C-10 BOB Home and Limit Switches Connection

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    Default C-10 BOB Home and Limit Switches Connection

    I am new to this but learning quite a bit. I have spent some time with the Mach3 manual, the CNC4PC documentation and have read post of other people on their preferences on how to best set the Limit and homing switches. The more I reed the more confused I have become. I know it is something simple, but I am missing something here and it is not quite clicking for me. I understand that I do not have to set up separate switches for the homing and limit, my confusion lies on the best approach. I also understand that Mach3 can distinguish based on operation if the same switch is used for homing or as a limit switch. My problem is the connection.
    I have induced two possible scenarios on how to connect. Keep in mind that I would like to have at least one free input to connect a Z probe. I believe both setups allow for that. Which setup do you recommend? Do you suggest another. Any help is greatly appreciated.
    Thaks

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    Default Re: C-10 BOB Home and Limit Switches Connection

    Ok I see nobody is answering the question. It does not matter I figured it out after a week of tinkering with it. I selected the second diagram and setup where I used one pin input for each axis for the homing, ++ and -- limits of each axis. It homes now nicely with limit switch protection. So if anyone ends up having the same dilemma, and that is to know how to best connect limit and home switches, you could find this post useful. Thanks



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    Default Re: C-10 BOB Home and Limit Switches Connection

    Hi pianori

    I didn't see your first post but,

    I find it easier to decide how to connect the BOB inputs by following block diagrams in the manual

    revised C10 BOB inputs
    C-10 BOB Home and Limit Switches Connection-c10-input-circuit-jpg


    John



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    Default

    Quote Originally Posted by john-100 View Post
    Hi pianori

    I didn't see your first post but,

    I find it easier to decide how to connect the BOB inputs by following block diagrams in the manual

    revised C10 BOB inputs
    C-10 BOB Home and Limit Switches Connection-c10-input-circuit-jpg


    John
    I have seen the block diagrams in the manual. Thanks. The issue for me is to understand what is the preffered way to connect the limit/homing switches? What I have now works fine for me. That is one pin per axis for min, max and homing on NC switches. The question is now if I need a pull down resistor like the Mach3 manual shows, or is not needed because the C10 has an internal resistor already? All my wiring is shielded to the BoB ground. The same resistor question applies for the probe for the Z axis. If you know the answers to those questions that would help a bit. Thank you.



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    Default Re: C-10 BOB Home and Limit Switches Connection

    Here is my setup working.





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    Default Re: C-10 BOB Home and Limit Switches Connection

    Hi pianori

    the preferred home / limit switch wiring will depend on various factors such as -

    1) is it a hobby self built machine or commercial industrial machine -
    the hardware used will influence your choice of limit switch (N/O or N/C) and if you need to use pull up or pull down resistors

    2) what's the function of the limit switch - protect the operator , protect the machine or just signal the position of part of the machine to the control software


    cables that are subject to repeated flexing will break so the control will not see a normally open limit switch being operated
    from this point of view series connected normally closed limit switches are safer

    on the other hand if a cable is crushed and the wires short circuited ,
    the control will not respond to a normally closed switch when its operated (and is now open)


    Mach 2 & 3 was developed to use one or more PC parallel printer ports to provide the required I/O signals to machine
    the printer port ground being the common terminal
    inputs had their own pull up from +5V that can provide about 1.6 mA when the pin is grounded

    the original IBM printer port was built using standard TTL 5V logic IC's - on new motherboards its part of a 3.3v super IO chip
    this can be a possible source of problems

    modular industrial programmable logic controllers very often have 24V inputs and outputs
    ( a wide range of options are available )

    it looks like CNC4PC's C31 breakout board has compatible 24V I/O

    I expect other members can add more examples


    John


    PS

    I've just seen your last post



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    Default Re: C-10 BOB Home and Limit Switches Connection

    Thanks for the info, but I am not sure that I understand what is your answer for the resistor. In the Mach3 manual it says to use a 470 ohm resistor, a generic answer. however, on a diagram on the cnc4pc it shows a 100 ohm resistor. That is why I am confused. Also, I am using the C-10 BoB and not the C-31 that you mentioned on your post. So my question refers to the appropriate resistor for the C-10, if any at all. Do you know if it is best to use a resistor for the C-10 BoB for the homing switches?
    Thanks



