Chapter 4
Software drivers for some USB virtual COM ports don’t allow applications to
read CTS directly. An application that requires direct reading of an input bit
can use DSR instead.
RI and CD are outputs on the DCE and can have any use if not used for their
original purposes of detecting a ring signal on a phone line (RI) and detecting
the presence of a carrier (CD).
All RS-232 inputs and outputs must be able to withstand a short circuit to any
other RS-232 signal, including ground, without damage.&
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The Break signal supports a rarely used form of signaling. Setting the break sig-
nal in a UART causes TX to remain at a positive RS-232 voltage, which is the
opposite of the idle-state voltage.
The break signal enables in-line signaling to a microcontroller or other device
that has no input buffers or flow-control lines. The signal also provides a way to
toggle the TX line as desired for any purpose. $
In the 25-pin connector, pin 1 is a shield connection to allow grounding of a
cable shield. The pin isn’t always connected inside the device, and the 9-pin
connector doesn’t include the pin. Many shields instead use the connector shell
to connect to a grounded chassis. The shield normally connects to the chassis, r
frame, of the DTE only, not the DCE.
Earlier versions of the standard called pin 1 protective ground, and the wire
sometimes connected the chassis of the equipment on both ends. Each chassis
in turn connected to a safety ground at the equipment’s power plug. TIA-232
recommends using a separate wire (not one of the wires in the connector) to
connect the grounds of the two chassis if needed.:
RS-232 logic levels are defined as positive and negative voltages rather than the
positive-only voltages of TTL and CMOS logic (Table 4-2). At an RS-232 data
output (TX), logic 0 is defined as equal to or more positive than +5V, and logic
1 is defined as equal to or more negative than -5V. In other words, the data uses
negative logic, where the more positive voltage is logic 0 and the more negative
voltage is logic 1.