gnome-ppp and the mac serial ports

gnome-ppp and the mac serial ports

[Anthony Lester]

I've finally got gnome-ppp working after commenting out a section of the source code which
checks the DCD (Data Carrier Detect) line of the serial port. The kernel code
corresponding to this check is in the file "macserial.c" and sets various bits of a status
byte according to the value of a register in the zilog uart which handles the standard mac
serial ports.

However the standard RS422 mac ports only have handsake-in and handshake-out lines
(corresponding to RTS and DTR for the RS232 I think) so there is no line corresponding to
DCD. So how would it get DCD?

The most surprising thing is that the kernel source contains explicit treatment for DCD,
so unless I am very much mistaken (or I have a dodgy modem cable) there is something
strange.

Any comments, what should be modified, applications, kernel, or both?

Anthony

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Re: gnome-ppp and the mac serial ports

[Michel Lanners]

On  17 Aug, this message from Anthony Lester echoed through cyberspace:
> 
> I've finally got gnome-ppp working after commenting out a section of the source code which
> checks the DCD (Data Carrier Detect) line of the serial port. The kernel code
> corresponding to this check is in the file "macserial.c" and sets various bits of a status
> byte according to the value of a register in the zilog uart which handles the standard mac
> serial ports.
> 
> However the standard RS422 mac ports only have handsake-in and handshake-out lines
> (corresponding to RTS and DTR for the RS232 I think) so there is no line corresponding to
> DCD. So how would it get DCD?

IIRC, DCD is wired to the GPi pin (is that the right name?). That's
some kind of multipurpose pin, that can be used for various things like
sync clocking, remote power-on, etc.... Have a look at your hardware
with a tool like TattleTech; the section about serial ports should show
if/whether DCD of the SCC is wired to the GPi pin.

You might want to search the TIL (http://www.apple.com/til) about
articles about the subject.

Michel

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Re: gnome-ppp and the mac serial ports

[Anthony Lester]

Thanks for the reply, but it doesn't clear up the affair. The serial interface is
documented in Inside Macintosh which explicitly states that DCD is not used. I've even got
a copy of the old Inside Macintosh volume 4, which has a schematic of the serail hardware,
DCD simply does not exist.

As for the general purpose pins, I had a quick look at Zilogs data sheets for the 8530.
There are no pins dedicated to general purpose use but some of the control pins (e.g. DCD)
can be used as such. Therefore it is possible that the Mac uses the DCD pins for something
else but it is more likely that it is simply hardwired to the supply rails.

This would explain what I see on stepping through gnome-ppp when it interrogates the
serial port, simply that DCD stays off whatever the status of the modem. This brings me
back to the original question. Should applications relying on DCD be modified so that they
do not rely on the value of this control line, or should the kernel be modified so that it
sends something other than always off.

Anthony

P.S. I downloaded TattleTech. Quite a useful tool, but it tells me only a little more than
Apple System Profiler, I couldn't see anything near as detailed as hardware connections.

Michel Lanners a écrit:
> 
> IIRC, DCD is wired to the GPi pin (is that the right name?). That's
> some kind of multipurpose pin, that can be used for various things like
> sync clocking, remote power-on, etc.... Have a look at your hardware
> with a tool like TattleTech; the section about serial ports should show
> if/whether DCD of the SCC is wired to the GPi pin.
> 
> You might want to search the TIL (http://www.apple.com/til) about
> articles about the subject.
> 
> Michel
> 


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