SERIAL PORTS NOTE: The IBM PC uses a DTE (Data Terminal Equipment) Connection standard. Most Modems are DCE (Data Communication Equipment). I/O Map ----------------------------- COM1: $3F8-3FF COM2: $2F8-2FF COM3: $3E8-3EF COM4: $2E8-2EF DB-9 Connector Pin Signal Pin Signal ------------------------------------------------------------------- 1 CD Carrier Detect 6 DSR Data Set Ready 2 RXD Receive Data 7 RTS Request to Send 3 TXD Transmit Data 8 CTS Clear to Send 4 DTR Data Term. Ready 9 RI Ring Indicator 5 GND Signal GND DB-25 Connector Pin Signal Pin Signal ------------------------------------------------------------------- 1 GND Ground / Shield 14 {2nd TXD} 2 TXD Transmit Data 15 Transmit Clock 3 RXD Received Data 16 {2nd Rec. Clock} 4 RTS Request to Send 17 {Receive Clock} 5 CTS Clear to Send 18 {Unassigned} 6 DSR Data Set Ready 19 {2nd RTS} 7 GND Ground 20 DTR Data Term Ready 8 DCD Carrier Detect 21 {Sig. Quality} 9 {Reserved} 22 RI Ring Indicator 10 {Reserved} 23 {Data Rate Sel.} 11 {Unassigned} 24 {Transmit Clock} 12 {2nd CD} 25 {Unassigned} 13 {2nd CTS} IBM PC-AT Serial cable DB25 to DB9 25 pin 9 pin connector serial adapter on computer (cable connector is female) Signal Direction DCE-ADAPTER Cable-Computer DB25 DB9 8 Carrier Detect 1 > 3 Received Data 2 > 2 Transmitted Data 3 < 20 Data Terminal Ready 4 < 7 Signal Ground 5 - 6 Data Set Ready 6 > 4 Request to send 7 < 5 Clear To Send 8 > 22 Ring Indicator 9 > All other pins on the 25 pin connector are unused. RS-232C INTERFACE STANDARD Data terminal equipment (DTE) is typically a computer. Data Communications equipment (DCE) is typically a modem. DTE | DTE Modem Data Terminal Equipment | Data Communications Equipment | Signal EIA / CCITT Pin Telephone Co. Signal Direct Line Number No. Lead Number Direction - Protective Ground 1 AA/101 - - Signal Ground 7 AB/102 - - Transmitted Data 2 BA/103 > < Received Data 3 BB/104 - - Request to Send 4 CA/105 > < Clear to Send 5 CB/106 - < Data Set Ready 6 CC/107 - - Data Terminal Ready 20 CD/108.2 > - Connect Data Set to line 20 * /108.1 > < Received Line Signal Detector 8 CF/109 - - Speed Select 23 CH/111 > < Transmit Signal Element Timing 15 DB/114 - < Receive Signal Element Timing 17 DD/115 - - Select Standby 11 * /116 > < Ring Indicator 22 DE/125 - - Test 18 * /* > * means non-defined/standardised by EIA / CCITT Switched Timing Sequence (dial-up line communication) ___________________________________________ Data Terminal Ready ___| ________________________________________ Data Set Ready ______| ______________________ Request To Send _________| |______________ _______________________ Clear To Send ____________| |__________ ________________ Transmitted Data _______________| |______________ Nonswitched Timing Sequence (direct line connection) ___________________________________________ Data Terminal Ready ___| ________________________________________ Data Terminal Ready ______| ______________________ Request To Send _________| |______________ _______________________ Clear To Send ____________| |__________ ____________ Transmitted Data _______________| |__________________