For other data and timing signals, the limit
is 4% of a bit width, which works out to 2 ?µs at 20 kbps or 0.33 ?µs at 120 kbps.
An output that switches between -5V and +5V in 0.33 ?µs is changing at the
specified upper (fastest) limit for the slew rate. The signals??™ rise and fall times
should also be as nearly equal as possible.
"#
Many microcontrollers have asynchronous serial ports that use TTL logic levels
based on a 5V power supply or CMOS logic levels based on a power supply of
3V, 3.3V, or 5V. Interfacing to an RS-232 port requires converting between
these voltages and RS-232 voltages.
Table 4-3 shows logic levels for different chip families and supply voltages. 5V
logic refers to the logic levels used by TTL or CMOS logic chips powered by a
single +5V power supply with signal voltages referenced to ground. 3V and
3.3V logic refer to the logic levels used by CMOS logic chips powered by a sin-
Inside RS-232
49
gle +3V or +3.3V supply with signal voltages referenced to ground. Microcontrollers
typically use positive logic, where the more positive logic level, or logic
high, is logic 1.
With TTL logic, a logic-low output must be no higher than 0.4V, and a
logic-low input must be no higher than 0.8V. A logic-high output must be at
least 2.4V, while a logic-high input must be at least 2V. Using these specifications,
an interface can have 0.
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