What is pin 1 on the DB25 connector used for?

HomePage Forums HobbyCNC PRO 3 & 4 Axis Board Assembly What is pin 1 on the DB25 connector used for?

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  • #18999
    Peter Gregson


    I want to optically isolate the inputs to my HobbyCNC Pro 4 board. I have searched for photovoltaic opto-isolators but they do not seem to be fast enough. They all have bandwidths less than 25 KHz which is not nearly enough to pass high-frequency step/direction signals.

    To get suitably fast isolators, I need to use isolators that have powered output amplifiers. As a result, I need to provide the output circuits of all isolators with the voltage to which the HobbyCNC Pro 4 inputs are pulled up, 5 volts IIRC.

    The assembly instructions that I received with my HobbyCNC Pro 4 kit does not specify any use for Pin 1 on the DB25 connector. Some controller boards use Pin 1 as an “enable” signal, but I was thinking of using it to provide 5 volts to the isolator board. Is this doable, or is there a better solution?

    Thanks, and
    Best regards,




    Let’s start with Pin 1 on the D connector.

    • PRO Rev 1 boards, Pin 1 (and pins 14, 16 & 17) are NOT used, however these pins are available via solder pads, near the 25-Pin D connector. Most often these are used as ‘output’ to drive spindle control relays, mist/lubricant pumps, etc.
    • PRO Rev 2 boards, similar with ONE exception: pins 14, 16 & 17 are NOT used, and these pins are available via solder pads (these pads are on 0.100″ spacing, so a standard pin-header can be used), near the 25-Pin D connector. Difference – Pin 1 can be either a generic output OR you can add a small ‘solder-blob-jumper’ to turn this into an enable signal.

    The optoisolation:
    I’d never considered using them for step and direction. Since these signals are “inside” the control box, between a computer and the PRO board, I’ve never even suggested putting opto’s there.

    Where I do recommend opto isolation, is anytime a wire leaves the safety of the control box (particularly limit and home switches), where there is a possibility of induced noise or (worse) a high voltage being applied on accident.

    See my post: To Shield or Not To Shield (your wiring)


    • This reply was modified 1 month, 1 week ago by BrianV.
    Peter Gregson

    Hello Brian,

    Thank you for your prompt answer. It is helpful.

    Just a couple of clarifications, please, as follows:

    1. I appear to have a Rev 1 board. Is there any way that I can run a jumper to some part of the HobbyCNC Pro 4 circuit from this pin so as to gain the Enable-pin function?

    2. Is the Enable pin on the Rev 2 board an input or an output?

    3. I like to provide isolation between different power domains whenever possible. My concern here is that I would like to limit the damage if, for some reason, one of the SLA7078MR chips were to fail. By using optoisolators, I limit the damage to the HobbyCNC Pro 4 in that eventuality and protect the controller board from damage. Since my controller board requires me to make an adapter card, this would be an opportunity to put optoisolators on all signals to/from the HobbyCNC Pro 4 as well as on all limit-switch, home switch and tool touch-off points.

    4. How much power, if any, can I draw from the +5VDC pad? I could mount a heatsink on the LM317. Would it be preferable to use a separate regulator for my adapter card?
    Thanks for your help.

    Best regards,


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