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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.