Serial Port

RS232 Serial Port Description

The IBM compatible series of personal computers use the RS232 interface as the serial port.
The RS232 serial port was also found on the Apple series of computers, but no longer.
The serial port on the Apple computer is now made up of either a USB port or an Ethernet port.
Information and pinout data for the older Apple computers are found on the Apple Computer RS-232 Buses page.
The 9-pin serial port pinout is located in the table below.
RS-232 uses Asynchronous Framing [Known data width, 8 bits] with NRZ encoding.
Normally the RS232 interface is asynchronous. RS232 is rated to operate up to 20kbps.
The maximum RS232 cable length is a little less then 20 meters.
For a detailed description of the RS232 bus standard refer to the EIA-232 Bus page.
The RS232 interface is no longer being supplied with many new computers.

RS232 Null Modem Pinout

RS232 9-Pin Null Modem cable Pinout [DB9] is use when connecting two computers together.
A Null Modem Adapter converts any straight-through wired cable into a null modem "cross-wired" configuration.

A Null Modem cable is identified by the gender of the connectors attached to the cable ends.
A Null Modem cable will have either a Male-to-Male or Female-to-Female interface on both ends of the cable.
A Male-to-Female cable would be a 'normal' RS232 cable.

RS232 Pinout

The RS232 specification only defines the pin-out for a 25 pin D connector; how ever, the 9 pin is used more often (defined by EIA-574).
The serial port found on Personal Computers uses a 9-pin connector.
Normally the RS232 serial port is used for external modems, and in older systems for the mouse and printer interfaces.
Most computer systems have only one serial port. The RS232 pinout for a 9-pin D-Sub connector is provided in the table below.
Also refer to D-Sub Shell Dimensions, or 9-pin D-Sub Insert Arrangements.

RS232 Serial Port Signal Description

TXD: Transmit Data; The data sent from the Data Terminal and received by the Data Set.
RXD: Receive Data; The data sent from the Data Set and received by the Data Terminal.
DTR: Data Terminal Ready; Used by the Data Terminal to signal to the Data Set that it is ready for operation, active high.
DSR: Data Set Ready; Used by the Data Set to signal to the Data Terminal that it is ready for operation and ready to receive data, active high.
RTS: Request To Send; Used by the Data Terminal to signal the Data Set that it may begin sending data. The Data Set will not send out data with out this signal, active high.
CTS: Clear To Send; Used by the Data Set to signal the Data Terminal that it may begin sending data. The Data Terminal will not send out data with out this signal, active high.
CD: Carrier Detect; Used by the Data Set to indicate to the Data Terminal that the Data set has detected a carrier (of another device).
RI: Ring Indicator; Used by the Data Set to indicate to the Data Terminal that a ringing condition has been detected.
GND: Ground; The common return for all signals on the interface.
When using Software Flow Control (XON, XOFF); you only need 3 lines, TX (data), RX (data), and GND. XON being equal to ready, XOFF equal to not ready.

For reference, the pinout for the 25-pin connector and the RJ-45 connector is provided below.
The RJ-45 connector is a Ethernet interface on most new computers, and not a RS232 interface.
25-pin D-Sub Insert Arrangements

RJ45 Pin-Out

{RS232 Pinout top}

Home
Distributors	Components	Equipment	Software	Standards	Buses	Design	Reference