henry@utzoo.UUCP (Henry Spencer) (07/24/84)
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!henryhenry@utzoo.UUCP (Henry Spencer) (07/26/84)
Predictably, the "arc" instruction on the 7470A has the same problem
as the "circle" instruction. (For those of you who didn't read the
earlier article, it puts the plotter into relative-coordinates mode
even if it was in absolute-coordinates mode to start with.)
--
Henry Spencer @ U of Toronto Zoology
{allegra,ihnp4,linus,decvax}!utzoo!henry