[plug] Macs and Linux (OLD macs)

[Harry McNally]

On Mon, 29 Jul 2002 13:01:29 +0800 Brad Campbell <brad at seme.com.au> wrote:

> Harry McNally wrote:
> > 
> > On Fri, 26 Jul 2002 16:15:30 +0800 Brad Campbell <brad at seme.com.au> wrote:
> > 
> > > I am the LinuxAlien wrote:
> > > >
> > > > >
> > > > >Hell, I had it running on a IIsi with 4 meg of ram.. bit slow in X
> > > > >though:p) Even bodged up a serial null modem cable to my laptop to give
> > > > >it a network, though ppp at 19,200 baud is a bit painful.
> > 
> > Could this be tweeked to a faster baud ?
> 
> It may do 38400 but you must remember the MAC Serial hardware has no FIFO and
> with a slow processor, the kernel has a hard time keeping up with the data.

Got you. No FIFO is difficult.

>  
> > > > how did you get the serial cable connected, what port?
> > > > Tim
> > > Printer or modem port.. Just used some nice fine wire and poked it in
> > > there. Pinouts are available on the net. The mac uses a varian of rs-422,
> > > so you connect the Data + line to ground and connect the Data - line to
> > > the Rs-232 pin for both tx and rx.
> > 
> > Oooo. That makes the engineering brain cells cringe. Methinks you may be
> > hurting the RS422 driver that way. Actually no. If the parts have
> > overcurrent protection it probably hangs in there. Still not nice if the
> > equipment shares a common ground because RS232 won't get tidy negative-going
> > swings.
> 
> Correct, no negative swings. Practical experience has shown me that most (if
> not all)"Modern" rs232 chips will read under about 2 volts as a 0 and not
> require a negative.

Yup. The standard is -15 to -3 and +3 to +15 for the mark/space levels. This
normally means that the signal has a 6 volt hysteresis but modern parts seem to
bias the input to prevent very noisy floating inputs from generating crazy
output chatter. That might explain why your setup can detect the 422 input.

> RS232 (EIA-232) drivers are supposed to be limited to 8ma source, and 422(485)
> drivers can sink/source 60ma and cope with many volts common mode, so 8-10v
> from a standard pc rs252 is not going to do any damage.

Ok. I thought the 485 common mode was max about 7 volts but RS232 drivers don't
send out +/-12v as they used to in the " << isSentimental() ? "good" : "bad" <<
" old days so that's probably why it goes. Be interesting to stick it on a scope
and see what's what tho'. 

All the best Brad
Harry

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