Houston Instrument HiPlot DMP-29 Plotter. (evaluation) David H. Ahl.

In most cases, the lead-in to a review leaps to assert itself. In some cases the product has one or more outstanding characteristics that beg to be mentioned; in others, the company is strange or unusual; in still others, the reviewer has some biases that just must be put on the table.

In the case of this review, all are operative. Here are three possible lead-ins:

* Houston Instrument is an inwardlooking company. They have spent nary a dime on advertising with Creative Computing even though plotters are high on the list of peripherals that many readers wish to purchase. They rarely send out a press release; and when they do, it is certainly not to us. We always make a point to stop by their booth at NCC, but we are usually made to feel like dolts by the engieners at the booth who speak an obsure dialect of English called Plotter-Speak.

* For many years, Houston Instrument has been making a line of plotters know as HiPlot. They have gone their merry way, designing one innovation after another into their plotters, until today, the DMP-29, little-known and little-heralded, is one of the most advanced, user-friendly, and cost-effective plotters on the market.

* In the early 70's, I worked for Digital Equipment Corporation as manager of the Educational Products Group. To help users of computers in educational institutions, I wrote a series of booklets called Basic Applications Programs--(Subject). One was on plotting. It described how to use two plotters, the TSP and Fasplot, with PDP-8 Basic.

Among other things, I wrote a simulator in Basic which would make the plotter execute the turtle graphics portion of a virtually unknown (at the time) language called Logo. When Logo became more widely known, I asked some of my friends at DEC about my simulation routines. Alas, they had gone the way of other blue sky projects and had long since been forgotten and tossed out.

It was just as well. I was up to my eyeballs in other things and could ill afford to get hooked on plotters again as I once had been. Nevertheless, I still have a fond place in my psyche for plotter graphics. I am appalled at what many people do with plotters. I feel that plotters represent a vast, untapped resource for artistic expression, and should be the computer peripheral of choice after a disk drive. A Friendly Package

The Houston Instrument DMP-29 plotter is a flat bed plotter; this means that the sheet of paper lies on a flat surface and the pens move in two directions to trace a plot upon this surface. In contrast, on a drum plotter, the paper is wrapped around a drum and the pens move across the paper while the drum rotates to produce a two-dimensional plot.

On the DMP-29, the bed accommodates a piece of paper measuring 11" X 17". It is possible, in either hardware or software, to select a half-size plot measuring 8-1/2" X 11". On a full-size plot, the plotting area is 10" X 15" and on a half-size plot, 7" X 10". The paper is held in position by a lip on the plotter bed at the bottom and by permanent strips of adhesive tape at the top.

All the manual plotter controls are on a touch sensitive front panel similar to the control panel on a microwave oven or an Atari 400 computer. There are 14 controls arranged in seven groups.

This may sound like a complicated set of controls, but it is really quite straightforward during normal operation. The first group of controls contains the power on/off switch, reset (clears memory and resets the plotter), load (pen moves to a corner to facilitate paper loading), and chart size ntoggles back and forth between a small and large chart). For normal plotting, these are the only controls that will ever be used.

"Select enable" lets the plotter respond to computer input. When the plotter is powered up, this is on. LED indicators show the status of this and the remaining controls, as well as five other aspects of the plotter.

Two buttons select either remote or local operation; on power-up, remote operation is the default mode.

The remaining controls are used for local mode and digitizer mode. Two buttons raise and lower the pen, while a nine-button keypad is used to move it manually about the plotting surface.

On the rear of the plotter are two RS-232 connectors, a male marked "Modem," a female marked "Terminal," and a dip switch with eight switches to select communications characteristics. Connecting to a Computer

The instructions with the plotter suggest that "any standard DB 25-pin flat ribbon cable can be used to connect the serial RS-232C interface to a computer." Maybe so, but this approach did not work on any of the four computers we checked out with the DMP-29. In desperation, we tried a "reverse connection" cable and it worked fine.

The computer must be connected to the modem port (male) of the plotter. Although the pin-outs have the same description as those on the computer (Pin 2 is transmit data, Pin 3 is receive data, Pin 4 is request to send, Pin 5 is clear to send and so on), the lines must be reversed. In other words, Pin 2 goes to Pin 3 and 3 to 2, Pin 4 goes to 5 and 5 to 4, and Pin 6 goes to 20 and 20 to 6. Pin 1 (protective ground) and Pin 7 (signal ground) are tied together. We made up a cable to provide these exchanges, but such cables are available from suppliers such as Wireworks Corp. (DC9-2, 9-cond male/male, #810937).

