David Wilson wrote:
Michael J. Mahon wrote:Modems require both polarity supplies to do RS-232 signalling. I'm sure you'll find +5v on the connector, too.No, according to the Tech Ref Manual (page 406) the signals on that connector are: 1 -5V 2 RxD 3 TxD 4 DCD 5 DTR 6 DSR 7 GND Note: All signals except -5V and GND are TTL levels. Even the circuit diagram (sheet 7) lists it as -5V (generated on sheet 2 from the -12V supply via a 200 ohm resistor and pin 7 of the VID HYBRID circuit).
Oops--clearly this is *already* a serial port, and the modem has only to drive to/receive from a phone line. For that purpose, any supply polarity is fine (though the choice of -5v is curious). -michael Music synthesis for 8-bit Apple II's! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it is seriously underused."