RS-232 | Tech Info Home Page |

RS-232 ( Recommended Standard 232 )

The RS232 standard was original released in 1969 by the EIA (Electronic Industries Association). The CCITT (Consultative Committee on International Telegraphy and Telephony also release a similar standard covering the "Interface between Data Terminal Equipment and Data Communications Equipment employing Serial binary Data Interchange."

RS-232 cables are either a DB-9 (9-pin) connector, or a DB-25 (25-pin) connector.

A typical use of an RS-232 cable is connecting an external modem to the computer.

Terms:

• DCE - Data Communication Equipment
• DTE - Data Terminal Equipment

• DB9 - Serial Port Connector plug on the back of a computer for connecting external modems, mice

The Connector

DB-25 Serial Port Pin-out

< means to DTE,

 > means to DCE

1. -  FG Frame ground
2. > TD Transmitted Data
3. < RD Received Data
4. > RTS Request To Send
5. < CTS Clear To Send
6. < DSR Data Set Ready
7. - SG Signal Ground
8. < DCD Data Carrier Detect
9. < DC +
10. < DC -
11. < QM Equalizer Mode
12. < S DCD Secondary Data Carrier Detect
13. < S CTS Secondary Clear To Send
14. > S TD Secondary Transmitted Data
    > NS New Sync
15. < TC Transmitter Clock
16. < S RD Secondary Received Data
    < DCT Divided Clock Transmitter
17. < RC Receiver Clock
18. < DCR Divided Clock Receiver
19. > S RTS Secondary Request To Send
20. > DTR Data Terminal Ready
21. < SQ Signal Quality Detect
22. < RI Ring Indicator
23. > DRS Data Rate Selector
    < DRS Data Rate Selector
24. > ETC External Transmitter Clock
25. > BUSY Busy

Loopback plug pin-out - (Female DB-25)
1-7
2-3
4-5-8
6-11-20-22
15-17-23
18-25

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Some quick notes on RS-232

• Pin 7 on cables is always connected
• On DTE equipment, pin 2 transmits, pin 3 receives
• On DCE equipment, pin 3 transmits, pin 2 receives
• The general discussion of what pins is transmitting implies that the equipment is DTE.
• Usually DCE equipment will have female connectors, DTE male.
• Modems are usually DCE
• RTS is a DTE output
• CTS is a DTE input
• DSR is monitored by equipment for status
• DCD is an output
• DTR is outputted by DTE and monitored by DCE

In RS-232 devices:

For outputs

• Negative voltage is 1 (-1 VDC to -5 VDC )
• Positive voltage is 0 ( 1 VDC to 5 VDC )

For inputs

• Negative voltage is 1 ( -3 VDC to -15 VDC )
• Positive voltage is 0 ( 3 VDC to 15 VDC )

Signal Conditions and associated Logic names

Logical 0	Logical 1
Space	Mark
Perforation	No Perforation
Off	On
Start	Stop
Low	High
False	True
Reset	Set
Positive	Negative

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