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The bias resistance needs to be taken into account when selecting the termination resistor. If you are trying to get two ancient pieces of equipment (both of which use true RS-232 drivers and receivers) to work with a long cable, the capacitance limit in RS-232 may apply, but RS-232 does not take bit rate into account. If this is required, it can be accomplished by any means desired including RS-485 drivers on an additional network, RS-232 drivers on extra wires, TTL levels, or really anything you can dream up since it won't work with any other RS-485 network except the one you are designing. All of the application guides and data sheets that say RS-485 has a limit of 1200 meters or 10Mbit are flat out wrong. Articles, application notes, even data sheets from semiconductor manufacturers discuss both the data rate and line length limits in RS-485. Low capacitance cable becomes important when pushing line length and/or data rates to the maximum. The appendix discusses capacitance and resistance of the cable, then gives an example calculation where the capacitance of the cable per foot (30pF/foot) multiplied by the cable's length, plus the capacitance of the receiver (100pF) gives a maximum cable length of 80 feet.

Stub length, termination, and biasing resistors can have a significant impact on the performance of the network. Pull a cable between the two devices and see if they can talk to each other. See the sections on termination and biasing for more information. If an RS-232 to RS-485 converter is used, things start to get even more murky. 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. If you keep the cable length at 70% or 80% of this limit you should expect the network to work with a true RS-232 driver and receiver. Section 3.2 clearly states that "The maximum length of the cable is not defined". That being said, what is the practical line length limit? In this figure the driver has two leads and connects via a stub to the transmission line at points "A" and "B". In reality, if you are using relatively modern equipment (say 1990 or later) and a low baud rate, line lengths of 1,000 feet (300 m) or more are possible. The logic function of the driver and receiver are not defined, only the binary state of the differential voltages on the wires.

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.". As mentioned above in the Voltages section; On the RS-485 network there will be times when the two wires are not driven by a transmitter. Voltages of less than 200mV are undefined. The receiver is designed to operate with voltages between 3 and 15V in magnitude (i.e. both positive and negative voltages) but must be able to handle an input of 25V without damage. When a binary 1 is transmitted, the signal is spacing, and the voltage on the wire must be more negative than -3V. When a binary 1 (aka OFF or marking) is being transmitted the voltage on TXData must be less than -3V.

This limited the open circuit voltage to ±12V. Lower voltage drivers are now available to allow operation from battery powered devices and the open circuit voltages may be lower than ±12V. In addition to using the best cable, there are many different drivers, receivers, and transceivers available. See the instructions on the converter you are using. When 5V drivers are used, the driver typically pulls one wire to common, and the other wire to 5V (and vice-versa for the opposite data). When the input of an RS-485 driver goes high, RS485 standard one of the outputs will go high in reference to circuit common and the other will go low. This difference in earth ground may be high enough to cause damage to the RS-485 devices, but connecting a third wire between their circuit commons/earth grounds would try to bypass the power earth common (this is often called a ground loop) causing excessive current in the third wire.