Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!ntg!dplatt
From: dplatt@ntg.com (Dave Platt)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Help needed with HP-Deskwriter & Localtalk
Message-ID: <961@goblin.ntg.com>
Date: 6 Jun 91 18:01:26 GMT
References: <3592@opal.cs.tu-berlin.de>
Distribution: comp
Organization: New Technologies Group, Inc.  Palo Alto CA
Lines: 39

In article <3592@opal.cs.tu-berlin.de> aes19@opal.cs.tu-berlin.de ( aes19) writes:

>Can anyone tell me how to connect an Deskwriter to appletalk ? Because
>this printer is the only device on the net i won't buy the appletalk-
>kits. I need this only for work with System 7 and virtual memory on an
>SE/30. The serial driver for the Deskjet won't work work in this confi-
>guration ( and i've heared that the appletalk driver does ). 

My understanding is that you _will_ need some form of LocalTalk kit
(either Apple's, or Farallon's PhoneNet, or something equivalent).
Someone I spoke with recently tried to set up a "minimal" LocalTalk
network between his Mac and DeskWriter, for the same reason you've
mentioned, and was unable to get it to work.

The reason is this: the DeskWriter (recent ones, at least) are designed
to detect whether they are connected to a LocalTalk network, or directly
to a Mac via a serial null-modem cable.  They apparently do this by
sensing certain electrical characteristics of the RS-422/423 interface.
I infer that they're measuring the impedence of the circuit they are
driving... a direct serial connection will appear to have a fairly high
impedence, while a LocalTalk connector's transformer will have a rather
low DC impedence.

If you connect a DeskWriter directly to your Mac, it will insist on
going into direct-connect mode.  If you tell your Mac to turn on
AppleTalk, and go into the Chooser, the DeskWriter driver will send out
NBP lookup packets... but the DeskWriter itself will ignore them.

There might be some way to "spoof" the DeskWriter's connection-sensor...
but as I don't have detailed information about how the sensing is
actually done, I can't recommend a specific spoof.

I'd suggest that you buy two inexpensive PhoneNet-clone connectors.


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Dave Platt                                                VOICE: (415) 813-8917
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