ProfitPress Mega CDROM2 Shareware Freeware (MSDOS)(1992)(Eng)

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Text File | 1991-02-21 | 17.0 KB | 595 lines

PHOTOPLOTTING PRINCIPLES What's a Photoplotter? A photoplotter is just what the name implies: an plotter that writes using light. A plotter has to be told: Which tool to use. When to use the tool, and when not to. Where to go next. Whether to go there in a straight line or along an arc. For a photoplotter, "tool" means specially shaped apertures through which light passes to create a given shape on film. An aperture can be used without movement to make a shape (a "flash") or with movement to make a line or an arc. There are two major types of photoplotters, "Vector" and "Raster" (or "laser"). Each handles apertures differently. Vector Photoplotters Aperture Wheels Traditionally, the photoplotter counterpart to a pen plotter's pen rack has been the aperture wheel. The aperture wheel is a disk with 24 or 70 apertures arrayed radially along its circumference. When the photoplotter selects an aperture, the aperture wheel is rotated to place the desired aperture between the light source and the film. Apertures are themselves pieces of film and can be made to any shape required, although in practice this is a time- consuming process and there is a physical limitation on size. Flash and Draw Apertures To achieve constant exposure on a vector photoplotter, apertures used for flashing pads are filtered differently than those used for drawing traces. Therefore, Flash and Draw apertures cannot be used interchangeably without risk of localized over-exposure and under-exposure. Aperture Wheel Setup for Vector Plotters The setup of an aperture wheel is an exacting and time consuming process since each aperture in the wheel must be hand-mounted and aligned. In order to avoid repeated setup costs, designers have the photoplotting vendor keep a wheel on file and are forced to always use that same set of apertures. This has obvious drawbacks, both in terms of design flexibility and the ease of migration to other vendors. Raster (Laser) Plotters Aperture Lists Increasingly, vector photoplotters are being replaced by the laser photoplotter, which emulates the older style machine in a raster (bit-map) fashion. While use of the term "aperture" to describe a pad or trace shape persists, the term "aperture wheel" is now being replaced by "aperture list", which implies the greater flexibility now available to the designer. There are three principle advantages with aperture lists on raster plotters: Aperture shapes can be easily generated in software, thus eliminating the need to design a physical wheel. More apertures can be defined on a list. Allowable apertures sizes are typically (but not always) greater than those imposed by the physical dimensions of an aperture wheel. Flash and Draw Apertures No distinction need be made between Flash and Draw aperture types since the light source intensity is constant. Speed Advantage of Laser Plotters Laser plotters operate much quicker than vector machines. A complex plot that required hours on a vector machine can usually be performed in ten minutes or less on a laser photoplotter. This decreases turnaround time and in many markets has driven photoplotting costs down. Talking to Photoplotters The de facto standard for photoplotter data is the Gerber format, more properly known as RS-274D. The term Gerber refers to the Gerber Scientific Instrument company, a pioneer and leader in photoplotter manufacturing. RS-274D is a variation on traditional Numerical Control (NC) machine tool languages. It differs from traditional NC formats (i.e. drill data), as far as its use of tool selection codes but is otherwise compatible. RS-274D data is organized in "blocks". A block consists of a combination of codes: Tool selection Setup Movement And, an End Of Block (EOB) character, which only follows a combination of the above codes. An EOB character is usually an asterisk ('*') or dollar ('$'), optionally followed by a carriage return and line feed. An RS-274D code consists of a letter D,G,M,X,Y,I or J followed by a numerical value. These codes designate the following: * - End of Block (end of command) D - Select aperture, or set aperture use mode X - Move to X value Y - Move to Y value G - Various setup codes M - Various control codes I - Relative X location for arc center J - Relative Y location for arc center D Codes D codes have multiple purposes. The first is to control the state of the light being on or off. Valid codes for light state are D01, D02, and D03. D01 - Light on for next move. D02 - Light off for next move. D03 - Flash (Light On, Light Off) after move (effect is limited to block in which appears, ie non-modal). You can also think of a D03 as D02, D01, D02 series of commands linked together. D codes with values of 10 or greater represent the aperture's position on the list or wheel. It is very important to understand that there is no universal "D10" or "D30". Unlike the D01 , D02, and D03 counterparts which have a fixed meaning (draw, move, flash), D10 and higher values have aperture shapes and dimensions assigned to them by each individual user. Hence, one job's D10 could be a 10 mil Round, when another job's D10 could be a 45 mil Square. There are two distinct ways to number an aperture list. The traditional 24 aperture system started with D10 - D19, jumping suddenly to D70 - D71, then back to D20 - D29, ending with D72 - D73. This is still a common format for output for CAD packages, and is still mandatory for old 24 aperture Gerber vector photoplotters. It is now common to start with D10, then increase numerically in steps of 1 (D10, D11, etc.) continuing up to D70 and beyond, rarely beyond 1000 individual apertures. X & Y Codes The X & Y values in the Gerber file determine where the aperture shape and dimension will be positioned and drawn. X & Y values are used as coordinate pairs to determine where the light will be exposed, using the D codes shapes (i.e. D10) and light exposure status (i.e. D01, D02, D03) for drawing lines and arcs, as well as moving between drawing entities. Here are a few examples of using X & Y codes with D codes. D10* { Select aperture D10} X1000Y1000D02* { The D02 tells us that the light will be off, and we move to coordinate position X1000 and Y1000} X2000Y3000D01* { The D01 tells us that we will draw (light on) to coordinate position X2000 and Y3000} X5500Y100D03* { The D03 tells us to move to coordinate position X5500 and Y100 with the light off, then flash (turn the light on and off)} G Codes G codes are used to configure the photoplotter. Commonly implemented codes include: G01 - Future X,Y commands are