Stable development, elegant design, and high-precision output. As soon as upon a time, Hewlett-Packard made test-and-measurement gear that was beloved by working engineers. Certain, drop a type of infants in your foot and also you had been taking a look at a damaged toe. However that’s a small worth to pay for reliability and a few character constructing. So after I lately got here throughout an early Nineteen Seventies HP 7041A X-Y recorder whereas clearing out my mother and father’ attic, I knew that I simply needed to see if I might get it up and working once more.
What’s the distinction between an X-Y recorder and a plotter?
X-Y recorders had been designed to chart information from analog devices in actual time, versus plotters, which plot the outputs of digital computer systems. The fundamentals are the identical: There’s a mechanism to boost and decrease a pen, and motors to maneuver the pen throughout the floor of a web page. The massive distinction is {that a} plotter sometimes makes use of digital instructions to manage the pen’s movement, whereas the movement of a recorder’s pen is managed by analog voltage inputs.
So all I wanted to do was feed the HP recorder the proper voltages, and I might draw something I needed! Only some obstacles stood in my method.
The recorder interface permits Python code written on a Raspberry Pi [bottom right] to create analog alerts which can be stage shifted up and down to satisfy the completely different ranges required by the plotter’s x- and y- axis. James Provost
The primary impediment was getting the factor residence, as a result of my mother and father’ attic was in Ireland and becoming a 13-kilogram, 48-by-36-by-17-centimeter behemoth into my suitcase for my flight again to NYC wasn’t occurring. About US $300 in packing supplies and transport charges solved that downside, and I used to be nonetheless somewhat forward of the sport financially in comparison with acquiring an identical classic recorder from eBay, and method forward of shopping for a brand new Bantam Instruments NextDraw plotter with an identical drawing space. (To be truthful to Bantam, its plotters are modern plug-and-play units that may deal with a a lot wider vary of pens).
The second impediment was that the recorder didn’t work. As soon as I bought it residence, I found that the y-axis could possibly be adjusted manually utilizing a knob on the management panel, however the x-axis was lifeless. The mechanism for elevating and decreasing the pen made a weak clunking sound and barely twitched.
“Had I simply spent some huge cash to ship residence an HP-model boat anchor?”
I opened up the case, which was fast work with a Phillips screwdriver—no bizarre safety screws, no glue, no fragile plastic, no yellow stickers warning that your guarantee can be voided and also you’d in all probability get boils when you dared to look inside. On the fantastically laid out printed circuit boards inside, I noticed some resistors that had been clearly not a part of the manufacturing unit set up. Had been they modifications or repairs? If the latter, had they been profitable or had I simply spent some huge cash to ship residence an HP-model boat anchor? And I had no concept how the 40-pin interface connector on the again of the recorder was alleged to be hooked as much as management alerts.
All these issues had been solved when I discovered the recorder’s handbook on eBay. Oh, what a handbook. Not simply working directions, however detailed illustrations for taking the recorder fully aside and placing it again collectively. It listed each part, with images of the circuit boards and digital schematics on gatefolds. And it included instructions for modifying the circuitry when you needed to measure completely different voltage ranges than the manufacturing unit settings–extract a resistor right here and there and solder in some new ones. This defined the resistors I’d noticed. In a world the place licenses and software program locks forbid folks from simply plugging in a replacement component, the considered a serious firm encouraging its prospects to interrupt out a soldering iron is thoughts boggling.
Quickly, the applying of some instrument oil, silicone grease, and Kimwipes had the x-axis and pen lifter working once more. I connected a variable energy provide to the recorder’s connector and slowly introduced the voltage up as I watched the pen holder transfer in response. This allowed me to find out the recorder’s enter ranges, which turned out to be 0 to 1 volt for the y-axis, and 0 to five V for the x-axis, overlaying 25 and 38 cm of movement, respectively, with about 0.2 millimeter accuracy.
The following step was to construct an interface. Though microcontrollers typically have digital-to-analog capabilities built-in, there’s typically just one true analog output pin. I wanted two. A method like pulse-width modulation would let me output an analog-ish voltage on a number of pins, however sometimes with only 8-bit resolution or 256 distinct voltage ranges. I wanted not less than 1,900 ranges to match the recorder’s accuracy.
The Raspberry Pi makes use of an enlargement “HAT” and two 12-bit digital-to-analog converters commanded by way of I2C connectors to create 0- to three.3-volt management alerts. These are shifted to 0- to 1-V and 0- to 5-V ranges utilizing a voltage divider and amplifier respectively, in addition to a logic gate used to transform a 3.3-V digital sign that raises and lowers the pen to a 5-V stage.James Provost
So I purchased two $5 Adafruit MCP4725 breakout boards. These are 12-bit digital-to-analog converters—every offering 4,096 distinct ranges—managed over an I2C serial connection, and two boards can share the identical I2C bus. I related them to a Raspberry Pi Model B+ I fished out of a drawer by way of a $6.60 SparkFun Qwiic HAT.
The DACs put out a sign within the vary of 0 to three.3 V, so I despatched one board’s output by a voltage divider to scale it right down to 0 to 1 V for the y-axis. For the x-axis, I fed the opposite board’s output by an LMC6484 amplifier, powered by a 5-V pin from the Pi, to carry it as much as 0 to 4.8 V—not fairly the total vary, but it surely’ll do till I give you a extra refined interface. I introduced a sign to boost and decrease the pen out from one of many Pi’s GPIO pins, passing it by a 74LS08 AND gate IC used as an inexpensive and cheerful 3.3- to 5-V digital stage shifter.
I then wrote code on the Pi to place the plotter by its paces, utilizing parametric equations written in CircuitPython to attract swirling hypotrochoids and different geometric curves. In the end, it must be attainable to have the Pi settle for and translate instructions written in a plotter-control language comparable to HP-GL. Then I’ll be capable to plot vector graphics and textual content from drawing software program like Inkscape. However for now, I’m pleased to simply have my recorder buzzing away beside me, hale and hearty and constructed to final.
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