Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!hp-pcd!hpvcfs1!neff
From: neff@hpvcfs1.HP.COM (Dave Neff)
Newsgroups: comp.sys.mac.misc
Subject: Re: Deskjet printers
Message-ID: <32730002@hpvcfs1.HP.COM>
Date: 1 Jun 90 17:08:33 GMT
References: <1990May22.221510.25732@csuchico.edu>
Organization: Hewlett Packard, Vancouver, WA
Lines: 46

>So, we use our HPDJ driver with ATM and we are quite happy with it.
>Sometimes we hope that HP would have used something more powerful than
>a Z80 as the CPU of this magnificent printer.

Having co-written the DeskJet Epson emulation cartridge firmware,
written some of the DeskJet+ printer firmware and much of the
DeskWriter AppleTalk firmware I am acutely aware of the limits of the
Z80 (in a DeskJet) and the Z180 (in the DeskJet+ and DeskWriter).

However, an 8 megahertz Z180 is totally adequate for a graphics only
printer like the DeskWriter and does fine for a bitmap font and graphics
printer like the DeskJet+.  The DeskWriter is hardly ever limited by
the Z180, rather it is limited by the Mac imaging or the physical speed
that the mechanism can move paper and move the print head over the
paper.

If you are using the HPDJ driver runing a DeskJet+ at 19.2 K baud
you are limitted by the speed of the IO port (although the classic DeskJet
could still be faster in this case, explained below).  Not even a 25 megahertz
68000 would make the DeskJet+ any faster in this case.  Over the
parallel port, the DeskJet+ can process graphics data as fast as most
hosts can send it and can keep the printer "mechanism limitted". At
19.2 K baud the DeskJet+ is IO limited.

I would much rather write firmware for the 68000, but really the
performance limitations of the DeskJet+ and DeskWriter printer are
not generally due to the fact that they contain a Z180.

Now if we are talking about the classic DeskJet, it only has a 4 megahertz
Z80 and it was optimized for text not graphics.  I can make a DeskJet 
classic print graphics 3 times faster if I use the input buffer for
graphics rather than buffering up data and make a couple other modificatoins
to the way incoming data is handled (or if I had a bit more RAM).
In other words, even for the DeskJet classic the Z80 isn't really
the problem (although doubling the clock speed would help), its graphics
performance is partly limited by insufficient RAM and a particular
firmware architecture emphasizing text over graphics.  The DeskJet+
modified the architecture to significantly improve graphics performance
and the Z180 was quite adequate for the task.  The DeskWriter architecture
was further tuned to be a graphics only printer and the Z180 is fine for
this task as well.

I'm glad you think its a "magnificant printer". 

Dave Neff
neff@hpvcfs1.HP.COM