Modifying an Old HP7475a Plotter to Work Over USB

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.

Comments

[Bobby Bland]

Hi
I have a HP 7470A with RS232. I saw this last night and ordered an adapter, thinking it would use the same board, but no. I can't find a diagram, but here is a picture. Any ideas on which chip I need to connect to?
I already have a 7570A, but I hadn't found this hack, so I gave up on getting to work and swapped out the motors and board, it is now a 2D 3D printer, but I have to run everything through Mach3, so this would be much easier.
Also, on your set up, can you change the paper size to custom, or is it still limited to A4?
Many Thanks for this, and for any help with the 7470A.

[chrismajda]

Hi,
my sincerest apologies for replying so late.

I have found the technical manual for your type of plotter.

I believe that you need to connect to U13.
Refer to pdf page 26, 27 and 24.
It looks like it is the same chip as mine.

Kind regards
Chris

[jeffstyle62]

Any advise to update an HP7550A?
Same way?

[xtalgrwr]

Excellent stuff!

[ertugrulbeyaztas]

Hi Chris,

Firstly thanx for this great project which brings the old tech back to future...

I have some questions if you can help me with

1- i am bit confused. Your sparkfun module is converting the usb signal to RS232 or to UART ?

Because, if it is rs232, then you could directly connect it to the connector with no solder. But from your solder, i see that you solder them to UART and you also mentioned that the module is usb to rs232 converter.


2-i have HP7550 which has RS232, HPIB and i will try your method with that one. Hence, if you have any documentation etc for HP7550, i would really appreciate that.

3- i also have a Mutoh IP210 which also has the RS232 port and is based on HPGL. I believe i can apply the same method to this one. Some documentation for this one would really help me, too.

[molnarszabolcs89]

Hi there!
I guess I have a software question, but Im not sure. I have a hp7475A since a few days and more or less I think I managed to set it up (I found the right port and got every driver, Inkscape) already start to plot some drawings. But I have an issue that every time when I execute the draw (even on a very simple draw like a circle or a word) it starts to draw properly but on some point the pen start to do zig-zags and then stops in the middle of the line. Usually it does with arcs or the letter "S".
Does anyone had the same problem, or has some solution for it?
I have already tried every possible setup on the plot page.
Thanks in advance!
Cheers,
Szabolcs

[datoo786]

Hi,
I am in the UK and wondered if DB25 to DB9 Cable would do the job. Attached are links for the relevant cables:
https://www.amazon.co.uk/C2G-Female-DB25-Modem-Cab...

https://www.avpartsmaster.co.uk/p-56096-c2g-2m-db9...

https://www.ebay.co.uk/itm/293041780863?chn=ps&nor...

https://www.ebay.co.uk/itm/313390367682?chn=ps&nor...

Thanks!

[chrismajda]

Hi,

if your computer still has an original serial COM port with the correct voltage levels then it should still work.

The problem that I had was that nowadays only USB ports are available one can only use virtual USB COM ports.

Hope that helps !

[ffaat]

Do you think this would work with a PL2303HX based USB-TTL UART programmer rather than the Sparkfun module?

[chrismajda]

Sure it will, if all the drivers for it are installed on your PC.
I just had a Sparkfun module from a previous project, but any USB to UART device will work.

[El_mot]

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

[michels57]

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

[DomenicoL10]

Salve ,mi puoi dare delle indicazioni il mio traccia solo righe senza senso che programma usi grazie

[michels57]

hello ; adressez vous directement à l'auteur de l'article c'est lui le concepteur de ce programme génial

[russ.hughes]

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.

[RobA24]

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.

[El_mot]

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

[DomenicoL10]

salve, vorrei sapere come hai fatto a stampare su plotter serve comunque un driver per plotter e so che il drive e fino a windows XP.grazie

[DomenicoL10]

Salve e' possibile sapere che versione inkscape ha usato non riesco a caricare sia l'immagine da errore

[brightled]

very nice indeed,I have one of these and the converter board,you can also use this module to connect a serial port printer to a single board computer.