No matter how powerful your design software may be, you must eventually create a photoplot file in Gerber format to create film. Aggravation can be reduced by better understanding what is contained in a photoplot file. Misunderstandings between the PCB designer and photoplot operator account for a large percent of the bad film produced in this industry.

Before diving into the details of the photoplot file a short discussion of photoplotters will make the following material easier to digest. Early photoplotters consisted of a precision servo controlled X-Y table to which a piece of high contrast film is attached. A bright light source is directed through a shutter, through an aperture wheel and focused onto the film. A controller converts Gerber commands into the appropriate table movements, wheel rotation and shutter opening.

When the shutter is open light shines through the aperture exposing the aperture's image onto the film. When the table simultaneously moves while the shutter is open, a line or stripe is imaged onto the film. Given the proper commands for table motion, aperture selection and shutter operations, one can construct just about any image on film.

Simplicity: A double-edged sword

The beauty of the Gerber database is its simplicity. There are only four essential commands plus coordinate data. The database had to be simple and compact because the first machines were driven by a fragile punched paper tape. The need to compress as much information as possible into as few bytes as possible explains many of the "problems" we encounter today when storage space is measured in hundreds of megabytes instead of hundreds of bytes.

But simplicity exacts its own price. The Gerber file lacks essential information necessary to run the plotter. This missing data is handed from the designer to the plotter operator separately, and is often the source of errors. Further, the temptation to create non-standard extensions is impossible to resist. Every photoplotter manufacturer supports the basic Gerber commands plus whatever features they feel necessary to differentiate their machine. The result is that one man's feature is the next man's unsupported problem.

Gerber File Contents

Below is a simple Gerber file illustrating the structure and content of the format:

G90*                   1
G70*                   2
G54D10*                3
G01X0Y0D02*            4
X450Y330D01*           5
X455Y300D03*           6
G54D11*                7
Y250D03*               8
Y200D03*               9
Y150D03*               10
X0Y0D02*               11
M02*                   12

The line numbers at far right are not part of the file. Examining this file without any prior knowledge of Gerber one would correctly deduce that each line represents a particular machine command and that the asterisk (*) is the end of command character. There seems to be different kinds of commands: instructions beginning with G, D, M and x,y coordinate data.

G-Codes: Setting the Initial Conditions

Gerber calls the Gxx commands preparatory codes. In most cases these codes are used to prepare the machine's state prior to drawing. Several G codes are important to recognize:

G90/G91 Incremental vs. Absolute Coordinates

The G90 command in line 1 tells the machine that data coordinates are absolute. Each set of coordinates is referenced to the table's origin (0,0). The converse to absolute is incremental -- each coordinate is measured relative the previous coordinate value and is set by issuing the G91 command.

You may never encounter the G90 command in any Gerber file since most programs assume that absolute coordinates are the default and do not bother to write the G90 command. What's worse is that many incremental databases don't bother to use the G91 command either. If you see a "starburst" display similar to figure 3 on your CAM station you are probably trying to read a file written with incremental coordinates.

Figure 3 Left: correctly displayed. Right: Viewer set to incremental when data is absolute.

The G70* (line 2) indicates that data following is in units of inches. This is another G-code that rarely appears in Gerber files. In the U.S. one assumes that Gerber files are output in inches, but files generated in the rest of the world may be in inches or millimeters. G71 indicates units of millimeters.

G54, tool select, (line 3) is the most commonly encountered G code and instructs the plotter to rotate the aperture wheel to the position described by Dxx immediately following the G54 command. If you find no G54's in a particular file don't panic. G54 is an optional command on many photoplotters; some programs don't use it and the photoplotter recognizes that by specificity Dxx (where xx is not 01,02,03) that the correct tool should be selected.

D-codes are instructions to the photoplotter that naturally include the letter "D." The first three D-codes control the movement of the x-y table.

D01 (D1): move to the x-y location specified with the shutter open.

D02 (D2): move to the x-y location specified with the shutter closed.

D03 (D3): move to the x-y location specified with the shutter closed;
          then open and close the shutter -known as flashing the exposure.

D01 is the command that "draws" lines.

