N nodes are connected in a multipoint RS485 network. The RS485 network must be designed as one line with multiple drops, not as a star. Although total cable length maybe shorter in a star configuration, adequate termination is not possible anymore and signal quality may degrade significantly. In a typical applications several address able devices may be linked to a single controlled (PC), and in this way a single line may be used for communication. Therefore RS485 can overcome the practical communication speed limit of RS232. In general if you are not connected to a modem the handshaking lines can present a lot of problems if not disabled in software or accounted for in the hardware (loop-back or pulled-up). Of course, many other problems can cause a communications failure and these will be addressed in the troubleshooting section. In its simplest form, a pair of converters from RS232 to RS422 (and back again) can be used to form an "RS232 extension cord." Data rates of up to 100K bits / second and distances up to 4000 Ft.

The second optional grounding arrangement is to connect the circuit common directly to the frame ground without a resistor. Differential signals can help nullify the effects of ground shifts and induced noise signals that can appear as common mode voltages on a network. Differential data transmission (balanced differential signal) offers superior performance in most applications. If the voltage on the RS-232 line is positive or high, the signal is a binary 0. Should this signal be inverted back to a low? We also see that the maximum allowed voltage levels for all interfaces are in the same range, but that the signal level is lower for the faster interfaces. The "idle" state (MARK) has the signal level negative with respect to common, and the "active" state (SPACE) has the signal level positive with respect to common. For an off, mark or logic 1 state, the driver's A terminal is negative relative to the B terminal. PURPOSE AND SCOPE OF COVERAGE - This standard is intended to provide a method of interconnecting data terminal equipment and a data communication channel when each is furnished by different companies. Differential signals and twisting allows RS485 to communicate over much longer communication distances than achievable with RS232.

RS422 (differential) was designed for greater distances and higher Baud rates than RS232. Standards have been developed to insure compatibility between units provided by different manufacturers, and to allow for reasonable success in transferring data over specified distances and/or data rates. RTS (Request to send) does have some utility in certain applications. Here the master initiates a communications request to a "slave node" by addressing that unit. The request and acknowledge data needed in most protocols does not consume bandwidth on the primary data channel of RS232. This is, because on the other interfaces the communication channel is shared by multiple receivers and-in the case of RS485-by multiple senders. Because there is a chance of data collosion with this implementation, RS485 standard theory tells us that in this case only 37% of the bandwidth will be effectively used. This means that there can be 32 nodes capable to both transmit and receive. RS485 is able to provide a headline data rate of 10 Mbps at distances up to 50 feet, but distances can be extended to 4000 feet with a lower speed of 100 kbps.

The specification allows for data transmission from one transmitter to one receiver at relatively slow data rates (up to 20K bits/second) and short distances (up to 50Ft. @ the maximum data rate). With RS485 communication distances of 1200 m are possible. When using the default RS485 receivers with an input resistance of 12 kΩ it is possible to connect 32 devices to the network. It is also possible to convert between RS485 and RS232 using simple interface converters that may include optical isolation between the two circuits as well as surge suppression for any electrical 'spikes' that may be picked up. 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. Often RS-485 links are used for simple networks, and they may be connected in a 2 or 4 wire mode.