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 We recently received a Houston Instrument DMP-6 8-pen plotter (purchase
 price about $2300).   This is  one of 4  similar inexpensive  multi-pen
 models made  by  Houston.  According  to  the Hiplot  Digital  Plotters
 Operator's Instructions manual, the  Houston Instruments DMP-L  plotter
 models have the following characteristics:

 DMP-3, DMP-4:
 plot surface 10  in horizontally x  7 in vertically  (25.40 cm x  17.78
 cm); paper size 11 in x 8.5 in; 6 pens; 200 steps/inch.

 DMP-6, DMP-7:
 plot surface 15 in  horizontally x 10 in  vertically (38.10 cm x  25.40
 cm); paper size 17 in x 11 in; 8 pens; 200 steps/inch.

 Experimentation with the DMP-6 revealed that the plotter simply ignores
 vectors with out-of-range coordinates, and that the x and y  coordinate
 ranges are 0..2850 and 0..1970,  corresponding to a display surface  of
 14.25 in horizontally by  9.85 in vertically (36.195  cm x 25.019  cm).
 Plotting speed for horizontal and vertical lines is about 2.4  inch/sec
 (by stopwatch).  For comparison, the larger CalComp pen plotters in the
 $20K range  are  2  to 4.5  inch/sec,  while  a 10  inch/sec  model  is
 available for about $25K, and a 30 inch/sec model for about $43K.   The
 string generator  displays 94  printable  ASCII characters;  caret  and
 underscore are not available.

 The DMP-L plotters support X-on/X-off  transmission protocol, and on  a
 transmission delay raise the pen to prevent ink bleeding, and lower  it
 when data is again available.   The X-on/X-off protocol is  exceedingly
 useful, because it makes  it possible for the  plotter to throttle  its
 input to prevent buffer overflow, completely eliminating the  necessity
 on the host of elaborate padding and time-out mechanisms to avoid  loss
 of data through buffer overflow.

 Multiple pen-up moves are actually executed, even though only the  last
 is actually  required.   On  a  pen  change  request,  the  plotter  is
 intelligent enough to  raise the pen,  change it, and  lower it  again;
 unfortunately, it seems to  lose one or a  few increments in doing  so.
 It is therefore desirable to retransmit the absolute coordinates  after
 a pen change.  The  movement for the  pen change is  not optimal --  it
 does a horizontal move to  the pen bank, then  a vertical move to  find
 the desired pen.  A  diagonal move would be  faster.  In practice,  pen
 changes are almost invariable followed by move commands;  nevertheless,
 the DMP-L dutifully restores the pen to its last position, even  though
 this is probably wasted motion.

 The manual  was  not  clear  about where  spaces  are  required  around
 commands; experimentation showed that A and  R do not need a  preceding
 blank if  they  have  a  preceding letter  command,  but  other  letter
 commands appear to always require a preceding blank or comma.

 The plot paper is held down by a vacuum system, which necessitates  the
 presence of small holes on the plot surface.  This has occasionally led
 to puncturing of the  paper when repeated raising  and lowering of  the
 pen occurred over a hole, as in the drawing of Hershey characters.   An
 electrostatic hold-down would be preferable, and interestingly  enough,
 is used in the Houston  Instrument Hiplot dumb plotter model  retailing
 for under $1100.

 The command  sequences  are  simple  - U/D  for  pen  up/down,  Pn  for
 selecting pen  n,  A/R  for  absolute/relative  interpretation  of  all
 following coordinates,  Ln  for  line  style  n,  S  for  text  strings
 (available in 4 sizes and  orientations), and M for markers  (available
 in five sizes and shapes).  Coordinates are simply represented as ASCII
 strings.  Other letters, and control characters such as ASCII CR LF are
 simply ignored.  A sample command sequence  to draw a box with  colored
 edges might be

 P1 UA100,100 D250,100 P2 250,500 P3 100,500 P4 100,100

 Experiments have shown  that making absolute  moves and relative  draws
 reduces the plot file byte count to  about 2/3 of what it is with  pure
 absolute data.  Purely relative  commands are undesirable because  loss
 of an increment or coordinate point would destroy the remainder of  the
 plot.  We therefore issue  an "A" (absolute)  command before the  move,
 and an "R" (relative)  command after the move;  the relative mode  will
 remain in effect for all subsequent draws.