Re: Apple IIc+ modem connector

[Eric Smith <eric@brouhaha.com>]

Michael J. Mahon wrote:
> That sounds right.  You should be able to treat the connector as
> a "5v RS-232" port.

Nope.  It's a TTL-level asynchronous serial port.  RS-232 is not involved.

Note that 5V is a perfectly valid RS-232 level.  But 0.0 to 0.8V (TTL
low) and 2.0-3.0V (TTL high) are not.  Aside from which, a normal TTL
(or NMOS) logic output doesn't meet other RS-232 requirements, including
short circuit protection.

However, it is possible to drive an RS-232 signal from a 5V CMOS output
with a series resistor, if you don't actually need to meet the RS-232
(or more recently, TIA/EIA-232) specifications.  This works because
most line receiver chips have their threshold somewhere between
+1V and +2V.