D02 is the command to move the table without exposing any film. D01 and D02 correspond to moving the paper on a pen plotter with the pen down and pen up.

D03 is the "flash" command. The table is moved with the shutter closed. When the desired x-y coordinates are reached the shutter opens and closes leaving the image of the aperture on the film. The flash instruction is an efficient way to image the thousands of pads present on most circuit boards.

D01, D02 and D03 commands follow their coordinate data. For example the following sequence of commands:

X0Y0D02*
X450Y330D01*
X455Y300D03*

would move the table position to 0,0 with the shutter closed and then would draw a line from 0,0 to 450,330. A flash would be located at 455,300.

Wheel Positions/Apertures D10-D999

Unlike D01, D02 and D03 the D-codes with values from 10-999 are data, not commands. They represent apertures or positions on the photoplotter's wheel. Early photoplotters used a wheel with 24 positions.

Each slot is filled with a piece of film. The wheel rotates, positioning the desired aperture in the light path. Table 1 shows the correspondence between D-code and aperture position.

D-code Aperture   D-code   Aperture
       Position            Position
  10       1        20        13
  11       2        21        14
  12       3        22        15
  13       4        23        16
  14       5        24        17
  15       6        25        18
  16       7        26        19
  17       8        27        20
  18       9        28        21
  19       10       29        22
  70       11       72        23
  71       12       73        24

One can see that it proceeds logically from D10 through D19. Then, instead of D20 mapping into position 11, D70 and D71 intrude. The mapping continues, now skewed by two places until reaching the point where one would expect to see D30. Instead D72 and D73 jump in. Most photoplotters and CAM software ask you to enter the aperture descriptions by D-code; a few reference by aperture position.

There are D-codes between 3 and 9 but they are specialized commands for rare or obsolete machines.

Miscellaneous M Codes

At the end of the file we see the command M02*. Gerber calls the M codes, miscellaneous codes. The only common M-code used is at the end of the file- M00, M01 and M02 which are all different types of program "stop" commands. Occasionally one sees an M02 at the beginning of a Gerber file. Apparently some applications issue this to insure that any previous file running is stopped - however many CAM softwares see the M02 at the beginning of the file and ignore anything following it.

X,Y Coordinate Data

Coordinate data makes up the bulk of the Gerber file. It is difficult to manually follow table motion from a printout because Gerber uses several techniques to minimize the number of bytes required to represent the data. These are:

• Suppress the decimal point in the x,y data
• Suppress either the leading or the trailing zeros
• Only output changes in coordinate data

The decimal point is redundant if you know in advance where it will be. The decimal point needs to be reinserted by the photoplotter control software in the correct location. One of the most common errors that novice designers make is to assume that the person receiving their data knows you data format. Consider the following Gerber commands:

X00560Y00320D02*
X00670Y00305D01*
X00700Y00305D01*

The table moves along X from 00560 to 00670 during the first two commands. But what does 00560 represent? It could be 5.6 inches, 0.56 inches, 0.056 inches or even (but not likely) 0.0056 inches. No way to tell. If the designer tells you that there are two integers before the decimal point and 4 integers after the decimal point then you know that 00560 represents 0.56 inch.

Leading and Trailing Zero Suppression

The designers of the Gerber database didn't rest after eliminating the decimal point. They must have looked at a printout and thought,

"What good are all those extra zeros in front? suppose we cut them off. You can still figure out the coordinate value if you count decimal points from the right side of the number."

           No Zero                    Leading Zero
          Suppression                  Suppression

          X00560Y00320D02*             X560Y320D2*
          X00670Y00305D01*             X670Y305D1*
          X00700Y00305D01*             X700Y305D1*

Without zero suppression 48 bytes are used. With leading zero suppression 33 bytes are required to represent the same information. In the days of paper tape this was a very significant reduction.

Depending on the data you might be better off leaving the leading zeros on and suppressing the trailing zeros.

           No Zero                   Trailing Zero
          Suppression                  Suppression

          X00560Y00320D02*             X0056Y0032D2* 
          X00670Y00305D01*             X0067Y00305D1*
          X00700Y00305D01*             X007Y00305D1* 

To correctly interpret the data you must count from the left side of the number to locate the decimal point. Confusing? Yes.

