Date: Thu, 13 Feb 1997 16:11:56 -0600 (CST)
   From: Ty Sarna <tsarna@endicor.com>
   Newsgroups: netbsd.tech.net
   Organization: Endicor Technologies, Inc., San Antonio, Texas
   Sender: tech-net-owner@NetBSD.ORG
   Precedence: list
   Delivered-To: tech-net@NetBSD.ORG

   In article <4315.855866721@ux2.sp.CS.CMU.EDU> you write:
   > Why is a line discipline not sufficient?  why do you need special
   > interrupt handlers, etc?  I understand that localtalk runs at a high
   > speed (though i forget what exactly the speed is 8-), and therefore
   > the port needs to be configured specially, but i'd think that the line
   > discipline could do the appropriate stuff without relying on special
   > handlers to receive and transmit data.

   LLAP has a peculiar on-the-wire format.  I'm not sure it can even be
   implemented on arbitrary serial chips.  On some chips it may require
   some expensive operations in software (like inserting a zero bit after
   every 5 one bits, and removing a zero bit after five ones on reception). 

Sounds like synchronours HDLC to me, at the lowest level. If this is
true, you need special chips to do it... or maybe additional hardware
tranforming async HDLC to sync HDLC. In the latter case, a line
discipline to talk to these beasts would be sufficient. In the former
case you need special hardware (e.g., Zilog 8530, which is used in
Sparcstations) as cheap serial interfaces don't have any HDLC mode,
and enhanced drivers which accept some ioctl to set the hardware into
HDLC mode.

No, a line discipline won't be sufficient in the general
case. However, if we define a set of ioctls to put the driver into
HDLC mode, a line discipline should be enough to transfer the data
between any of these HDLC drivers and the Appletalk protocol.

	-is