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    Default Re: C-10 BOB Home and Limit Switches Connection

    Hi pianori

    in the early hour of this morning I forgot to give any reasons for the value of the pull up / down resistor to use

    low value resistors like the 100 ohm pull up resistor in one of your diagrams
    can help to give some immunity to stray signals picked up by your wiring
    also speed up the switching time as the pull up resistor and cable capacitance forms an RC network that will slow the positive going transition when the limit switch opens

    the upper limit will depend on the circuit details and the IC's used

    on some systems you may find a 1000 ohm pull up resistor used when an original 74xx series TTL device has been used

    if you replace the old IC with a pin compatible CMOS device - eg 74ACTxx ( ACT = Advanced, CMOS , TTL compatible)
    you could increase the pull up resistor to 10000 or even 100000 ohms provided it does not make the circuit susceptible to stray pickup on the cables

    ( very high value pull up resistors are very often used with conductive rubber key board switches)

    with a the C10 that you can only select to have 5 pull up or 5 pull down resistors on the inputs
    ( the on board pull up / down resistors are 4700 ohm )

    if you have selected to have 5 pull down resistors but need a pull up for a touch probe for example
    you can over ride the 4700 ohm pull down resistor with a lower value pull up

    for the 74ACT series devices used on the C10
    a logic high input needs to be more than +3.7V
    logic low is less than +1.3V

    disregarding any currents in or out of the logic IC input

    as the two resistors form a potential divider you need to choose a value that gives you a valid logic high

    an external 1650 ohm pull up resistor will result in a 3.7v volt drop across the C10's 4700 ohm pulldown resistor - right on the minimum for a logic high !!!

    using an external 470 ohm pull up with the 4700 ohm pull down results in a voltage of +4.54V

    a 100 ohm pull up results in a logic high = 4.89V


    John

    Last edited by john-100; 07-04-2016 at 07:07 AM. Reason: add details of the effects external resistor value


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    Default Re: C-10 BOB Home and Limit Switches Connection

    Thank you for your answer.
    So to recap, you are saying that I need a 470 ohm resistor like the Mach manual suggests and ignore the cnc4pc diagram indicating of 100 ohm resistor for a NC pull down system? Also, how are you deriving to the values of "...an external 1650 ohm pull up resistor will result in a 3.7v volt drop across the C10's 4700 ohm pulldown resistor..." . Are you using Oms' law? Can you give an example of deriving the numbers?
    Thank you



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    Default Re: C-10 BOB Home and Limit Switches Connection

    Hi pianori

    the value of the pull up resistor is not that critical and I would not change it in a working system

    using a 100 ohm pull up resistor has a slight advantage in that the logic high is a little higher but
    using a 470 ohm resistor is OK as the logic 1 level will be approximately 4.5V which is about 0.8V more than the minimum

    yes I did use ohms law after looking up the data for the 74ACT245 used in the C10 break out boards circuit

    with the boards jumper set to pull down the input pins to ground via the 4700 ohm resistors
    assuming I can ignore any current going into the IC's input pin

    the minimum current through the pull down resistor = 3.7V (from the 74ACT254 data sheet) divided by 4700 ohms = 0.787 mA

    knowing the voltage across the pull up resistor will be 5 - 3.7 = 1.3V and the current through it is the same as the pull down resistor

    the maximum value of the pull up resistor = 1.3 V / 0.787 mA = 1650 ohms

    I then re calculated the voltages using a 100 ohm and a 470 ohm pull up resistor


    quite often the values of components can have a wide tolerance and the circuit still works as designed

    over the weekend I have been working on a wartime civilian radio from 1944
    most of the resistors are to a + or - 20% tolerance and are easy to replace with 5% resistors
    but the electrolytic capacitor are values not made today
    the old +50% -20% 8uF 425V capacitors have been replaced by modern 10uF 450V capacitors

    John



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C-10 BOB Home and Limit Switches Connection

C-10 BOB Home and Limit Switches Connection