network has a period of marking before the start bit was transmitted, eliminating the need for a bias resistor to force the network to a marking state. Forcing the wires to the idle condition when no driver is active is the most common reason to need biasing on the network. Consider factors like the distance between devices, the data rate, the noise environment, the need for multi-point communication, and the power supply availability. Shielded cables and proper grounding can also help reduce noise. This can be used for the sender to confirm that the data was received correctly, or for the user to see what data was sent. However, if the receiver considers the undriven wires to be a binary 0, when the driver is turned on and set to transmit a start bit, which is also a binary 0, the receiver will not see a transition, and therefore will not see the start bit.