RS-232 Connections/Operation
56
Chapter 7
Manual Documentation Number 3PCIoUx-1008
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.
