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The RS-485 standard requires a minimum performance, but many drivers exceed this performance and some have quirks such as slew rate limiting. 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. Section 1.3, Data Signalling Rates, states; this standard is applicable for use up to a nominal limit of 20,000 bits per second. The TX output of a first device connected to the RX input of a second device, and the TX output of the second device connected to the RX input of the first device. To understand how to terminate a network (the wires) you must first understand transmission lines. If the length is short enough and the bit rate is low enough, completely unbalanced loose wires will work.

Adding a resistor to the end of the cable that matches the value of the cable's impedance will absorb the signal traveling down the line and reduce or prevent reflection. This should give you an approximate limit for that cable in feet. 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 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. Pull a cable between the two devices and see if they can talk to each other. The primary difference between RS-422 and RS-485 is that RS-485 requires two wires in a single pair which all devices transmitters and receivers are connected to, and RS-422 typically uses two or more pair in which only one transmitter and up to ten receivers are connected to a single pair.

RS-485 or its annex. The annex says this connection can be wired directly or made through a 100 Ω resistor. POTS telephone cable's impedance is less controlled than CAT cable and can vary from 600 Ω at lower audio frequencies to less than 100 Ω at higher frequencies. 120 Ω cable should provide the best performance, but the 100 Ω CAT-X cable may you have laying around may also work. Care must be taken that an SC connection, especially over long cable runs, does not result in an attempt to connect disparate grounds together - it is wise to add some current limiting to the SC connection. When a binary 1 (aka OFF or marking) is being transmitted the voltage on TXData must be less than -3V. 200mV as a binary value, but different manufacturers can set the threshold where ever they want. Articles, application notes, even data sheets from semiconductor manufacturers discuss both the data rate and line length limits in RS-485. RS-485 serves as a physical layer for the AES3 digital audio interconnect. All of the application guides and data sheets that say RS-485 has a limit of 1200 meters or 10Mbit are flat out wrong. The legends and folklore, not to mention the flat out wrong information that has grown around the line length and data rate limits inherent in RS-485 are truly astounding.

The VR-18 Paperless Chart Recorder can be connected to a PC with an Ethernet cable allowing students to use the Brainchild's FREE software, allowing for streaming live data or downloading the data to an Excel spreadsheet to further manipulate the information for reports and assignments. RS-485 has even less information. Figure 1 of the RS-485 standard is a diagram (not shown for copyright concerns) which shows the "balanced interconnecting cable" as a transmission line of two wires. Worse is the fact that annex A states that this graph is a conservative guide based on empirical data of 24AWG telephone cable (POTS). The graph in RS-422's annex A is not an absolute limit, but a guide to what should always work with cheap telephone wire. RS-422's annex has a chart of empirical data using 24 AWG copper UTP telephone cable. In this case your best bet for defining how long the cable can be is to try it.

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