Re: Appletalk/Phonenet box wiring (Re: ...phoneNet connection to Win2K?)

[dempson@actrix.gen.nz (David Empson)]

Matthew Russotto <russotto@wanda.pond.com> wrote:

> In article <3b507efb.217582471@news.mw.centurytel.net>,
> Mister Boffo <mister_boffo@hotmail.com> wrote:
> >Honestly, I don't think there is anything at all different between the
> >serial and modem ports on a Macintosh if this is all there is to it.
> >Appletalk appears to involve no additional hardware beyond the normal
> >serial port, and AppleTalk may be a completely software-based network.
> 
> Appletalk requires RS-422 drivers and a serial controller capable of SDLC
> communication at 230Kbps (I think using NRZi encoding, but it's been
> way too long since I traced this).

It is FM encoding, but I forget whether it is FM0 or FM1.

The RS-422 transmit driver chip must have an enable input (controlled by
the RTS signal from the Zilog SCC on the Mac and IIgs), so that the
transmitter can be tri-stated when the computer isn't sending data.

(The transmitter is permanently enabled when the port is being used for
normal asynchronous communication.)

There are also some specific requirements of the LocalTalk protocol
which impose further restrictions on the SDLC implementation (to do with
the synchronization mechanism, mainly).  A Zilog SCC or compatible
device is OK, but some other devices are not, without additional
hardware.

> A simple asynchronous RS-232 port won't work.  But you're right -- there is no
> hardware difference between the serial and modem ports on the Macintosh.

That's "printer" and modem ports.

> >Apple always claimed the printer port was for AppleTalk, but I don't
> >see any objective reason why it matters which port get used. Perhaps
> >Apple used more expensive serial hardware for the printer port, so
> >that it could signal much faster than the modem port? Anybody know
> >what the max speed is of the modem port vs printer port?
> 
> For the Lisa, only one of the ports was RS-422, so you had to use that one.
> For the Mac, the difference had to do with the floppy disk driver.  It
> disabled interrupts, which could cause lost characters.  This didn't
> matter much for AppleTalk, which is expecting that sort of thing.  But
> it causes problems with modems.  So the floppy driver polls the modem
> port periodically while interrupts are disabled.  The printer port is
> left to its own devices.

The early AppleTalk implementation (prior to Open Transport) probably
also disables interrupts for the duration of a packet being sent or
received (as does the IIgs), so floppy activity and AppleTalk will be
mutually exclusive.

On later Mac models, Open Transport can presumably use DMA for
LocalTalk, so there should be such a performance it.