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Therefore the total bias resistance is effectively in parallel with the termination resistor as a load to the cable. These examples are for bias resistors that just barely meet the 0.2V differential voltage. 0.2V the receivers output switches to a 1 and when the voltage goes below -0.2V the receivers output switches to a 0. (or vice versa if there is inversion on the receiver) Since the last bit from a UART will be the stop bit (1), then the transmitter is turned off (the differential voltage goes to 0V, but not having gone less than -0.2V), this should leave the receiver with a 1 being output to the receiving UART. 12V. The receiver must recognize a differential voltage of greater than ±200mV as a binary value. The logic function of the driver and receiver are not defined, only the binary state of the differential voltages on the wires. Any receiver connected at some point in the cable will change the impedance of the cable at that point on the cable. All information below this point is legacy and very questionable. The result of this looks like two 120 Ω resistors in parallel (60 Ω) to either end or to any point 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. These capacitors will act as a short to AC signals. If the driver is transmitting asynchronous start stop data (from a UART and no inversion of the driver's input) the idle condition of the two wires will be a 1. If you bias the network at the receiver so that the receiver see's a 1, you could not tell when the driver was disconnected (without additional hardware) If you bias the network to force the two wires to a 0 (when the transmitter is not connected), the receiver will see a 1 on the wires (when the receiver and the driver are connected without an open in the wires). 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. For those designing or documenting a system, it is a bit more complicated than simply swapping pins.

TODO: add connectors and pins from manufacturers such as NI, B&B, ADAM, etc. Volunteers? The higher the data rate and the longer the cable, the more likely it will be that you have to add termination resistors. RS-232 does not define a protocol, but the protocol that is almost always transmitted on these connectors is asynchronous start-stop ASCII (data from a UART). The signal will eventually stabilize (until the next bit transition) so the slower the bit rate the more time the signal will have to stabilize before it is sampled (usually by a UART). When an RS-485 driver gets its data directly from a UART (with no added inversion), you would expect the "A" and "B" wires to match the voltages in the RS-485 standard for the voltages on the wires, but they will not (unless the driver inverts its input). The RS485 standard (TIA-485) specifies differential data transmission over a terminated twisted pair.. The purpose of handshaking is for the receiver to tell the transmitting device to "shut up, I'm full of data and will lose anything more you send me". When the microprocessor's in the computers and peripheral's speed was increased to a blistering 1MHz, handshaking was still necessary, but the stop sending / OK to send lines would toggle less often.

Each pair can transmit at full speed whether or not the other pair is transmitting (full-duplex). Then the software will work unchanged when connected to a full-duplex RS-485 network, a RS-232 network, and a variety of other communication media. Alas, a long 3-conductor cable intended for RS-232 can not be switched to full-duplex RS-485, which requires 5 conductors. 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. Before we discuss polarity lets take a look at logic levels and binary states. In most digital logic a binary 0 is usually considered OFF and a binary 1 is ON. The first is that RS-485 denies any control of the logic function of the generator and receiver. 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. You would think that this would mean that when a 1 is being transmitted "A" would be high and "B" would be low.