9. User EnhancementsSerial port pin assignments

The previous section presupposes knowledge of the correspondence between terminal signals and serial port pins. Here's a reference for that correspondence, taken from David Tal's <GSRGAAO@TECHNION.BITNET> ``Frequently Used Cables and Connectors'' document. I'm including a diagram illustrating the connectors, and a table listing the correspondence between pin numbers and terminal line signals.

DB-25 DB-9 Name EIA CCITT DTE-DCE Description
Pin # Pin #

---

1 FG AA 101 --- Frame Ground/Chassis GND
2 3 TD BA 103 ---> Transmitted Data, TxD
3 2 RD BB 104 <--- Received Data, RxD
4 7 RTS CA 105 ---> Request To Send
5 8 CTS CB 106 <--- Clear To Send
6 6 DSR CC 107 <--- Data Set Ready
7 5 SG AB 102 ---- Signal Ground, GND
8 1 DCD CF 109 <--- Data Carrier Detect
9 -- -- - - Positive DC test voltage
10 -- -- - - Negative DC test voltage
11 QM -- - <--- Equalizer mode
12 SDCD SCF 122 <--- Secondary Data Carrier Detect
13 SCTS SCB 121 <--- Secondary Clear To Send
14 STD SBA 118 ---> Secondary Transmitted Data
15 TC DB 114 <--- Transmitter (signal) Clock
16 SRD SBB 119 <--- Secondary Receiver Clock
17 RC DD 115 ---> Receiver (signal) Clock
18 DCR -- - <--- Divided Clock Receiver
19 SRTS SCA 120 ---> Secondary Request To Send
20 4 DTR CD 108.2 ---> Data Terminal Ready
21 SQ CG 110 <--- Signal Quality Detect
22 9 RI CE 125 <--- Ring Indicator
23 -- CH 111 ---> Data rate selector
24 -- CI 112 <--- Data rate selector
25 TC DA 113 <--- Transmitted Clock

Pin Assignment for the Serial Port (RS-232C), 25-pin and 9-pin

        1                         13         1         5
      _______________________________      _______________
      \  . . . . . . . . . . . . .  /      \  . . . . .  /    RS232-connectors
       \  . . . . . . . . . . . .  /        \  . . . .  /     seen from outside
        ---------------------------          -----------      of computer.
        14                      25            6       9

   DTE : Data Terminal Equipment (i.e. computer)
   DCE : Data Communications Equipment (i.e. modem)
   RxD : Data received; 1 is transmitted "low", 0 as "high"
   TxD : Data sent; 1 is transmitted "low", 0 as "high"
   DTR : DTE announces that it is powered up and ready to communicate
   DSR : DCE announces that it is ready to communicate; low=modem hangup
   RTS : DTE asks DCE for permission to send data
   CTS : DCE agrees on RTS
   RI  : DCE signals the DTE that an establishment of a connection is attempted
   DCD : DCE announces that a connection is established