Using a plotter with RS-232C input with a voltage source (block diagram sought)

Thread Starter

richard3194

Joined Oct 18, 2011
179
Hi. I have a vintage X-Y plotter (IWATSU SR-10). It has a RS-232C input. I'd like to use it, to plot a varying DC voltage wrt frequency so I can plot the selectivity of IF stages of receivers (and stuff) . Can someone please suggest a possible block diagram for a potential system I can employ? And suggest specific IC components? Perhaps this is something for a microcontroller? Just to get me started. Thank you.
 
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Jon Chandler

Joined Jun 12, 2008
1,053
As a first pass overview, you'll need ADCs (analog to digital converters) for each axis, a microcontroller to read the ADC values then convert those values into an (X, Y) coordinate, and then convert that value into code the plotter understands. Not a trivial task, especially since different brands of plotters require different commands. There's no one-chip plug and play solution.
 

Thread Starter

richard3194

Joined Oct 18, 2011
179
Mmm, seems it could be quite comlicated. It takes HP-GL commands. Might be better to simply record DC voltages on my PC and use a USB to RS-232C converer. Oh, that would work if I was printing CAD files and the like. What about recording on my PC the DC voltage as a tone of varying intensity (or frequency) and using that as input to PC run software that converted effectively tone loudness or frequency to an X position that the plotter could understand?
 
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MrChips

Joined Oct 2, 2009
30,823
It appears that you want to build a frequency spectrum analyzer.
Plotting on paper would be too slow and inconvenient in many respects. What you need is real-time display updates.
If you don't mind doing some programming on a PC, I would suggest trying out MATLAB or Octave. Octave is free. MATLAB isn't free.
 

Thread Starter

richard3194

Joined Oct 18, 2011
179
What I'd like to do is use a sweep generator to feed an intermediate frequency amplifier and have the y plot respond to it's output. The x plot would be set to some timebase. I could employ the audio card in my PC to save a copy (as a file) of the IF output. The trick then is get movement of the y plot (HP GL commands) as this file is played. I'd then have a plot of the IF selectivity response, I'm no expert, but that appears to require software that converts a digital signal (possibly an audio signal, but not neccessarily so) ) into the appropriate HP GL command(s). The IF output is going to be tone modulated, or simply a DC voltage.
 
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MrChips

Joined Oct 2, 2009
30,823
Why are you fixated on plotting on a sheet of paper?
Presumably you will have to do this many times if you are attempting to align some kind of frequency dependent circuit.
Why not display the graph on a computer screen?
 

Thread Starter

richard3194

Joined Oct 18, 2011
179
I see this project as a challenge. And a means of re-using this vintage plotter. I've got a thing about plotters. If it's going to be too involved, I'll not bother. It's true though about the fact that to align an IF amplifier, you need an oscilloscope.:)
 

MrChips

Joined Oct 2, 2009
30,823
I had similar thoughts about repurposing a HP pen plotter. I wanted to turn it into a PCB milling machine.
Then I acquired an inexpensive CNC router. It does the job well. The plotter is still in the junk pile waiting for another bright idea.
 

Thread Starter

richard3194

Joined Oct 18, 2011
179
Given what has been said, the plotter is only going to work as intended, that is by printing graphics files. If that is true, the task turns out to be that of creating graphics files from a voltage input sent to the PC. In theory that requires a program, and I'm assuming, that would be no easy task. I don't think there is any application that will produce the needed graphics file, unless an oscilloscope app can be made to. I mean a regular oscilloscope can be made to show an IF selectivity curve. Is that the answer then to install an osilloscope app on the PC? It's looking like it. Simply print the screen. Not what I was seeking to do, but seems I'd be able to plot my selectivity curves. I'd need to install the HP printer drivers, because the plotter is treated as if it is a printer (I'm assuming). Of course I could use a regular printer. :)
 
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Jon Chandler

Joined Jun 12, 2008
1,053
If you're certain the plotter speaks HPGL, and you have some means to get data into a PC, the HPGL language isn't too difficult to understand.

There's 4 major commands (speaking from a foggy past):

Move (x, y) – MOVE the pen from its current location to position (x, y) with the pen UP

Draw (,x, y) – DRAW from the current location to position (x, y) by a straight path with the prn down.

PENUP, PENDOWN – lifts or lowers the pen.


Getting to a real-time x-y plotter is very non-trivial. Using saved data to print a graph or picture (line art) isn't too difficult.
 

ericgibbs

Joined Jan 29, 2010
18,869
I could employ the audio card in my PC to save a copy (as a file) of the IF output.
Hi Richard,
If you could save the data as a wav file, use LTSpice to read and plot the wav file, then take a screenshot of the plot.

The Audacity Program can convert most audio file type to a wav format.

E
 

Thread Starter

richard3194

Joined Oct 18, 2011
179
Sticking to the example of taking the output from an Intermediate frequency amplifier: What seems to be at least a first step of any possible solution is to first data log the output. I think maybe there is a consensus on that. Strictly of course, a WAV would be overkill for the sample rate. Noted: LTSpice can read and plot the data in a WAV file. I wonder how low the sample rate could be. Probably something like 10 samples a second. That would depend on the generator sweep rate. I'll do some more diggin'. I may be better employed for the moment just seeking to get my plotter working with graphics files, then considering how I might be able to plot a data log. EDIT: It's looking like a USB voltage data logger would come in handy, along with the software. My potential solution, which is a solution more to do with data logging and the convenience of associated software, rather than making the plotter work (I mean, originally the task of mine, was to discover how to get a plot without simply processing a graphic file or screen shot, which is obviously what the plotter can do).
 
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