HP 7475A Plotter [Part 2]

It has been quite some while since I made my first post on the 7475a, it has been sitting in my closet neglected for months because of my move and I am back in classes now. But in need of several pcb’s for current projects I made a point to dust it off last weekend and see if we could etch some copper.

I had already done quite a bit of experimentation before I had packed it away over the summer and was expecting to be able to get it up and running quite easily. I had scoured the internet for some time as for software to drive the device with really no luck. This process is a little bit more difficult that just finding a driver for your old Epson inkjet, the 7475a is a vector graphics “printer” and thus a driver for one is quite different. There are commercial products such as winLINE, however I found them expensive and the trial prints with a large watermark.

I had been printing using the eagle CAM tool and a command line, the HPGL text file generated from eagle can be simply copied to the plotter via command prompt. I found this to be by far the best method, there are no crazy old drivers needed and the process should work regardless of operating system.

It is important to first ensure you have your serial communication settings correctly set. I have successfully used the following port settings:
Bits per second – 9600
Data bits – 8
Parity – None
Stop bits – 1
Flow control – None
The picture below also shows the corresponding dip switch settings on the plotter, now using your null modem cable that we talked about some time ago in my first post we are ready to communicate.
IMGP5681

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If you want some test HPGL code you can use some of mine, other wise fire up eagle and use the CAM tool to generate a new one. You need to make sure your generating HPGL instructions, not HPGL-2/X this thing is almost as old as me.

Once you have generated your HPGL file just open up a command prompt and navigate to the directory it was saved to. Type “copy ‘filename’ com’x'” replacing file name with whatever you saved your file as and comx with the com port you will be using, in my case this looks like “copy 4780main com3″. This will push the file to the plotter and it should start printing immediately. If it does not and the error light rapidly flashes, check your com settings again and make sure the roller lever on the plotter is engaged.

HPGL is a very simple instruction set, you can easily edit the files eagle outputs in notepad. I make use of this to delete all the “sp” instructions, I remove the pen carousel and attach a normal height permanent marker to the pen holder. we will only use the “pu” pen up, “pd” pen down, and “pa X,Y” commands there but there is a wealth of other commands in the HPGL manual.

From here on its a lot of experimentation to get the process right for you, I wanted to be able to do some SMT work. SOIC has pin spacing of 1.27mm and PLCC, and SSOP all looked like possibilities as well. Anyone who has done some PCB etching before has probably used a black sharpie for fixing little mistakes that toner transfer can have. So I first grabbed a pack of ultra fine point sharpies, that is a 0.8mm tip. I hold the pen in place by sliding a rubber grip from a mechanical pencil around the body of the marker, then simply push the pen into the plotters pen holder with the tip about 0.5″ off the surface.

The sharpie worked great as expected however I found the 0.8mm trace width too bulky to work with. After reading around some forums I found some Staedtler Lumocolor permanent markers, the small size has a 0.4mm tip and they are recommended for all surfaces. These produce a much finer trace and I as the shadow in the picture below shows will work well with SMT components. I cannot confirm this inks compatibility as etch resist just yet though.
IMGP5675 IMGP5678 IMGP5679

Sadly as of October 2008 I am having issues with my plotter, when trying to continue my experimentation I have been plagued by erratic plotting behavior. That is after following the instructions that are sent to the plotter for a few seconds it will start plotting random vectors all over the page as shown below. I have spent some time working with the plotter trying to diagnose the issue and I am coming up blank. I feel it could be my communication hardware as I am using a usb serial adapter now, and the plotter still makes great test pages.
IMGP5677

However this project may lay dormant for some while now, I am moving on to a CNC milling process for several reasons. It would take significant modifications to the plotter to be able to use for hole drilling, a cnc process can guarantee much smaller trace width and repeatability, and because this will totally eliminate the need for FeCl or other etchants. My ultimate goal is to simplify and speed the pcb prototyping process as much as possible, I feel an all in one machine needing no chemicals is currently the best answer. I will be posting up on the beginnings of my cnc build soon.

I have enjoyed working with the 7475A and hope to revisit it with new possibilities in the future, if you have any questions or suggestions please post them.

87 Comments

87 Comments

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