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Chapter 3: PORT CONNECTIONS
DTE/DCE Explanation
In order to determine the proper port connections to the 232BSS4, it is
necessary to have a basic understanding of the terms DTE and DCE. RS-
232 was designed, using DB-25 connectors, for connecting a DTE (Data
Terminal Equipment) device to a DCE (Data Communication Equipment)
device. Each device will have inputs on pins that correspond to outputs on
the same pins of the other device. For example, a DTE device will transmit
data out on pin 2 (on a DB-25) and a DCE device will receive data in on
pin 2 (on a DB-25). IBM PCs and serial printers are DTE devices, modems
are DCE devices.
Originally the RS-232 standard specified only a 25 pin D-sub
connector. Since then, the use of a 9 pin D-Sub supporting only a portion of
the original RS-232 signals has been used extensively, starting with the
IBM PC and migrating into other peripherals. The pinouts for this 9 pin
connector have since become the EIA/TIA 574 standard. This standard
specifies a DTE device that transmits on pin 3 and receives on pin 2, with
the DCE having the opposite configuration.
25 Pin DTE
DEVICE
LINE
2 (TD)
T
3 (RD)
R
4 (RTS)
T
5 (CTS)
R
20 (DTR)
T
6 (DSR)
R
8 (DCD)
R
7 (GND)
T = RS-232 Transmitter
Documentation Number 232BSS4-0812m
&B Electronics Mfg Co Inc – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104 – www.bb-elec.com
B&B Electronics – Westlink Commercial Park – Oranmore, Galway, Ireland – Ph +353 91-792444 – Fax +353 91-792445 – www.bb-europe.com
25 Pin DCE
LINE
DEVICE
2 (TD)
R
3 (RD)
T
4 (RTS)
R
5 (CTS)
T
20 (DTR)
R
6 (DSR)
T
8 (DCD)
T
7 (GND)
Figure 3.1. DTE/DCE Port Diagrams
9 Pin DTE
9 Pin DCE
DEVICE
LINE
LINE
3 (TD)
T
2 (RD)
R
7 (RTS)
T
8 (CTS)
R
4 (DTR)
T
6 (DSR)
R
1 (DCD)
R
5 (GND)
R = RS-232 Receiver
DEVICE
3 (TD)
R
2 (RD)
T
7 (RTS)
R
8 (CTS)
T
4 (DTR)
R
6 (DSR)
T
1 (DCD)
T
5 (GND)
21
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