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Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.periphs
Subject: undocumented `feature' in HP7470A plotter
Message-ID: <4135@utzoo.UUCP>
Date: Tue, 24-Jul-84 16:36:37 EDT
Article-I.D.: utzoo.4135
Posted: Tue Jul 24 16:36:37 1984
Date-Received: Tue, 24-Jul-84 16:36:37 EDT
Organization: U of Toronto Zoology
Lines: 18

Folks with HP7470A plotters (and perhaps other similar HP plotters),
beware.  I just ran down what looked like a software bug, only to
discover that it results from an undocumented hardware `feature'.
The circle instruction (CI) draws the circle just like it should,
but it *also* puts the plotter into "relative plotting" mode, where
X,Y pairs are increments rather than absolute positions.  This is
lots of fun if you're running the thing in absolute-plotting mode
and you trust the circle instruction to leave it that way.

The fix is easy enough -- just bang the thing back into absolute mode
after drawing a circle -- but you have to know it's necessary!

I don't know, actually, whether this is a firmware bug or a documentation
bug.  My 7470A manual is dated May 1983 (on the back cover); anybody
have a more recent one?  Any of the HP folks care to comment?
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry