Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!rutgers!mcnc!ncsuvx!ncsuvm!netoprbh
From: NETOPRBH@ncsuvm.ncsu.edu (Brandon Hill)
Newsgroups: comp.sys.amiga.tech
Subject: Re: A Cost Effective MULTI-SERIAL Device Proposition
Message-ID: <90037.161429NETOPRBH@NCSUVM.BITNET>
Date: 6 Feb 90 21:14:29 GMT
References: <76649@tut.cis.ohio-state.edu>
Distribution: usa, world
Organization: North Carolina State University Computing Center
Lines: 38

I (along with some others around here) have contemplated the same idea.
However, to convert the game port to a serial port is not as trivial as
it may first sound.

First, the game port cannot alone drive a serial connection, as it is not
set up to sink or source the currents required for a serial connection.

Second, you can't directly receive RS-232 standard signals, because they
can be anywhere between -12 and +12 volts (way out of line with what you
can expect to apply directly to the game port)

Therefore, you must have something to transmit and receive signals.  I have
heard that the Maxim Max-232 can transmit and receive standard signals more
or less from a 5 volt source using an internal DC-DC converter to drive the
lines at near 12 volts.  If such a chip could service all the needed signals,
it could probably be stashed inside the hood of a db25 connector...which would
make the cable look like a simple db9 - db25 converter.

The hardware end, therefore, is almost trivial...or at least as near as can
be expected for such a project.  The software is not nearly as trivial.

The first problem is that there are no lines on the game ports that will
trigger an interrupt in the cpu.  This means that if you wish to receive data,
you will have to poll the receive line constantly.  You might be able to get
by with simply polling at the bit rate (1200 times per sec at 1200 baud),
but you would be much safer polling at twice the bit rate to take care of
any switching transients.  This will result in quite a bit of software
overhead which gets you nothing unless you are actually receiving or
transmitting data.  I am not even sure that the amiga is capable, without
external hardware, of interrupting exactly a certain number of times per
second. (At least, the master int disable would reek major havoc on a software
driven bit level protocol)

In short, you might be able to do it, but you would be better off hanging a
uart off of the parallel port to get an extra serial port.  And you would be
able to run at measurable speeds using a uart too :-)

                                                         BlH