rg.jones@rogers.com wrote:
Michael J. Mahon wrote: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).This is where I was getting confused. Even though it is a "modem" port ... wouldn't +5 have come in handy to drive that circuitry? Since the manual stated pins 2 - 6 were TTL levels I assumed I could directly attach a UART from a microcontroller to the TxD/RxD pins without the need of a device like the MAX232 in between the microcontroller and the modem port. Is that a correct assumption? When looking at the schematic it shows the signals being pulled out to the header after the 6551 and before the passthrough into another chip (sorry I don't have the schematic on hand right now so i can't say which chip it is - level translator?) before being run out to the mini din connector.
That sounds right. You should be able to treat the connector as a "5v RS-232" port. -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."