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 DEFINITION OF THE CONNECTOR PINS

 Nearly all companies fail to implement the full standard.  Only
 a subset of the full 25 pins are functional.  This is what makes
 RS232C connections such a headache.  RS232C connections are
 unavoidable with modems, but NEVER use serial RS232C printers if
 you can possibly get parallel interfaces instead.

 In theory you can have 4 separate conversations going on
 simultaneously.  The primary and secondary send and primary and
 secondary receive.  However I have never seen the secondary used
 except in Mohawk equipment.


 +---------------------------------------------------------------------------+
 | Pin | Pin | Real|     Common Name      |       Comments                   |
 | (25 | (9  | Name|                      |                                  |
 | pin | pin |     |                      |                                  |
 |conn)|conn)|     |                      |                                  |
 ------+-----+-----+----------------------+----------------------------------|
 | 01  |     |  11 |    Protective Ground | Usually the shield.  Sometimes   |
 |     |     |     |                      | jumpered to signal ground.       |
 ------+-----+-----+----------------------+----------------------------------|
 | 02  | 03  |  BA | TD  Transmitted Data | Usually Marking -- Binary 1s     |
 |     |     |     |                      | while idling.  Data from         |
 |     |     |     |                      | terminal to modem binary 0 start |
 |     |     |     |                      | bit, then data for a character,  |
 |     |     |     |                      | then binary 1 stop bit then the  |
 |     |     |     |                      | sequence repeats for the next    |
 |     |     |     |                      | character.                       |
 ------+-----+-----+----------------------+----------------------------------|
 | 03  | 02  |  BB | RD  Received Data    | Usually Marking -- Binary 1s     |
 |     |     |     |                      | while idling.  Data from modem   |
 |     |     |     |                      | to terminal.                     |
 ------+-----+-----+----------------------+----------------------------------|
 | 04  | 07  |  CA | RTS Request to Send  | ON means terminal wants to send  |
 |     |     |     |                      | data.  The terminal will wait    |
 |     |     |     |                      | until the modem says it is ok by |
 |     |     |     |                      | raising CTS.                     |
 ------+-----+-----+----------------------+----------------------------------|
 | 05  | 08  |  CB | CTS Clear to Send    | ON means modem says it is ok for |
 |     |     |     |                      | the terminal to start sending.   |
 |     |     |     |                      | Most of the time CTS should be   |
 |     |     |     |                      | on.  Only in very old fashioned  |
 |     |     |     |                      | modems is there any reason not   |
 |     |     |     |                      | to send at any time.  With       |
 |     |     |     |                      | printers the RTS CTS pair can be |
 |     |     |     |                      | used for flow control.  The      |
 |     |     |     |                      | printer can pretend to be the    |
 |     |     |     |                      | modem.  When it is getting behind|
 |     |     |     |                      | printing it drops CTS, to stop   |
 |     |     |     |                      | the flow of incoming data.  But  |
 |     |     |     |                      | most printers are more lunatic   |
 |     |     |     |                      | and pretend to be the Terminal.  |
 |     |     |     |                      | Then they play silly games with  |
 |     |     |     |                      | DTR or RTS hoping the other end  |
 |     |     |     |                      | will understand these non-       |
 |     |     |     |                      | standard signallings for flow    |
 |     |     |     |                      | control or leave it up to you to |
 |     |     |     |                      | design a cable to make the       |
 |     |     |     |                      | printer look like a modem.       |
 ------+-----+-----+----------------------+----------------------------------|
 | 06  | 06  |  CC | DSR Data Set Ready   | ON means the modem is powered    |
 |     |     |     |                      | on.  In the olden days modems    |
 |     |     |     |                      | were called Data Sets.  Then IBM |
 |     |     |     |                      | started calling Files "Data      |
 |     |     |     |                      | Sets".  To avoid confusion we    |
 |     |     |     |                      | started calling Data Sets        |
 |     |     |     |                      | "modems".                        |
 ------+-----+-----+----------------------+----------------------------------|
 | 07  | 05  |  AB |     Signal Ground    | Common ground for all signal     |
 |     |     |     |                      | lines.                           |
 ------+-----+-----+----------------------+----------------------------------|
 | 08  | 01  |  CF | CD  Carrier Detect   | The modem is receiving whistles  |
 |     |     |     |                      | from some remote modem.  It      |
 |     |     |     |                      | turns this ON to let the         |
 |     |     |     |                      | terminal know about it.  We are  |
 |     |     |     |                      | connected.                       |
 ------+-----+-----+----------------------+----------------------------------|
 | 09  |     |     |     testing          | Often +12 volts but don't count  |
 |     |     |     |                      | on it.                           |
 ------+-----+-----+----------------------+----------------------------------|
 | 10  |     |     |     testing          | Often -12 volts but don't count  |
