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.