++
%*(
,-.
%*(
,-.
%/(
,-.
%/0/(
,-. +
%*(
Logic-low output (max.) (V) 0.4 0.1 0.1 0.1 0.1
Logic-high output (min.) (V) 2.4 4.9 2.9 3.2 3.5
Logic-low input (max.) (V) 0.8 1 0.6 1.0 0.8
Logic-high input (min.) (V) 2.0 3.5 2.1 2.3 2.0
Chapter 4
50
$ #()*)
The MAX232 (Figure 4-1) includes two drivers that convert TTL or CMOS
inputs to RS-232 outputs and two receivers that convert RS-232 inputs to
TTL/CMOS-compatible outputs. The drivers and receivers also invert the signals.
The chip contains two charge-pump voltage converters that act as tiny, unregulated
power supplies that enable the chip to support loaded RS-232 outputs of
?±5V or greater. Four external capacitors store energy for the supplies. The recommended
value for the capacitors is 1?µF or larger. If using polarized capacitors,
take care to get the polarities correct when you put the circuit together.
The voltage at pin 6 is negative, so its capacitor??™s + terminal connects to ground.
Because the outputs can be as high as 10V, be sure the capacitors are rated for a
working voltage direct current (WVDC) of at least 15V. (Most are.)
+$ " $
Interface chips to suit just about any application??™s needs are available.
The MAX232A was an early improvement that uses smaller, 0.1?µF
charge-pump capacitors and can operate at up to 200 kbps. The MAX233 and
MAX233A (Figure 4-1) require no external charge-pump capacitors at all.
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