Pixy 3 desktop plotter by Mannesmann Tally. (evaluation) David H. Ahl.

Pixy 3 Desktop Plotter by Mannesmann Tally

Have you been intrigued by the capabilities of a plotter but put off by the typical multi-thousand dollar prices? If so, you may want to consider the Pixy 1 and Pixy 3 plotters from Mannesmann Tally. Both cost under $1000, have a fair amount of built-in "intelligence,' and produce more-than-adequate plots. The Pixy 3 has three pens, while the Pixy 1 has just one; we obtained a Pixy 3 and put it through its paces.

Compact Physical Package

The plotter is remarkably compact, measuring 17 X 10.5 X 4.7 . It accepts an 8 1/2 X 11 piece of paper on the flat plotting surface. However, the plotting area is smaller, 180mm (m.1 ) X 250mm (9.8 ). If you have been resisting the metric system, this plotter will force you to reconsider--all the measurements and parameters must be specified in millimeters.

The paper is secured to the plotting surface by overhanging lips on the top (11 side) and upper part of the left of the sheet of paper. The right side and lower portion of the left side of the sheet are held down by flexible magnetic strips (see photo).

There are remarkably few controls. An on/off button switch is at the left rear. Four touch-sensitive directional controls and a pen up/pen down control for manual pen movement are at the left front. Turning the power switch on with one of the directional controls depressed activates one of three alternate functions: self-test, print mode 1 (plotter acts like a printer), and print mode 2 (plotter acts like a printer but does not respond to carriage returns--useful for checking plotter input data.)

A red LED indicates an error condition. This occurs when a non-command character has been sent to the plotter or when the input data are in the wrong format.

Three types of pens are available for the Pixy plotters: fiber tip water base (for paper), fiber tip oil base (for aceate or overhead transparencies), and ceramic (for paper). Eight colors are available. The pens are held to the pen holder at the left of the plotting surface by a permanent magnet. The plotter head also has a magnet with which it picks up a pen.

The mechanism is amazingly simple. The plotter head moves vertically on a bar which moves horizontally. Upon power up, the head picks up pen 1, moves to the home (0,0) position, and awaits commands from the computer.

Repeatability (accuracy) with the same pen is within 0.3mm and with a second pen is within 0.4mm. Our plot of lines within a circle (Figure 1) verified this accuracy.

Easy Interfacing?

The Pixy plotters are normally equipped with an 8-bit parallel interface. This means that to hook up the plotter, you would have to disconnect your printer. We judge that most people would not want to do this, and, therefore, ordered our plotter with the optional factory-installed RS-232 interface.

The manual devotes 11 pages to describing the two interfaces--wiring the connector, timing diagrams, pinouts, and data protocols. It also states that "connection cables for all the major personal computers are available from Mannesmann Tally or your local dealer.' Well, maybe. If not, a standard reversed RS-232 cable will work (Pin 2 to 3, 3 to 2, 4 to 5, 5 to 4, 6 to 20, 20 to 6).

But for us, that was just the beginning of the fun. Next, the DIP switch on the bottom must be set. It specifies baud rate (600, 1200, 2400), stop bits (1 or 2), parity, and data word length (7 or 8 bits). Next, your computer must be set to match in either hardware or software.

We went through all this and thought that all was OK, except that the plotter kept doing strange things. We set it to print out the incoming data and got pure hieroglyphics--well, maybe ASCII hieroglyphics. As it turned out, certain plotting commands must be enclosed in quotation marks and others not. This is not at all clear in the manual.

Another problem we encountered was that the plotter would not "handshake' correctly with our computer. According to the manual, when the 256-character buffer is full, an XOFF is transmitted to the computer, and when it is half empty (128 characters), an XON is transmitted. Maybe so, but our computer was having none of it. The computer kept the data flowing which led to the plotter buffer overflowing. When this occurred, the plotter sort of gulped, returned to the x or y origin (with the pen down), digested some new data, and started plotting from that point (see Figure 2).

Perhaps not the most elegant solution to this problem, but one that worked for us, was to set the slowest baud rate (600), and put an occasional delay loop in our plotting programs. Line 80 in Listing 2 is an example of such a delay loop.

Plotting Commands

Once the plotter was interfaced and receiving the data sent by the computer, we found it had a rich and varied set of commands.

For doing line graphs, the Pixy can produce nine types of line: solid, dashed, long-short dash, dash-double dot, etc. Moreover, the pitch (distance between successive line segments) can be varied over a wide range (1 mm to 25.5mm; 10mm is standard). As might be expected, the more complex line patterns cannot be differentiated with pitches under 10mm.

As mentioned earlier, the lotter surface is 180 X 250mm. Coordinates within this area are specified in 0.1mm increments. In other words, the live plotting area varies from x=0 to 2500 and y=0 to 1800. There is no provision for resetting the origin; 0,0 is always at the lower left.

