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How Serial Ports Work

        Tech | Buses

The Serial Connection
Close-up of 9-pin and 25-pin serial connectors
Close-up of 9-pin and 25-pin serial connectors

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.

9-pin connector:

  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:

  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...