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From: neff@hp-vcd.HP.COM (Dave Neff)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Deskwriter: serial vs PhoneNet
Message-ID: <1170010@hp-vcd.HP.COM>
Date: 28 Jan 91 19:51:35 GMT
References: <1991Jan25.102811.4724@usenet@scion.CS.ORST.EDU>
Organization: Hewlett Packard, Vancouver, WA
Lines: 54


>Here is my question:  given a 32K file, which would give the quickest
>output:
>A Deskwriter connected to a lone mac using the serial interface or
>B Deskwriter connected to a lone (just one) mac using PhoneNet.


The DeskWriter typically has virtually identical performance both on
LocalTalk (assuming the LAN is not real busy) and on 57K baud serial.
The different input buffer size between LocalTalk and serial is not
generally a performance concern, neither is the different effective
IO transfer rates due to the fact that the speed of the DeskWriter
is not IO limitted, rather it is either limitted by the speed of the
Mac (on a Plus, SE, or Classic), or the actual speed the printer can
fire dots.  The DeskWriter makes heavy use of data compression techniques
to eliminate the IO from being the bottleneck in either IO mode.

There is one significant exception:  Data that does not compress well
(full page scanned images are a notable case) can be IO limitted, and
in that case LocalTalk can be significantly faster than serial.
If you do print these kinds of images, the speed improvement might
be worth the cost of the PhoneNet conectors.  

One final note to confuse things further:  Activating AppleTalk on
the Mac will eat up some system RAM, that can then slow down the
DeskWriter driver if you have only a 1 meg system.  If you have 2 or
more meg this is not a consideration.  One RAM related speed up
trick for systems with 2 or more meg is to be sure 64K or more of RAM cache
is enabled in the control pannel.  This can speed up the DeskWriter
driver (along with other things).

So in conclusion, if you have at least 2 megs of RAM, a fast Mac 
(16 megahertz CPU or more), enable some RAM cache, and compare 57K
baud serial with 230K baud LocalTalk (and an idle LAN), performance will
generally be virtually identical but in some cases LocalTalk can be faster.

The 8K vs. 16K buffer in the two modes is not really a performance
consideration at all.  Basically, input buffer size is mainly an issue
of how fast you get your Mac "back" if not using a spooler.  With
a character based printer, 16K can be several pages of a document.
But on a graphics based printer (like the DeskWriter), the difference
between 8K and 16K of input buffer is just a couple of seconds of
print time, so in serial mode your Mac might be ready to use again
a second or two before in LocalTalk mode, but the actual time for
the printer to finish printing will not be affected by input buffer
size.

I hope this helps.  I designed the DeskWriter LocalTalk and serial
IO processor firmware, wrote a large amount of the firmware, and
characterized the performance in both LocalTalk and serial modes,
so I think you can believe my answer :-).

Dave Neff
neff@hpvcfs1.HP.COM