Inside RS-232Instead of requiring or generating their own negative voltage sources,
Maxim/Dallas Semiconductor’s DS275 and DS276 borrow the voltage from
the interface at the opposite end of the cable.
Circuits that have access to higher-voltage positive and negative supplies don’t
need voltage converters. The MAX1488E contains four drivers and requires
±12V supplies. The complement to this chip is the MAX1489E, which con-
tains four receivers and operates from a single +5V supply. (The receive circuits
don’t require higher-voltage supplies.) The MAX3314E requires dual power
supplies of ±5V, doesn’t use charge pumps, and has RS-232 output swings of
slightly less than ±5V.
+
Many chips incorporate extra protection from electrostatic discharge (ESD)
transients on inputs and outputs. Circuits that require electrical isolation can
use the MAX250/MAX251 pair with four optocouplers and a transformer.! "
If you examine the data sheets for the MAX232 and similar chips, you’ll see that
the specifications for their RS-232 inputs are less stringent than required by the
RS-232 standard. As Figure 4-2 shows, the input thresholds are similar to TTL
logic, with a logic low input defined as 0.8V or lower and a logic high input
defined as 2.0V or higher..
For cables of 10 ft or less, you may be able to communicate with an RS-232
port by using an inexpensive interface that uses 5V logic rather than RS-232
voltages.
Figure 4-3 shows an option for connecting a 5V port to a remote RS-232 inter-
face. This circuit is intended only for short links because it doesn’t meet
RS-232’s voltage and other specifications.
At the driver, any inverted 5V logic can provide the interface. Figure 4-3 uses
Q1, a PN2222 or other NPN general-purpose or switching transistor that func-
tions as an inverter. A TTL/CMOS output drives the base of the transistor,
with R1 limiting its base current. When the TTL/CMOS output is low, Q1 is