Re: Apple IIc+ modem connector

[rg.jones@rogers.com]

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.


Glenn