The longer the wires are, the less signal there will be at the end. The longer the cable length, the more effect the termination resistor has on the signal level. However the bias resistors effect the total termination resistance. Therefore the total bias resistance is effectively in parallel with the termination resistor as a load to the cable. Increasing the frequency will increase the inductive reactance, thereby increasing the series resistance in this model. A termination resistor can increase the signal's quality (reduce ringing), but it will also reduce the signal's amplitude. Lowering the bias resistor values will increase the bias voltage which will increase the noise immunity at the cost of increased current from the 5V supply. 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.

At the other end of the cable is (typically) the same thing -- the connectors, the RS-485 interface IC, and a UART inside a microcontroller. A RS-485 interface will usually use pins 7 and 8 for the two data lines, since they comprise a twisted pair. TODO: add connectors and pins from manufacturers such as NI, B&B, ADAM, etc. Volunteers? Devices with the same connector but from different manufacturers are probably not directly inter-connectable. Which fault conditions are to be detected and what is to be done when a fault is detected is application-dependent. Determining which fault has occurred would take additional hardware. This scheme will automatically take control of the network when a byte is transmitted, but there will not be any guaranteed marking time so bias resistors may be required. A single set of bias resistors could be used at one end of the network, if the resistance was halved to 720 Ω.

A pair of wires has capacitive coupling and a finite (usually very high) resistance between the wires. The other pair is dedicated to peripheral-to-PC communication. The CAT-5 cable allows you to relatively quickly switch -- to half-duplex RS-485, or the 3 wires of RS-232, or a variety of other communication protocols -- without pulling any new cables. 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. Biasing has a number of uses on a RS-485 network, but first lets look at what RS422 and RS-485 have to say. The RS-422 and RS-485 standards do not include a connector. ↑ "RS-422 And RS-485 Applications Ebook" (PDF). The RS-485 driver's enable is controlled by the timer. RS-422 drivers are always connected and act as their own termination at the driver's end of the wire. RS-422 is full duplex so it can use the X-On / X-Off protocol, if needed. If handshaking is required, it can be attempted using using X-On / X-Off handshaking protocol, but it is unlikely to work. When it was able to receive more data, the receiver would use the handshaking lines to signal the transmitter that it was OK to send more data.

This stopped the data flow to the receiving end, allowing the receiver to process the data in its buffer. USB to serial converters come with ADDC (Automatic Data Direction Control) to automatically sense and control data direction, making the handshaking control method obsolete. There is no hardware handshaking in the RS-485 standard and in most cases it is no longer needed. Another RS-232 to RS485 scheme is to monitor the data stream going into the RS-485 transmitter and trigger a one-shot timer when an edge occurs. From a hardware point of view, full-duplex RS-485 has some major advantages over RS-232 -- it can communicate over much longer distances at higher speeds. RS-485 allows a host CPU to talk to a bunch of sensors all connected to the same cable. If CAT-5 (100 Ω cable) cable is used, the termination resistors should be 100 Ω. This means that the lower the bit rate the less termination resistors are needed.

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