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  1. #1

    Breakout board pull up resistors?

    I have one of the generic Chinese breakout boards like this
    https://www.ebay.co.uk/itm/274524902217
    When it is powered up but not connected to the PC, the input pins go high. This is fine except that when pin 17 goes high the relay switches on, and hence the spindle. So if the PC fails while the CNC is switched on, the spindle will start. I don't think this is safe or convenient. I assume that the BOB has pullup resistors on the inputs. Seems to me I have two options. One is to desolder the resistor for pin 17 and replace with a pulldown resistor. But I don't know where the pull up resistor is, or whether there is one for each input pin. The other is to try to pull the pin low with a lower value resistor than the pull-up, in the hope of bringing the voltage below the ON threshhold, while still allowing the PC to pull it high when needed.
    Thoughts?

  2. #2
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    Re: Breakout board pull up resistors?

    Quote Originally Posted by robertjenkins View Post
    I have one of the generic Chinese breakout boards like this
    https://www.ebay.co.uk/itm/274524902217
    When it is powered up but not connected to the PC, the input pins go high. This is fine except that when pin 17 goes high the relay switches on, and hence the spindle. So if the PC fails while the CNC is switched on, the spindle will start. I don't think this is safe or convenient. I assume that the BOB has pullup resistors on the inputs. Seems to me I have two options. One is to desolder the resistor for pin 17 and replace with a pulldown resistor. But I don't know where the pull up resistor is, or whether there is one for each input pin. The other is to try to pull the pin low with a lower value resistor than the pull-up, in the hope of bringing the voltage below the ON threshhold, while still allowing the PC to pull it high when needed.
    Thoughts?
    Those relays can be a bit of a chore. There is a jumper next to the relay, so:
    What you could do is change the jumper setting so it uses pin17 screw terminal instead of the onboard relay for spindle on/off.
    Get a seperate 5v relay with NO/ COM /NC contacts and hi/low trigger option and conect it to pin 17 output.

    When PC is off mine sits in NC position. It will not trigger unless it has signal. Spindle switch is connected to NO. It only turns on spindle if Mach activates pin17 via 'spindle on/off'' button. It will NOT come on unless pc is on with mach running.
    This will solve the problem.
    I actually have a servo drive for mine. It's setup as: Pin17 is my on/off, Pin16 is my direction control.

    Relay like this one below.
    https://www.amazon.co.uk/Electromech...732588&sr=8-19
    You can get them cheaper, this was just for quickness. Made more sense to me to buy multiple channel ones.
    Being able to choose between high or low trigger I found quite useful.




    My same board is wired exactly like in the images:
    I use seperate 5v supply into bottom pins pc5v & pcgnd. Not via the usb.




  3. #3

    Re: Breakout board pull up resistors?

    Thanks. What difference does changing the jumper make? Won't pin 17 be the same whether or not the BOB relay is being used?

    Also, I don't quite understand the hi/low trigger thing. If I wire the new relay input to pin 17, and have the spindle connected to the NO contacts, won't the same thing happen, i.e. when the BOB is powered but the PC is off, pin 17 will be high and the relay will switch on? Or is it only a change from low to high that will turn the relay on?

  4. #4
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    Re: Breakout board pull up resistors?

    Quote Originally Posted by robertjenkins View Post
    Thanks. What difference does changing the jumper make? Won't pin 17 be the same whether or not the BOB relay is being used?

    Also, I don't quite understand the hi/low trigger thing. If I wire the new relay input to pin 17, and have the spindle connected to the NO contacts, won't the same thing happen, i.e. when the BOB is powered but the PC is off, pin 17 will be high and the relay will switch on? Or is it only a change from low to high that will turn the relay on?

    Jumper= Either relay or pin operation, one or the other, not both.
    If your drivers and other bits are wired as per the pictures, (which is called common cathode) it goes like this:
    1. 5v from psu into relay modules +/- terminals to power the relays
    2. Pin 17, connected into relay (trigger). Relay jumper set low because pin17 will be a 5V negative signal, relay is default in NC position when off.
    The modules power holds it there. (stays there without power too).
    If bob is powered and pc is off it wil NOT trigger relay. When asked, PC will trigger relay to NO position.

    That's how mine acts. However, I'm not powering via usb. Unsure if that is a factor or not.
    The main issue with these 5 axis bob's is the way the onboard relay acts on startups.
    Many a time I heard it clicking away when not in use, prob why I never even bothered with the onboard.

    Now then.
    If your bob is wired common anode, which some bobs are (meaning the +/- power are switched). The pin17 signal would now be a 5V+ve one.
    In this instance the relay trigger is high, jumper needs to be set to high.
    This is where it can be useful to have the option, say if you replaced a bob.

    I have various switches that output both +ve and -ve triggers on my relay module, high and low.

  5. #5

    Re: Breakout board pull up resistors?

    There's something I'm still not getting here.
    I removed the jumper. So the relay does not come on. But pin 17 still goes high as soon as the BOB gets power, even if the PC is not on. So a new relay on pin 17 will still turn on as soon as the CNC gets power, even if the PC is off

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