THE MAX232 &
MAX232A
The MAX232 from Maxim was the first IC which in one package contains the necessary drivers (two) and receivers (also two), to adapt the RS-232 signal voltage levels to TTL logic. It became popular, because it just needs one voltage (+5V) and generates the necessary RS-232 voltage levels (approx. -10V and +10V) internally. This greatly simplified the design of circuitry. Circuitry designers no longer need to design and build a power supply with three voltages (e.g. -12V, +5V, and +12V), but could just provide one +5V power supply, e.g. with the help of a simple 78x05 voltage regulator.
The MAX232 has a successor, the MAX232A. The
ICs are almost identical, however, the MAX232A is much more often used (and
easier to get) than the original MAX232, and the MAX232A only needs external
capacitors 1/10th the capacity of what the original MAX232 needs.
It should be noted that the MAX232 (A) is just a
driver/receiver. It does not generate the necessary RS-232 sequence of marks
and spaces with the right timing, it does not decode the RS-232 signal, and it
does not provide a serial/parallel conversion. All it does is to convert signal voltage levels. Generating serial
data with the right timing and decoding serial data has to be done by
additional circuitry, e.g. by a 16550 UART or one of these small
micro controllers (e.g. Atmel AVR, Microchip PIC) getting more and
more popular.
The MAX232 and MAX232A were once rather expensive
ICs, but today they are cheap. It has also helped that many companies now
produce clones (i.e. Sipex). These clones
sometimes need different external circuitry, e.g. the capacities of the
external capacitors vary. It is recommended to check the data sheet of the
particular manufacturer of an IC instead of relying on Maxim's original data
sheet.
The original manufacturer (and now some clone
manufacturers, too) offers a large series of similar ICs, with different numbers
of receivers and drivers, voltages, built-in or external capacitors, etc. E.g.
The MAX232 and MAX232A need external capacitors for the internal voltage pump,
while the MAX233 has these capacitors built-in. The MAX233 is also between
three and ten times more expensive in electronic shops than the MAX232A because
of its internal capacitors. It is also more difficult to get the MAX233 than
the garden variety MAX232A.
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