With this reversed cable, we were able to run the plotter from a TRS-80 Model III, TRS-80 Model 100, NEC 8201, and Apple II. Presumably, it will work with any computer with a serial RS-232 output port.

The rear panel dip switch may be set to any of six baud rates (300, 600, 1200, 2400, 4800, or 9600), parity (even, odd, or none), and metric or English incremental steps. Data word format is always eight bits with two stop bits.

Typically, the first command in a plotter program written in Basic will be: OPEN xxx FOR OUTPUT AS 1 in which xxx is the communications specificaiton. On the NEC 8201, for example, the specification would be "COM:6N82XN". This means 2400 baud (6), no parity (N), eight bits (8), two stop bits (2), and XON enabled (X). The final character, N, does not matter to the plotter. We found 2400 baud to be an ideal operating speed; at slower speeds, the plotter occasionally had to wait for data while at higher speeds, the buffer frequently filled up.

The DMP-29 may be ordered with one of two optional interfaces: Centronics-compatible parallel or IEEE 488 GPIB. Producing a Plot

At the left side of the plotter bed are eight pen holders. Each one holds a hard-nibbed pen in one of eight colors: black, red, magenta, orange, light green, dark green, blue, and brown. These pens are included with the plotter. Each has a nib protector which must be removed when it is inserted in the holder. Optional fiber tip and drafting pens are available.

The plotter can use almost any type of paper, although a highly-polished paper (or mylar) produces better plots than a porous or rough paper. A wide variety of quadrille, chart, and graph papers are available from Houston Instrument as well as independents such as Dietzgen, K & E, Charrette, and Bienfang. We tried several and got excellent results.

The plotter has two operational modes. The first allows the plotter and computer to communicate using the XON/XOFF protocol and requires very little in the way of handshaking. When the buffer is full (512 bytes) the plotter indicates to the computer with a code 13 to stop sending (XOFF). When the buffer is half empty (256 bytes), an XON (Code 11), is sent.

This is normally the preferred mode of operation.

Altenatively, Mode two lets you replace the XON/XOFF mode with your own handshaking, delay loops, and user-specified prompt codes. While you theoretically can gain some speed by using Mode two, it is considerably more complicated in programming and recommended only for serious users who want to control everything in sight.

The DMP-29 has its own plotting language which is elegant and powerful. The simple sequence of a semi-colon followed by a colon (;:) activates the plotter. Once the plotter is selected, it responds to everything sent over the communications channel until a deselect (@) or reset (Z) command is sent. Upon receiving the select command (;:), the plotter automatically selects Pen 1 as part of the initialization routine.

If you want a pen other than Pen 1, you simply send Pn, where n is the number of the pen you want. T selects the self-test routine. This tests the logic circuits of the plotter and causes it to draw the self-test plot which uses all the pens and all the functions. The self-test can also be selected manually.

Pen up is selected by U and pen down by D. An H causes the plotter to go to the home position (0,0) in the lower lefthand corner of the paper. The command O causes the plotter to specify the current pen location as the origin of your plotting system.

The plotter normally assumes a full-size plot; half-size is specified by the command EF. If you have specified half-size and want to get back to full-size plots, the command EH will do it. These commands override any that have been set manually on the plotter front panel.

The plotter normally draws continuous lines. However, if you want any of nine varieties of dotted line, they can be specified by the command Ln (n=1 to 9).

The pen may be positioned with respect to an absolute set of coordinates (A) or relative to the last point plotted (R).

To draw a line, you simply specify the x and y coordinates of the start point and end point that you want drawn. For example, ;: HAD 0,600 600,600 600,0 0,0U will cause the plotter to be selected, go to the home position, specify absolute positioning mode, lower the pen, and then draw a square of 600 units on a side.

To draw the same square using relative positioning would require the following: ;: HRD 0,600 600,0 0,-600 - 600,0U

In a sense, relative positioning is very similar to the turtle geometry commands in Logo although, of course, angular measures are not implemented.