 |     |     |     |                      | on it.                           |
 ------+-----+-----+----------------------+----------------------------------|
 | 11  |     |     |     unused           |                                  |
 ------+-----+-----+----------------------+----------------------------------|
 | 12  |     |  SCF|     secondary        |                                  |
 |     |     |     |     Carrier Detect   |                                  |
 ------+-----+-----+----------------------+----------------------------------|
 | 13  |     |  SCB|     secondary        |                                  |
 |     |     |     |     Clear To Send    |                                  |
 ------+-----+-----+----------------------+----------------------------------|
 | 14  |     |  SBA|     secondary        |                                  |
 |     |     |     |     Transmitted Data |                                  |
 ------+-----+-----+----------------------+----------------------------------|
 | 15  |     |  DB | TC  Transmit Clock   | Supplied by MODEM DCE.  Square   |
 |     |     |     |                      | wave signal so terminal knows    |
 |     |     |     |                      | when to start sending the next   |
 |     |     |     |                      | bit of transmitted data.         |
 |     |     |     |                      | (not used in Async transmission) |
 ------+-----+-----+----------------------+----------------------------------|
 | 16  |     |  SBB|     secondary        |                                  |
 |     |     |     |     Received Data    |                                  |
 ------+-----+-----+----------------------+----------------------------------|
 | 17  |     |  DB | RC  Receive Clock    | Supplied by MODEM DCE.  Square   |
 |     |     |     |                      | wave signal so terminal knows    |
 |     |     |     |                      | when to sample each bit of       |
 |     |     |     |                      | received data.                   |
 |     |     |     |                      | (not used in Async transmission) |
 ------+-----+-----+----------------------+----------------------------------|
 | 18  |     |     |     unassigned       |                                  |
 ------+-----+-----+----------------------+----------------------------------|
 | 19  |     |  SCA|     secondary        |                                  |
 |     |     |     |     Request to Send  |                                  |
 ------+-----+-----+----------------------+----------------------------------|
 | 20  | 04  |  CD | DTR Data Terminal    | Terminal is powered on.          |
 |     |     |     |     Ready            | Terminal drops this signal to    |
 |     |     |     |                      | persuade the modem to hang up    |
 |     |     |     |                      | the phone.                       |
 ------+-----+-----+----------------------+----------------------------------|
 | 21  |     |  CG | SQ  Signal Quality   | The modem turns this on to tell  |
 |     |     |     |                      | the terminal it likes the        |
 |     |     |     |                      | quality of the carrier coming    |
 |     |     |     |                      | in.  It turns it off if it       |
 |     |     |     |                      | suspects errors due to static.   |
 |     |     |     |                      | Rarely supported.                |
 ------+-----+-----+----------------------+----------------------------------|
 | 22  | 09  |  CE | RI  Ring Indicator   | The modem turns this on each     |
 |     |     |     |                      | time an incoming call rings the  |
 |     |     |     |                      | phone.  The terminal would turn  |
 |     |     |     |                      | on DTR to tell the modem it is   |
 |     |     |     |                      | ok to answer.                    |
 ------+-----+-----+----------------------+----------------------------------|
 | 23  |     |  CH | I   Speed Indicator  | This is a two way wire.  For     |
 |     |     |     |                      | outgoing calls the terminal      |
 |     |     |     |                      | turns this ON to tell the modem  |
 |     |     |     |                      | to use higher speed e.g. 1200    |
 |     |     |     |                      | versus 300 baud.  For incoming   |
 |     |     |     |                      | calls the Modem tells the        |
 |     |     |     |                      | terminal data is coming at at    |
 |     |     |     |                      | higher speed by setting it ON.   |
 ------+-----+-----+----------------------+----------------------------------|
 | 24  |     |  DA | (TC) Transmit Clock  | For Async communications, it is  |
 |     |     |     |                      | the duty of the terminal to      |
 |     |     |     |                      | provide the clock.  For Sync     |
 |     |     |     |                      | communications it is usually the |
 |     |     |     |                      | duty of the modem.  In Async     |
 |     |     |     |                      | communications the modem doesn't |
 |     |     |     |                      | care about the clock.  It        |
 |     |     |     |                      | doesn't need one.  So this pin   |
 |     |     |     |                      | is only useful in the rare       |
 |     |     |     |                      | situation the terminal is        |
 |     |     |     |                      | providing the clock to the modem |
 |     |     |     |                      | in synchronous situations --     |
 |     |     |     |                      | e.g. in back to back X.25        |
 |     |     |     |                      | between two computers.           |
 ------+-----+-----+----------------------+----------------------------------|
 | 25  |     |     |     unassigned.      |                                  |
 +---------------------------------------------------------------------------+