Acorn drive interface question

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cncian
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Re: Acorn drive interface question

Post by cncian »

cnckeith wrote: Thu Oct 14, 2021 2:26 am screw terminals are open collector, they use the power you provide 5,12,24 vdc .
see this post.
viewtopic.php?f=63&t=1699
Hello Kieth! The schematic shows +5VDC being fed into pin 24IN of terminal H4 (for inputs 1-4). Are the optocouplers in the input circuit happy at this voltage?
eng199
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Re: Acorn drive interface question

Post by eng199 »

cncian wrote: Thu Oct 14, 2021 6:43 am Hello Kieth! The schematic shows +5VDC being fed into pin 24IN of terminal H4 (for inputs 1-4). Are the optocouplers in the input circuit happy at this voltage?
No, the optocoupler inputs require 24V, as you pointed out.

It looks like kfedd modified the schematic to demonstrate 5V pul, dir, and en. He missed a few things during the change.
Richards
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Re: Acorn drive interface question

Post by Richards »

Let's review how an opto-coupler works in a typical stepper drive. The input of an opto-coupler is visualized as a photo-diode. A photo-diode is is current operated. That means that when a specified amount of current flows through the photo-diode, the photo-diode is bright enough to activate its companion part, the photo-transistor. The voltage is not important as long as there is enough voltage to supply the required current and not so much voltage to exceed the voltage specs of the opto-coupler. The manual does not list the maximum voltage, but is shows how to use 24VDC, so 24VDC is less than the opto-coupler's maximum voltage.

Page 4 of the Stepperonline manual states 4.5V-5V for voltage HIGH and 0-0.5V for voltage LOW with a minimum pulse width of 2.5 microseconds (400,000 steps per second). Running at 400,000 pps is not normally recommended. Test that speed carefully before assuming that the manual is giving best case data. Page 7 of the Stepperonline manual shows which value resistor to use if you use 12V or 24V instead of the recommended 4.5-5V.

The manual specifies that the photo-diode works with 7 to 16 mA, 10 mA being typical. The manual shows that if you use 12VDC or 24VDC, you must use an external resistor to limit the current. It specifies a 1K resistor if you drive the photo-diode with 12VDC or a 2K resistor if you drive the photo-diode with 24VDC. No external resistor is required if you drive the photo-diode with 5VDC.

As Keith said, the H2, H3 headers are Open Collector. That means that YOU supply voltage to the Anode side of the Opto-Coupler (+) and then connect the (-) side to Step or Direction on the appropriate H2 or H3 terminal. The Acorn pulls (sinks) current through the photo-diode. The reverse biased diode shown in the Acorn manual is a safety device to protect the Acorn if a surge occurs at the H2-H3 terminals.

I've used my DM542 stepper drivers with the H2-H3 terminals and with a DB-25 breakout board. As long as I set the pulse per second rate to 100,000 or 200,000, those drivers worked very well for me. Currently, I have those stepper drivers connected to a DB-25 breakout board on my Taig Mill.

Some people blame sub-par performance on Open-Collector circuits. I tend to blame Opto-Couplers. A typical Opto-Coupler is not a high speed device. Because it does not turn on instantaneously or turn off instantaneously, it rounds the corners of the step pulse. Too much rounding at too high a pulse rate can cause indeterminate signals. Set the pulse rate in Wizard to 100,000 pulses per second. If you have access to an oscilloscope, you can plainly see the step pulse's distortion at high speeds.
-Mike Richards
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