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Fundamentals of RS–232 · Serial Communications · APPLICATION NOTE 83 · 030998 1/9 · Due to it's relative simplicity and low
Summary:
RS–232 SPECIFICATIONS
RS–232 is a “complete” standard. This means that the
standard sets out to ensure compatibility between the
host and peripheral systems by specifying 1) common
voltage and signal levels, 2)common pin wiring configurations,
and 3) a minimal amount of control information
between the host and peripheral systems. Unlike many
standards which simply specify the electrical characteristics
of a given interface, RS–232 specifies electrical,
functional, and mechanical characteristics in order to
meet the above three criteria. Each of these aspects of
the RS–232 standard is discussed below.
ELECTRICAL CHARACTERISTICS
The electrical characteristics section of the RS–232
standard includes specifications on voltage levels, rate
of change of signal levels, and line impedance.
The original RS–232 standard was defined in 1962. As
this was before the days of TTL logic, it should not be
surprising that the standard does not use 5 volt and
ground logic levels. Instead, a high level for the driver
output is defined as being +5 to +15 volts and a low level
for the driver output is defined as being between –5 and
–15 volts. The receiver logic levels were defined to provide
a 2 volt noise margin. As such, a high level for the
receiver is defined as +3 to +15 volts and a low level is
–3 to –15 volts. Figure 1 illustrates the logic levels
defined by the RS–232 standard. It is necessary to note
that, for RS–232 communication, a low level (–3 to –15
volts) is defined as a logic 1 and is historically referred to
as “marking”. Likewise a high level (+3 to +15 volts) is
defined as a logic 0 and is referred to as “spacing”.
The RS–232 standard also limits the maximum slew
rate at the driver output. This limitation was included to
help reduce the likelihood of cross–talk between adjacent
signals. The slower the rise and fall time, the
smaller the chance of cross talk. With this in mind, the
maximum slew rate allowed is 30 V/ms. Additionally, a
maximum data rate of 20k bits/second has been defined
by the standard. Again with the purpose of reducing the
chance of cross talk.
The impedance of the interface between the driver and
receiver has also been defined. The load seen by the
driver is specified to be 3kW to 7kW. For the original
RS–232 standard, the cable between the driver and the
receiver was also specified to be a maximum of 15
meters in length. This part of the standard was changed
in revision “D” (EIA/TIA–232–D). Instead of specifying
the maximum length of cable, a maximum capacitive
load of 2500 pF was specified which is clearly a more
adequate specification. The maximum cable length is
determined by the capacitance per unit length of the
cable which is provided in the cable specifications.
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