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Physical Circuit

The Cable That Is Defined By The RS-232 Standard

A complex RS-232 circuit could include two separate channels of communications with separate data, control, and timing circuits. The data circuits could carry two separate bi-directional conversations at the same time. The control circuits could provide flow control and synchronization between the communicators as well as indications of telephone ringing and off-hook conditions. The timing circuits could provide bit-by-bit clock synchronization between the two devices (which would help avoid misinterpretation of bits due to out-of-step clocks). This type of complex interface would require that all the specified wires be used in the interconnection cable.

The most simple RS-232 circuit would only require two wires: Signal and Ground. If this were the connection between transmitter and receiver, then the coordination of the two stations would be highly critical. For example, with only a Signal and Ground wire, the receiver would have no way to signal the transmitter (in the middle of receiving data) to say, "Slow down, you’re overloading me!". Typically, two-wire RS-232 is implemented only at slower baud rates to allow a large margin of error for the two communicators. It is not uncommon to see that a PDA (Personal Data Organizer) like an Apple Newton, HP OmniGo, or other "palmtop" computer uses a two-wire RS-232 connection to transfer data to and from the desktop PC. The two communicators may have a variety of special requirements, and this is where the "handshaking" implementations defined in the RS-232 standard come into play.