Within this area, the most used commands will be M (move with pen up) and D (draw with pen down). These commands must be sent in the format:

LPRINT #1, "M' 100,300

LPRINT #1, "D' 255,782

There are also two relative move and relative draw commands. These move the pen from the current position for the relative displacement specified by the x and y arguments in the command. For example, if the pen is at 100,000, and the relative move command "R' 25,50 is sent, the pen will move to 125, 250.

Two exceptionally powerful commands are the ones to produce circles and curves. Both of these can be specified with either absolute coordinates or relative movement. The circle command can produce circles, arcs, and spirals. Listing 1 is a simple program using the relative circle command (line 60) to draw a spiral (see Figure 3).

A "factor' command can change the plotting magnification or scale factor in the x and/or y axis. Coupled with the circle command, it allows ellipses to be drawn. Listing 2 is an example of the use of the factor command (line 60) with the absolute circle command (line 70) to produce a series of ellipses (see Figure 4).

The curve command draws a smooth curve through a set of three or more points specified as parameters. Both open and closed curves can be drawn.

For producing line and bar graphs, the Pixy has several useful commands. The axis command draws x and y axes with hash marks at specified intervals. The grid command draws series of lines parallel to either or both axes. Points on line graphs can be differentiated with 15 different special marks specified by the mark command. The marks can be drawn in any of 15 sizes from 0.7mm high to 10.5mm high.

As with the marks, 96 ANSI characters, 47 Greek letters, and 6 mathematical symbols can be drawn in any of 15 sizes. Unfortunately, letters and symbols in the smallest two sizes (0.7mm and 1.4mm high) are barely legible. From the third size on, there is no problem (see Figure 5).

A font command selects any of ten international character sets. Well, actually not an entire character set; 11 letters and symbols are changed for various jucountries.

All characters and symbols can be printed in four directions: 0, 90, 180, and 270 degrees. There is no provision for printing in other directions.

As mentioned earlier, if the up directional key is held down when the plotter is turned on, the print mode will activate. This uses character size 3 (2.1mm high) and produces reasonably legible listings. As it is extremely slow, you would not want to use this mode for long program listings or lists of data. For example, the seven lines of Listing 1 took nearly five minutes to draw on the plotter.

While we are on the subject of speed, we should mention that any of ten pen speeds can be set by the program for multiple step plots. The default value is the slowest speed, probably because it gives the most accurate and detailed lines. The fastest speed is useful for debugging plotting programs, as the plot will be correctly drawn but with a few occasional skips or uneven lines. However, if the steps are reasonably short, the faster speeds are perfectly satisfactory.

Documentation

As we have observed with so many other products, the documentation falls far short of the excellence of the product itself. Furnished with the plotter is a 48-page Operator's Manual. The first seven pages are devoted to nomenclature, operating notes, loading of paper and pens, and the self-test.

As mentioned earlier, 11 pages describe interfacing in a thorough, but highly technical way.

Three pages list the input data codes, two pages list the plotter specifications, two pages describe error procedures, one describes maintenance and cleaning, and one lists available accessories.

The balance of the manual, 20 pages, provides descriptions of the plotting commands, examples, and notes about their use. This in no sense is a tutorial; rather, it is simply a reference section with a few examples (often incomplete) from which you must figure things out on your own.

The Final Line

The Pixy 1 and Pixy 3 plotters provide excellent plotting capability at a modest price. If you do not need plots larger than 8 1/2 X 11 , can get along with one or three colors, and are willing to devote a fair chunk of time to experimentation, these plotters would be an excellent choice. We are disappointed in the documentation, but the essential information is there. You will just need patience and experimentation to get it out.

Table: Listing 1. Program produces spiral pattern in Figure 3. Listing was done on the plotter, but took nearly five minutes.

Table: Listing 2. Program produces eight circles. Scale factor is set in line 60 and circle is drawn in line 70.

Photo: Pixy 3 plotter handles up to an 8 1/2 X 11 sheet of paper.

Photo: Power switch, error indicator, and manual pen controls. Paper is held down by magnetic strips (left).

Photo: Plotter head holds a pen by means of a small magnet.

Photo: Pixy 3 has three fiber or ceramic tip pens.

Photo: Figure 1. Lines in circle plot shows accuracy of Pixy 3 plotter.

Photo: Figure 2. When the buffer is overloaded, the pen will momentarily return to one or both boundaries.

Photo: Figure 3. Spiral can be drawn with just one command (see Listing 1).

Photo: Figure 4. Eight circles with horizontal scale halved for each successive one (see Listing 2).

Photo: Figure 5. Roman or Greek letters can be drawn in 15 sizes. Smallest two sizes are practically illegible.

Products: Mannesmann Tally Pixy 1 (computer apparatus)
Mannesmann Tally Pixy 3 (computer apparatus)