Modal Data Coordinates

After eliminating the decimal point and suppressing the redundant zeros you might think that the database designers would rest on their success. Not at all. One sharp eyed programmer noticed that the same coordinate would appear over and over again when the table moved only along X or Y.

"Why not remember the last value of X and Y; output a coordinate only if it changes!"

          All Coordinates             Modal Coordinates

          X560Y230D2*                    X560Y230D2*
          X670Y305D1*                    X670Y305D1*
          X700Y305D1*                    X700D1*    

The concept that the plotter remembers the last value of coordinates is called modality. PC boards often have hundreds of pads in a row along X or Y and a properly sorted Gerber file will be much smaller when the redundant coordinate is eliminated. This is such a fundamental concept that coordinate data is always modal. You don't need to inform your target site that data is modal or nonmodal - every photoplotter and CAM software supports modal data. In this example an additional 4 bytes were save by using modal coordinates.

Commands can be Modal

Modality is a good concept for data and works equally well for commands. For example, if you have a string of draw commands why repeat the D01 command again and again. Let it stay in effect until another command (D02 or D03) occurs to change it.

          D1 not modal                    D1 modal
          X560Y230D2*                   X560Y230D2*
          X670Y305D1*                   X670Y305D1*
          X700D1*                       X700*
          X730D1*                       X730*
          X760D1*                       X760*
          Y335D1*                       Y335*

It would appear that all commands in Gerber ought to be modal. Once issued the command is in effect until otherwise superseded or turned off. However there are a couple of erratic exceptions that occasionally cause trouble. The most curious is the D03 flash command.

Certain brands of photoplotters don't treat D03 as modal. That is, they expect to see D03 at the end of each flash command. We've encountered this on MDA's FIRE 9000 family of photoplotters.

Flashes that were displayed on our CAM software didn't appear on the film. The problem is easily solved by re-outing the Gerber data with explicit D03's - Lavenir has a utility that does this and many CAM software's can be configured to treat D03 as either modal or non-modal.

The other erratic exception to the standard modal behavior of commands is the G02/G03 command - circular interpolation. Many photoplotters revert to G01 (linear) after a G02/G03 (circular).

The basic format for circular interpolation is:

GNN   XNNNN   YNNNN   INNNN   JNNNN   DNN *

Example: G02X40Y30150J0D01*

It would take more room available in this paper to properly treat the various permutations that can occur. Circular commands may be limited to just quadrant data on older machines or can describe full 360 degree arcs on newer machines. The meaning of I, J changes depend on whether data coordinates are absolute or incremental.

Popular CAM programs don't always display or edit circular commands correctly. If you wish to test your CAM software, we have provide some text files on diskette. Test your software in advance. Don't wait until you're up against a deadline.

Header for MDA's FIRE 9000 Photoplotter

G04%PAR.%*
G04%MODE=ABSOLUTE;%*
G04%ZERO=LEADING;%*
G04%FORM=4.4;%*
G04%EOP.%*
G04%APR,10000.%*
G04%A010:CIR,250.%*
G04%A011:CIR,300.%*
G04%A012:CIR;-CIR,100.%*
G04%A013:SQR,300.%*
G04%A014:REC,300,1000.%*
G04%A017:POL,X200,Y83,STR,X83,Y200,STR,X-83,Y200,STR,X-200,Y83.%*
G04%STR,X-200,Y-83,STR,X-83,Y-200,STR,X83,-Y200.STR%*
G04%X200, 7-83, STR, X200, Y83.%*
G04%EOA.%*

%
FSLAX44Y44*                         
ADD10C,.025*                        
ADD11C,.030*                        
ADD12C,.030X.010*                  
ADD13R,.030X.030*                   
ADD14R,.020X.100*                   
ADD17P,.040X8*                     
%

Plain Text Meaning


Leading Zero Suppression, Absolute, 4.4
D10 Round 0.025 diameter
D11 Round .030 diameter 
D12 Donut OD=.030 ID 
D13 Square Side=.030 
D14 Rectangle X=.030 Y=.100 
D17 Octagon Dia=.040