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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. | |
+---------------------------------------------------------------------------+
See Also:
connectors
25 to 9 pin correspondence
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