MAX220-MAX249 datasheets
The MAX232 and MAX233 are parts commonly used to translate TTL-level
signals (0-5V) into RS232-level signals (in this case -10V to +10V).
Unlike cheaper parts, such as the 1488, the MAX232 and 233 do not
require +10v and -10v supplies to do this translation: instead, they
use two charge pumps to produce these voltage levels from a single 5v
supply. Charge pumps require capacitors: the MAX232 and 232A require
5 external capacitors (1uf in the case of the 232, and .1uf in the
case of the 232A). The MAX233 has its capacitors built-in, so
requires no external components.
The MAX232, 232A, and 233 also contain buffers to do the reverse
translation: +-10V to 0-5V. (Note that you can also easily do this
reverse translation with a 47K resistor and a CMOS inverter.
Reference to come.)
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Randy Sargent <rsargent@media.mit.edu>