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 | 25 Pin DCE | 9 Pin DTE | 9 Pin DCE |
DEVICE | | LINE | LINE | DEVICE | DEVICE | | LINE | LINE | DEVICE |
| T | | | 2 | (TD) | 2 | (TD) | | | | | R | | | | T | | 3 | (TD) | 3 | (TD) | | R | | |
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| R | | | 3 | (RD) | 3 | (RD) | | | | | | | | T | | | R | | 2 | (RD) | 2 | (RD) | | | | T | |
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| | | 4 (RTS) | 4 | (RTS) | | | | | R | | 7 (RTS) | | 7 | (RTS) | | | | |
| T | | | | | | | | | | T | | | | R | | |
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| R | | | 5 | (CTS) | 5 | (CTS) | | | | | | | | T | | | R | | 8 | (CTS) | 8 | (CTS) | | | | T | |
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| | | 20 (DTR) | 20 (DTR) | | R | | 4 (DTR) | | 4 | (DTR) | | | | |
| T | | | | | | | T | | | | R | | |
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| R | | | 6 | (DSR) | 6 | (DSR) | T | | | R | | 6 | (DSR) | 6 | (DSR) | | | | T | |
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| | | 8 | (DCD) | 8 | (DCD) | | 1 | (DCD) | 1 | (DCD) | | | |
| R | | | T | | | R | | | | | T | |
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| | | 7 | (GND) | 7 | (GND) | | 5 | (GND) | 5 | (GND) | | | |
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| | | | | | | | | T = RS-232 Transmitter | R = RS-232 Receiver | | | | | | | | | | | |
Figure 3.1. DTE/DCE Port Diagrams
Documentation Number 232BSS4-2907m | 21 |
&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
