Inside RS-232The RS-232 receiver is much like the previous circuit’s, but the driver’s circuit is
different. When TTL Serial Out is low, PNP transistor Q1 is on and RS-232
Out equals the supply voltage, minus Q1’s collector-emitter voltage (a few
tenths of a volt). When TTL Serial Out is high, Q1 is off, and the RS-232 link
loops back on itself through R2. RS-232 Out equals RS-232 In, minus a small
voltage across R2. For this interface to work properly, RS-232 In must be idle
(negative) whenever TTL Serial Out is transmitting.
The NTE2355 and NTE2356 transistors are designed for use as digital
switches. They have built-in biasing resistors and a typical maximum frequency
of 250 MHz. Their emitter-to-base breakdown voltage is higher than most, at
10V.$
Some low-power circuits that connect to an RS-232 port don’t need an external
power supply. Instead, they draw all the power they need from the interface
itself.Figure 4-3: This 5V-only interface will work on many short links.