A common industry-standard recognised by pcb fabrication plants is EIA RS-274X. 
Also called Extended Gerber, this format provides drilling and soldering data 
in a text file which allows drilling machines and photo-plotters to create a pcb 
from X-Y coordinates. 
 
GERB20.EXE (v1.5) is a PC program that converts a Newton fleXYcad circuit into Extended Gerber 
format. It creates 4 text files that can be zipped up and sent to a pcb manufacturer.

Three hole sizes:  0.036", 0.048", 0.126" are provided by gerb20. 
These are equivalent to Metric size: 0.9, 1.2 and 3.2 mm. 
All regular component legs in a fleXYcad circuit (e.g. chips, capacitors, resistors etc), 
and the empty round connector and transistor shape will be drilled to 0.036" hole size. 
The remaining single connectors and shapes solder-pads (e.g. for switches) will be drilled 
to 0.048" hole size. D9-serial connector mounting holes will be drilled to 0.126" hole size. 


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Instructions:

The fleXYcad circuit needs to be a .txt file located in the same folder as gerb20.exe.
The size of the circuit-board is defined by a cut in opposite corners of the fleXYcad 
circuit - if there are no cuts, please add them to the circuit.
 
  1. Run gerb20.exe and select the source file when prompted. Gerb20 will scan the 
     fleXYcad circuit and display a list of the components, and it will measure the 
     size of the circuit from the positions of cuts in opposite corners of the layout.

  2. Gerb20 will offer a choice of pcb track-size and solder pad-size. 
     Try using track-thickness: 3 (= 0.025"), pad-size: 6 (= 9x7 = 0.09"x0.07").

  3. Gerb20.exe will create 4 text files:
   
   - GBL.txt. This is the bottom solder-layer, showing solder-pads and track-lines.   
   - GBS.txt. This is the bottom solder-mask information. It creates a protective 
     cover for the copper tracks, with a circular window above each solder-pad. 
   - GTO.txt. This is the top-side overlay, just a silk-screen border around the circuit
     that defines the size of the pcb.  
   - DRD.txt. This is an Excellon drill file. It provides 3 hole sizes: 
     0.036", 0.048", 0.126" (0.9mm, 1.2mm, 3.2 mm).

  4. After making the 4 Gerber files, gerb20.exe will draw the original circuit on 
     the screen and show the pcb tracks and drill holes, but without any solder-pads. 
     This display should assist in finding any circuit design errors e.g missing tracks
     or missing holes.

  5. Exit gerb20. Transfer the 4 files to another folder and *rename* them 
     e.g. mycct.gbl, mycct.gbs, mycct.gto, mycct.drd. 
    (Each time gerb20.exe runs it will overwrite the files that it created previously).


Certain limitations in gerb20.exe:

  1. This version does not create top-side silk-screen component position or ID data. 
  2. Error analysis is minimal. If the circuit fails, some debugging information 
     is saved in an error-log file but it won't suggest what is wrong with the circuit.  


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Before sending the files to a manufacturer, the Gerber files need to be opened in a 
Gerber Viewer to check that the circuit has been correctly formatted.

Two free viewers have been tested: 

  - ViewMate (v10.0.10) 	from PentaLogix (http://www.pentalogix.com)
  - ViewPlot (v1.5) 		from Roneda 	(http://www.roneda.nl) 


Instructions for the viewers (using default file-names): 

ViewMate:
  1. File -> Import Gerber (F2) -> select either gto.txt, gbs.txt or gbl.txt. 
     Each file needs to be opened to a new layer. The new layer has to be selected 
     in the coloured list on the left-hand side before Importing the file.
  2. File -> Import Drill (shift-F2) -> select drd.txt (onto a new layer, as in 1). 
  3. View -> Grid -> Variable. Units -> 0.001. (Sometimes the grid doesn't display).


ViewPlot:
  1. Delete the viewplot.ini file in the viewplot folder, otherwise viewplot.exe 
     may refuse to open. Do this each time before opening ViewPlot. A simple .bat 
     file can automate this. 
  2. File -> Load File (F3) -> select gto.txt, gbs.txt or gbl.txt, select OK.
  3. File -> Load Global Drill Tools -> select drd.txt, change Units to inches, 
     select 'Read Drill Tools', select OK.
  4. File -> Load File (F3) -> select drd.txt again, select OK, 
     (confirm settings: 2,3 number format/normal/inches), select OK
  5. View -> Change Grid -> select 100 mls (or 2.54 mm)

NB for ViewPlot: 
  1. If drill holes are not seen, check Units in step 3 is in Inches.
  2. ViewPlot will only allow a couple of circuits to be analysed before it needs 
     closing and re-opening. 


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Gerb20.exe and Viewplot.exe will both run on a Linux PC: 
  - install 'dosbox' for Gerb20.
  - install 'wine' for ViewPlot.


Technical:

Gerb20 exports 2,3 Number Format, Leading Zero Suppression, Inch units.
Gerb20 is written in QBasic and compiled into an exe file using PowerBasic.

If the 4 files are sent to a pcb-maker it is necessary to include an extra readme.txt
file with important information. A sample file is included in gerb20.zip

------------------------------
v1.5    more changes  
v1.4    small changes
v1.3    small changes
v1.2    Added error-handling if no .txt files are present.
v1.1    Added more pad-sizes.

Date: 13 November 2008