The Serial Connection

The external connector for a serial port can be either 9 pins or 25 pins. Originally, the primary use of a serial port was to connect a modem to your computer. The pin assignments reflect that. Let's take a closer look at what happens at each pin when a modem is connected.

Close-up of 9-pin and 25-pin serial connectors

9-pin connector: (Pin Nos.)

1. Carrier Detect - Determines if the modem is connected to a working phone line.


2. Receive Data - Computer receives information sent from the modem.


3. Transmit Data - Computer sends information to the modem.


4. Data Terminal Ready - Computer tells the modem that it is ready to talk.


5. Signal Ground - Pin is grounded.


6. Data Set Ready - Modem tells the computer that it is ready to talk.


7. Request To Send - Computer asks the modem if it can send information.


8. Clear To Send - Modem tells the computer that it can send information.


9. Ring Indicator - Once a call has been placed, computer acknowledges signal (sent from modem) that a ring is detected.

25-pin connector: (Pin Nos.)

1. Not Used


2. Transmit Data - Computer sends information to the modem.


3. Receive Data - Computer receives information sent from the modem.


4. Request To Send - Computer asks the modem if it can send information.


5. Clear To Send - Modem tells the computer that it can send information.


6. Data Set Ready - Modem tells the computer that it is ready to talk.


7. Signal Ground - Pin is grounded.


8. Received Line Signal Detector - Determines if the modem is connected to a working phone line.


9. Not Used: Transmit Current Loop Return (+)


10. Not Used


11. Not Used: Transmit Current Loop Data (-)


12. Not Used


13. Not Used


14. Not Used


15. Not Used


16. Not Used


17. Not Used


18. Not Used: Receive Current Loop Data (+)


19. Not Used


20. Data Terminal Ready - Computer tells the modem that it is ready to talk.


21. Not Used


22. Ring Indicator - Once a call has been placed, computer acknowledges signal (sent from modem) that a ring is detected.


23. Not Used


24. Not Used


25. Not Used: Receive Current Loop Return (-)

Voltage sent over the pins can be in one of two states, On or Off. On (binary value "1") means that the pin is transmitting a signal between -3 and -25 volts, while Off (binary value "0") means that it is transmitting a signal between +3 and +25 volts...