straightline moves G02 - Future X,Y commands are clockwise arcs G03 - Future X,Y commands are counterclockwise arcs G04 - Ignore the rest of this block (used for Comments) G54 - Prepare to change apertures G74 - Future arcs are quadrant arcs G75 - Future arcs are Full 360 arcs G90 - Absolute data G91 - Incremental data Typically for laser photoplotters, G54 codes are rarely necessary. Older vector plotter controllers may require this preparatory G codes for changing apertures (i.e. G54D10*). A common situation where G codes are mandatory for all machines is when the data is switching from vectors to arcs and vice versa. When switching from drawing vectors (G01) to drawing arc (G02, G03), the controller must be informed of the change of mode. Another important case for G codes is when determining if the arc is a quadrant (G74) or Full 360 (G75). Quadrant arcs never cross quadrant boundaries, because the center coordinate offsets (I,J Codes) are always unsigned (even if they are negative!). Therefore, it requires at least four G74 arcs to draw one complete circle. Center coordinate offsets for 360 arcs (G75) can be positive or negative, allowing for a single command to draw a complete circle. In either case, the center coordinates are given relative to the start point of the arc. The most dramatic difference between Quadrant and Full 360 arcs is that a Quadrant arc with identical start and end points has a sweep of 0 degrees, whereas a similar Full 360 arc is a full circle. The G90 code tells the machine controller that all data following is absolute data. Hence, if following X & Y data follows, the controller will move to the absolute value given by the X & Y value. G91 tells the machine controller that all data following is incremental data. The machine will move the data by the amount of the X & Y value, rather than to the absolute coordinate point. Example: X1000Y1000D02* X3000Y3000D01* In absolute mode (G90), the machine will first move to coordinate point X1000 and Y1000 with the light off, then draw a line to coordinate point X3000 and Y3000 with the light on. In incremental mode (G91) the machine will first move to coordinate point X1000 and Y1000 with the light off, then draw a line to coordinate point X4000 and Y4000 with the light on. This was done by adding X1000 + X3000 = X4000 and Y1000 + Y3000 = Y4000. Here are some more examples of G code usage in conjunction with X, Y, and D code values: G54D10* { Prepare to change aperture position (G54), then select aperture D10} G01X1000Y1000D02* { Prepare to draw a vector (G01) then turn off the light (D02) and move to coordinate position X1000 and Y1000} G90* { This block (command) and all future commands will be absolute data} X2000Y3000D01* Turn the light on (D01) and move to absolute coordinate position X2000 and Y3000} G91* { The G91 command tells the controller that this command and all future commands that the data is incremental} X5500Y100D03* { Turn the light off and move incrementally by a value of X5500 and Y100, then flash (D03) (light on and off)} M Codes M codes are used for machine control. Here are the most commonly used: M00 - Full machine stop. Commonly ignored by many plotters. M01 - Temporary machine stop. Commonly ignored by many plotters. M02 - End of Plot. I & J Codes When you encounter an I & J code in a Gerber block, you have found an arc command. Arc commands come in two flavors, Full 360 or Quadrant. The Gerber arc command is very complicated, and this section will only briefly describe usage of the Gerber arc. Full 360 arcs allow the plotter to draw a full circle (360 degrees of arc) in one single command.. Quadrant arcs only allow for an arc to be drawn through a maximum of 90 degrees of arc, never crossing a quadrant boundary. Due to this restriction, I and J arc center offset codes can get away with never having a negative value, even if the offsets are negative! When in a Full 360 arc (G75), only one command is required to draw a circle. In Quadrant mode, the same circle would require at leaset 4 Quadrant arcs (G74), because a circle goes through all four quadrants. Quadrant arcs will always have positively signed I and J values, even if the center offset is actually negative. Full 360 arc center offsets can be signed positively or negatively. A negative I or J is a sure indicator of Full 360 arcs. Modality It is often the case with Gerber data that when moving from one XY coordinate point to another XY coordinate point, the X or Y value will not change. Likewise, it is likely that if the plotter is drawing a line with multiple segments, the segments will be connected and the light stays on from segment to segment. In both of these cases, there are redundant commands, making the plot data file larger than necessary. RS-274D allows you to omit this redundant data. This example shows a box being drawn with four corners. Non Modal Data Modal Data X0000Y0000D02* X1000Y1000D02* X0000Y1000D01* Y1000D01* X1000Y1000D01* X1000* X1000Y0000D01* Y0000* X0000Y0000D01* X0000* From this example, a large amount of data has not been written, thus reducing the final data file size. Establishing the Decimal Point A numerical value in RS-274D data has an integer and a decimal part, but the decimal point ('.') is not a valid RS-274D character. Thus, decimal values are written as a string of integers. The implicit position of the decimal point is determined by three parameters: Number of integer digits (whole digits) Number of decimal digits (precision) Zero suppression. For example: In a system with integer digits=n and decimal digits=m (an "n,m" system), a numerical value is written using (n+m) digits. For example, in a "2,3" format the value 12.345 is written "12345". In a "2,4" format, the same value is written "123450". Zero suppression comes in three flavors - leading, trailing and none. The idea of zero suppression is to reduce data file sizes by eliminating unneeded 0 characters. The simplest and most common form of zero suppression is leading zero suppression. In a "2,4" format, with no zero suppression, the value 0.0100 would be 00 + 01000, written as "000100", but with leading zero suppression the same value is written as "100". With trailing zero suppression the same value 0.0100 would be written as "0001". How To Describe Data Formats Gerber data and other XY languages use a standard method for describing the data format. Two examples include: "2,3 leading inch" or "3,3 trailing metric". The first number specifies the whole digits used. The second parameter states the precision. "Leading" and "trailing" pertain to the zero suppression. And the last part of the description indicates the units. Refer to the above sections if these concepts seem unclear.