In the absence of any specifications, the plotter uses step sizes of 0.005 inches (200 steps per inch). In other words, a full size (10" X 15") plot with 0,0 at the bottom left corner would have a maximum x value of 3000 (15" X 200 steps) and a maximum y value of 2000 (10" X 200). Step size may be changed with the command ECn in which n is 1 for 0.001", 5 for 0.005", and M for 0.1 mm (metric).

For some plots it may be desirable to specify the velocity of pen movement. In the absence of this command, the pen moves at two inches per second. However, it may be specified at 1,2,4,8, or 16 inches per second (or one of five metric speeds). Windows, Functions, and Symbols

An amazing feature of the DMP-29 is called the Window. This is defined as an area of a plot that you wish to reproduce or scale. A single command specifies the upper left and lower right limits of a window. Whatever plot data are in this area will then be reproduced in another area called the Viewport, also defined by the upper left and lower right limits.

Moreover, if the Viewport is a different size or shape from the Window, the plot will be automatically scaled to fit. Thus, if you have a plot of a circle with a square Window around it, and you specify a large rectangular Viewport, the new plot will become a large ellipse. This pair of commands opens up a world of possibilities.

Anyone who has used a plotter is familiar with the equations for a circle, ellipse, and arc, and probably has a favorite curve-fitting method, since these plotting techniques are so frequently needed. However, all are built into the software of the DMP-29. A circle is plotted with the command CC x,y r, where x and y specify the center of the circle (which may be off the plotting surface) and r specifies the radius. The commands to produce an ellipse and arc are similar.

The general curve plot requires you to specify only the high and low points of a curve, although the plotter will not object to more points being specified. The plotter then calculates and plots the best fit line between your points.

Letters, numbers, and 30 ASCII symbols can be reproduced on the DMP-29 in any of nine heights (0.07" to 1.12"), and in any of four degrees of rotation (0, 90, 180, and 270) by using a single command.

If you want to get even fancier, an extended text command is available. This command accepts up to six parameters which specify character height and width (255 alternatives), normal or italics, character set (only one is available at present), and the degree of rotation (0 to 360 degrees).

Six marker symbols are also available for marking points on a graph, axes, and boundaries. Like the ASCII symbols, these can be plotted in nine different sizes. Digitizer Mode

In addition to acting as a plotter, the DMP-29 can be used as a digitizer input tablet. In this mode, the plotter transmits x,y point data to the computer for processing. The computer must be programmed to receive two five-digit coordinates, followed by a carriage return.

After the digitize command (ED) is received by the plotter, the plotter bed is your "pad," and the pen is the cursor. You maneuver the pen using the nine directional movement keys on the keypad. When the pen is set correctly, you press REPORT, and the coordinates are sent to the computer. Documentation

The DMP-29 plotter comes with an 82-page manual. It is marked "Preliminary," but apparently the final one is not yet available. As computer peripheral manuals go, it is neither the best or the worst.

In its favor, most of the plotter commands are adequately described and examples are given. On the other hand, the examples stop far short of producing "real" plots with axes and labels. Furthermore, all the example commands are listed as LPRINT xxx, whereas most computers use PRINT #1 to send information to the RS-232 port.

The section on interfacing the plotter to a computer should mention that most computers will require a reversed cable, and that a pin-to-pin cable will not work.

Nevertheless, with a bit of patience and experimentation with the pen in the up position, the manual enabled us to draw some interesting plots in an hour or so. In Summary

The DMP-29 plotter is an amazing piece of engineering. The wide variety of built-in commands make it easy to draw complicated and exotic plots. The general curve-fitting feature is especially useful for producing graphs. Also useful are the ten types of dotted lines and six marker symbols.

For computer artwork, the DMP-29 boasts many valuable features including automatic reproduction and scaling of plots, easy resetting of the pen origin, and absolute or relative pen positioning. The three step sizes and five pen velocities allow either "quick and dirty" or extremely detailed plots. Repeatability with the same pen is quoted as 0.004" and with a different pen, 0.008"; we found that these slight inaccuracies were not noticeable on most plots.

The ability to use the DMP-29 as a digitizer is a nice extra. The documentation with the plotter is fair, but would benefit from some longer examples of "real" plots. All in all, the DMP-29 is exceptionally versatile, user-friendly, and cost effective.

Products: HiPlot DMP-29 Plotter (computer apparatus)