RS-232 Connections/Operation
Document Number 3PCI-0712m
Chapter 7
29
RS-232 Signal Designations
The primary RS-232 signals are
TD
(transmit) and
RD
(receive). Together with GND
(ground), they often are referred to as a ―3-wire‖ interface.
The
RTS
(Request to Send) and
CTS
(Clear to Send) signals are handshaking lines
used to indicate to the other device that data can be sent or received. These lines may
be enabled or disabled on a byte-by-byte basis and are used to prevent buffer overrun
or the loss of data.
Two secondary handshaking signals are
DTR
(Data Terminal Ready) and
DSR
(Data
Set Ready). They are usually enabled when the device is powered up and the port is
open. They may be used for flow control by some devices instead of RTS and CTS.
DCD
(Data Carrier Detect) is used by a modem to indicate Carrier to the computer so
data can be sent/received.
RI
(Ring Indicator) is output by a modem to indicate that the phone or FAX line has a
incoming call, so it can be handled.
DTE and DCE
There are two types of RS-232 ports:
DTE
(Data Terminal Equipment) which is typically a computer
DCE
(Data Communications Equipment) which is typically a modem.
Data Terminal Equipment (DTE)
When a DTE is connected to a DCE they are linked directly together using a modem
cable wired pin to pin so that the inputs match the outputs of the other.
Data Communications Equipment (DCE)
When two pieces of equipment of the same type are interconnected (a DTE connected
to a DTE, or a DCE to a DCE), a crossover (often called a null modem) cable is needed
to route the outputs of one to the inputs of the other. This type of cable is needed to
interconnect two computers with RS-232.
Note:
Refer to Appendix B for straight-through and null modem cable
diagrams and pinouts.
RS-232 Signal Levels
RS-232 signal lines are referenced to ground, and each signal can alternate above and
below ground. The RS-232 standard specifies output voltages must be no less than +5
volts and no greater than +25 volts to represent a Space on a transmit line (or an
asserted handshake line). Output voltages must be between
–5 volts and –25 volts to
represent a Mark on a transmit line (or an de-asserted handshake line).
