Re: Apple IIc+ modem connector

[et472@FreeNet.Carleton.CA (Michael Black)]

"Michael J. Mahon" (mjmahon@aol.com) writes:
> rg.jones@rogers.com wrote:
>> Hi,
>> 
>> bieling@terra.es wrote:
>> 
>>>Yes, that's rare.
>>>But if the reason why you asked is that you're thinking about some sort
>>>of serial port-based gadget, 5v can be easily obtained from the
>>>*external* serial connectors as long as the current needs are not too
>>>high.
>> 
>> 
>> Was looking at doing something inside the IIc+ - I searched and found I
>> can use http://www.maxim-ic.com/appnotes.cfm/appnote_number/3534 to
>> convert -5v to +5v or leech it off something else close by to get +5v.
> 
> Or you could treat -5v like ground and ground like +5v.  This would
> no doubt mean level-shifting the Tx/Rx lines, but otherwise should
> work fine.
> 
My first thought was that that was what they were doing, having decided
that the -5V line doesn't really see much useage so they might
as well drain it for the modem than the plus lines that are used
a lot.  I think even in the back of my mind I was thinking that
one of the IC modems did run off a negative supply, but the remembrance
is way too vague to know if there is reality there.

But I'd not thought of the level shifting, and that seems undue
complication for the sake of making use of the -5v line.

There has to be  a good reason why the designers went that route,
and it would be nice to know what it was.  The schematic of the modem
that's supposed to plug in there would explain what they used, and
from there might explain why they used -5v.

  Michael