Printing to AT / AT Coprocessing Card?

[William Roberts]

In article <4170 at uhccux.uhcc.hawaii.edu> david at uhccux.uhcc.hawaii.edu (David Lassner) writes:
>Page 10-1 of the A/UX 1.1 Network System Administration reference
>says "Because of the high processing requirements of both AppleTalk
>and the A/UX multitasking OS, AppleTalk under A/UX requires a coprocessing
>card. ...
>Is this for real?  Do we really need a card to print to an AppleTalked
>LaserWriter

Yes, it's for real. The problems is to do with interrupt service times
and the very feeble hardware provided for the LocalTalk
interface. Basically that thing has a three byte buffer inside
it, so you have to pull those bytes out very, very quickly if
you don't want to lose packets. I did hear something about AT&T
requiring a minimum interrupt service time from anything to be
called "UNIX" and it being very hard to meet this with the Mac
II hardware. They could use a newer version of the chip (bigger
buffer), but the current trend at Apple seems to be using
add-in hardware for comms. In particular, they are keen on
co-processor cards, i.e. where you make the card do some of the
work and so off-load processing effort from the Mac II.

> and if so, who makes them?

The manual entry you quoted says it all:

> Contact your Apple representative for a list of cards that are
>available from various vendors."

I admit it is rather bizarre (it can talk LocalTalk when it's a
MacOS machine, what happened under A/UX?) but there does seem
to be a reason for it. At least your Apple representative is
likely to tell you ALL of the vendors who make such cards :-)
-- 

William Roberts         ARPA: liam at cs.qmc.ac.uk
Queen Mary College      UUCP: liam at qmc-cs.UUCP    AppleLink: UK0087
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