Modifying an Old HP7475a Plotter to Work Over USB

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About: My passion is to design PCBs and to program microcontrollers. I have a M.Sc. in Mechatronics Engineering and a B.Sc. in Mechanical Engineering. I currently work as a Hardware and Project Engineer for a compa...

About a month ago I was at the local municipal dump when I came across this unusual looking printer on the sorting table for electronic waste about to be thrown into a container. I was curious, because I have never seen such an old device that had actual pens in it.

After taking it home, I found out that it was a plotter from the late 70s and that it was used to draw technical drawings, graphs, etc.

After turning it on, I was able to move the pen around with the keypad, meaning that it seemed to be functional.

My first thought was how on earth will I find drivers for this thing? and what kind of cable will I need? and what kind of software will I need?

The great news is that these types of plotters work on commands with HPGL (Hewlett Packard Graphical Language) which is literally just simple commands to do many graphical things and all over RS232 communication, meaning that it is really easy to modify these types of old plotters to work with modern computers over USB.

There is also a lot of original documentation available online, such as service manuals with circuit diagrams and part lists.

Important Note:

This modification is for the RS232 version only. If the plotter has a HP-IB port instead of the 25-pin RS232 port, then this modification will not work.

Step 1: Hardware Modification: Introduction

Even though the port has 25 pins, only 4 connections are needed. Since printers of the past operated with RS232 communication with +12 V and -12 V data signals, there is some additional circuitry on the plotter circuit board to achieve these voltages.

Looking at the part list in the service manual one can find the chip used for data communications. This chip operates on normal TTL communication, meaning that 0 V to +5 V are normal conditions for this chip.

Sparkfun has an RS232 module that runs over USB and has all the pins needed for successful data communication to this plotter.

Step 2: Hardware Modification: Soldering

By unscrewing the 3 screws on the rear side, one can open the top cover and expose the main circuit board (make sure the plotter is unplugged from the mains).

There are only 4 lines that will be needed: RX, TX, GND and RTS.

Since these plotters have very limited memory, it is very easy for data to be lost and have the plotter do unexpected things, therefore hardware handshaking will be required. This just means that the plotter will tell the computer over the RTS pin when it has enough memory available for new incoming data.

Solder 4 wires directly to pin 1, 2, 5 and 6 of the plotter's USART chip (GND, RX, RTS and TX of the chip respectively) and run them out the back to a header pin strip for the RS232 module.

I added a cable tie to prevent the wires accidentally being ripped out.

Gently close the top cover paying attention to the paper lever.

Step 3: Wiring

Connect as follows:

Plotter GND to RS232 GND

Plotter RX to RS232 TX

Plotter TX to RS232 RX

Plotter RTS to RS232 CTS

Adjust the DIP switches at the back as in the photo. The baud rate is set to 4800.

The plotter is now ready.

Step 4: Software: Inkscape

Inkscape is professional quality vector graphics software which runs on Linux, Mac OS X and Windows desktop computers. Most importantly, it's free and can create HPGL files that can be sent directly to the plotter.


To get started, a HPGL file is included. One might just need to change the page orientation to landscape.

Step 5: Additional Information:

Thank you for reading my instructable.

Here are some example plots (in blue) as well as additional documentation.

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    58 Discussions

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    TomH250

    4 weeks ago

    Nice work.
    Would this work with a hp7550A?

    1 reply
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    chrismajdaTomH250

    Reply 11 days ago

    Thanks

    Yes, it should also work, providing you have a terminal at the back that is labelled RS-232. If you only have an H-IB port, then sadly no.

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    JesusB59

    Question 2 months ago on Step 2

    Good work, may i ask you a question ?
    i have a SEKONIC XY PLOTTER SPL 430. It look like the HP 7475 but the mother card is not the same so i can't connect the FTDI Basic breakout at the same place like you.
    Th mother card of SEKONIC PLOTTER is : H PCB E 144 TK 00081-A
    Have you an idea of a side i can plug the FTI circuit ?
    Thank's for answering


    11 answers
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    chrismajdaJesusB59

    Answer 2 months ago

    Hello,

    I am having a hard time finding the circuit diagram for your plotter. Do you perhaps have any documentation that came with this plotter?

    I was only able to find the owner's manual.

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    JesusB59chrismajda

    Reply 2 months ago

    Thank's Chrismajda for answering and for the manual. If I push the button test, the machine works perfectly. But I can't connect the machine correctly to the PC because I haven't any driver for windows10. The hardware modification it's a good idea but I don't know where I must connect the FTDI Basic.

