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Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. The driver has the capability of driving 10 receivers of 4k impedance, but the actual number that can be driven depend on the actual input impedance of the receivers, bit rate, wire, stub lengths, biasing and termination of the network. The point-to-point full-duplex RS-485 network allows you to get the complete prototype system fully operational quickly, since it is easier to debug and more immune to certain common problems on other systems (noise problems on RS-232, turn-around problems on half-duplex RS-485, etc.). 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 reducing the bit rate is not an option, then you could try an RS-232 to RS-485 converter at both ends. 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). The UART connected to the RS-232 driver/receiver controls the protocol of the bits being transferred.

Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485. Since the majority of RS-485 applications are driven by a UART, the data rate is usually below 100kbit. In this case, POTS telephone wire should work for quite a long line. An advantage is that this allows the software (if properly written) to take control of the network and hold it for a short time before transmitting the data. 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. 120 Ω cable should provide the best performance, but the 100 Ω CAT-X cable may you have laying around may also work. It does not mean that this protocol is part of the standards or that only this protocol may be used with the 232/422/485 standards.

In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the DMX512 protocol. RS-422 specifies a number of other things, but they are all electrical values. The RS-422 and RS-485 standards do not include a connector. The 9-pin connector and electrical levels were eventually documented in EIA/TIA-574. EIA/TIA-574 is the standard for 9-pin connectors, RS485 standard and EIA/TIA-561 is a standard for 8-pin connectors that are commonly used with serial ports. RS-232 specifies two connectors, a 25-pin (DB-25) and an alternate 26-pin connector (pin 26 is "No Connection"). The RS-232 standard defines a DB-25 and a 26-pin alternate connector, the function of each of the pins in the connector (called circuits), and the electrical characteristics of the signals that are on the "circuits". PURPOSE AND SCOPE OF COVERAGE - This standard is applicable to the interconnection of data terminal equipment (DTE) and data circuit-terminal equipment (DCE) employing serial binary data interchange. Articles, application notes, even data sheets from semiconductor manufacturers discuss both the data rate and line length limits in RS-485. The longer the unterminated wires are the slower the bit rate needs to be.

This serial port was no longer a "RS-232 compatible" serial port since the connector and signal levels no longer matched the RS-232 standard. While this is incorrect, many to this day still believe that the RS-232 standard includes things such as baud rate and the bit stream protocol used in the IBM serial port. If the can't, try reducing the bit rate. This protocol may include things like the start bit, number of data bits, parity and stop bit(s). Layer two of the OSI model includes the bit and framing protocol, which is outside of these EIA/TIA standards. In summary, the EIA/TIA standards (which are commonly miscalled RS standards) are the physical layer which include electrical signal levels and RS-232 includes the connector. When IBM changed to the new connector they also changed the electrical signal levels. The 9-pin "RS-232" connector used for the IBM PC-AT serial port is not specified in RS-232. Other reasons that some RS-232 ports were half-duplex: some very old UARTs may be half-duplex limiting the system, and some very old computers drove the RS-232 drivers directly from the processor without a UART. If multiple devices start to talk at the same time, the data may be corrupted.