Connecting BOB one axis to 2 stepper drivers

[Noplace]

Hi everyone,

So I acquired this BOB recently to conduct my initial testing http://www.cnc4pc.com/Store/osc/prod...roducts_id=270 and you would notice it uses RJ45 for the axis connections.

in my design I have two motors for the Y axis so I was wondering can I just connect one axis port to 2 drivers?
do i need a 'buffer' circuit or something or can I just split the wires via a terminal block or something?

it has one more port for the 'A' axis but I dont think that would work.

thanks!

[Clive S]

Hi everyone,

So I acquired this BOB recently to conduct my initial testing http://www.cnc4pc.com/Store/osc/prod...roducts_id=270 and you would notice it uses RJ45 for the axis connections.

in my design I have two motors for the Y axis so I was wondering can I just connect one axis port to 2 drivers?
do i need a 'buffer' circuit or something or can I just split the wires via a terminal block or something?

it has one more port for the 'A' axis but I dont think that would work.

thanks!

You don't say which software you are going to use ie Mach3 or Linuxcnc etc. Remember this is only a breakout board to make it easy to get the pins from the parallel port.
If you are using two motor for one axis then you would use say the X or Y and the A.
So in your case Y and A (It is more usual to call the long axis the X in the UK) Then in the software like Mach3 you need to set it up as slaved. ..Clive

[Neale]

Best thing to do is connect the two drivers to the Y and A BOB outputs, and then configure Mach3 or LinuxCNC or whatever you are using to use both ports for Y. That also allows you to do proper homing of both motors as well (with Mach3, anyway).

[Noplace]

I'm still debating which to use (mach3 or linuxcnc) however linuxcnc looked a bit more complex. but thanks guess I have to utilize the A port

[Clive S]

I'm still debating which to use (mach3 or linuxcnc) however linuxcnc looked a bit more complex. but thanks guess I have to utilize the A port

If you want to use two motors slaved then use Mach3 ..Clive

[Noplace]

Guys sorry to re-ask the same thing but I would prefer if you could just explain why I can't split the pul/dir signals coming from the BOB to two drivers? what is exactly the harm I'm not seeing?

[Clive S]

Guys sorry to re-ask the same thing but I would prefer if you could just explain why I can't split the pul/dir signals coming from the BOB to two drivers? what is exactly the harm I'm not seeing?

Well in simple terms think what would happen if one motor failed, you would then rack the gantry. ..Clive

[Neale]

I'm not sure of the exact parameters of the hardware you are using, but typically putting two driver inputs in parallel will not guarantee that each driver will reliably see each pulse from the BOB. BOB outputs usually have an opto-isolator, and so do driver inputs. An opto-isolator in crude terms is an LED which needs a certain current to turn it on. That pulls down the voltage across the input terminals which might then be too low for the other LED to operate in parallel. If you really want to operate two opto-isolators from a single signal, you could wire them in series so they have the same current through them. But that means a higher voltage from the BOB would be needed and what this should be gets a bit more complicated. It's really a lot safer to use the X+A approach - this is exactly why it is there, and also allows independent homing of the two motors (as you cannot guarantee that they will stay exactly synchronised for ever, however carefully set up).

[Noplace]

Thank you, I appreciate you explaining it thoroughly :)