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    chrismajdaJesusB59

    Reply 2 months ago

    If you have a RS232 terminal at the back, then it should be possible to use the plotter without any drivers. I do not need any plotter drivers for my setup.

    I just need to see a circuit diagram or at least some really good photos of the circuit board so that I can find the UART chip and then possibly which pins you need to connect.

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    JesusB59chrismajda

    Reply 2 months ago

    Thank you very much. I'll send you this pics as soon as possible. I'm not at home. I'll be back the last week of may.
    It's very friendly to spend your time for my problem.

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    JesusB59chrismajda

    Reply 8 weeks ago

    Hello Chris, I'm at home today. This is the pics of the mothercard of my SEKONIC XY PLOTTER SPL 430. If you find where i can plug the FTI circuit, il will be your obliged.
    Have a good day

    20190527_081545.jpg20190527_082346.jpg
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    chrismajdaJesusB59

    Reply 8 weeks ago

    Hello,

    Thank you for the good photo of the circuit board.
    I believe that I have located the correct chip for the serial interface. I have attached pictures for you to see.

    Please take care of the DIP switch selection, at the back, that switch 10 is set to ON for serial communication and that the other switches are set to the same baud rate as the PC. The settings are in the plotter datasheet that I sent you earlier.

    Let me know if you have any further questions and if this solution worked.

    Good luck !!

    sekonic.png
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    JesusB59chrismajda

    Reply 6 weeks ago

    Hello Chris, I'm very disappointed. I can't do anything with my plotter. I respect all you make and put the same values but I have no result. Help !

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    chrismajdaJesusB59

    Reply 4 weeks ago

    Hello,

    Sorry for the late reply. I just noticed your message now.
    The only suggestion that I have is that you try to connect the sparkfun CTS pin to pin 24 DTR instead of the pin 23 RTS.
    I have another plotter that needed this connection. I hope this helps.

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    JesusB59chrismajda

    Reply 4 weeks ago

    Thank you for reply. I'm not a lucky man, I follow your advices and change the connection 23RTS to 24DTR but the plotter doesn't start. Have you the same parameters than me for the plotter ?

    20190619_112529 Para3.jpg20190619_112519 Para2.jpg20190619_112446 Para1.jpg
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    JesusB59chrismajda

    Reply 8 weeks ago

    Thank you very much for your answer. I'm not at home again this few days. But I'll tell you soon the result of this modification.
    Thanks again

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    El_mot

    Question 2 months ago on Step 5

    O wonder if it's possible to draw PCBs at home with this stuff. What is the actual resolution?

    4 answers
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    michels57El_mot

    Answer 2 months ago

    yes it's possible , I have used it with STAEDLER lumicolor pen ( black) these pens resist in FECL3 liquid without problem , there is also an another idea :1) paint the pcb ,2) replace the pen with a pointed steel rod and well sharpened, the rod will remove the paint before the acid operation

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    russ.hughesEl_mot

    Answer 2 months ago

    Yes it is. I have used mine to prototype small PCB's. I modified the pen holder to hold a ultra fine point sharpie and used a glue stick to mount the board to a piece of thick paper. I did 3 plots then etched the board.

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    RobA24El_mot

    Answer 2 months ago

    I used to do PCB's with one of these. I would draw each layer separately on mylar then get a photographic reduction done by the PCB shop. There is also a resist ink that can be drawn on copper if you are wanting to etch your own artwork. I would draw at about 2x scale, adding a scale to the drawing so the photographic reduction would give an accurate PCB. The pen is about 1 mm wide so I think the resolution would be about a mm.

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    El_motRobA24

    Reply 2 months ago

    Ok, thanks.
    Does not make sense then, at least for me.

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    Dragonavionics

    Question 2 months ago

    Hi Chris (or indeed anyone)
    When I finally dug out the plotter it turned out to be a ColourPro 7440 model, It doesn't have a separate USART chip and it looks like the main processer does all the work.
    I have checked out the diagram and the Tx and Rx pins seem fairly straight forward to identify but I am a bit confused about where the CTS line from the Sparkfun should attach.
    I have attached the relevant diagrams, would you be willing to take a look and perhaps give me some advice, Im reluctant to try trial and error as space around the chip is a bit tight for multiple attempts.
    many thanks in advance

    Best wishes
    Chris