control stepper motors with two controllers

Hello. I have a 3-axis milling machine controlled by parallel port (typical Chinese blue card) for my use, it works very well for me. 


I also have a controller with Arduino to control stepper motors that I use for quick tasks and not having to use the PC with LinuxCNC. 


I would like to know if there is any way to connect the two controls to control the motors with LinuxCNC or with the Arduino controller without having to change cables. 

I have to say that my intention is not to use both controls at the same time.

You'd have to implement a multipole switch, one pole for each signal that needs swapping over.

Or if you only need to switch low current, a demultiplexer could be used for inputs and a multiplexer for outputs, but that's going require you designing a circuit.

Open collector transistors are a possibility but either way its going to require some work.

JT wrote:

A plug for the step and direction signals is the simple way. With the power off just unplug the PC then plug in the Arduino

JT

That's the first thing I thought of, but I don't like the idea of ​​changing connectors all the time.

The topic of making a circuit sounds good, but I don't have much of an idea.

Would anything happen if I connected the negative signals from the PC and Arduino together, and the positive signals directly from an external 5V power supply with the PC board turned off and vice versa?

No don't do that. You would be feeding 5v into the computer, and breaking all the smoke valves.

I'd design the system so that all that has to be changed is the cable to the breakout board, so all that is required to change is one db25 connection.

So you have a situation that the grbl board or PC controls the machine via the breakout board. So one connection to change. The breakout board is powered by a separate power supply.

Why not just use an old DB25 2 way parallel port switch?
Just make sure that there is a permanent (non-switched)
common ground between all systems:

www.amazon.com/CableWholesale-Way-Switch...Female/dp/B000I9B2K0

Often you can simply put some (schottky) diodes in series in the step and direction signals in between both "step generators" and the stepper motor drivers, but it depends on the actual hardware you have.

Another option that can work is to put some 74HC series logic in between and use that to switch signals. For example with a 74HC541 you can switch 8 digital signals on/off via the "G" inputs. You can then use a simple double throw switch to switch the "G" inputs of the TTL IC's.

In a more elaborate setup, you can control the "G" inputs with some microcontroller and you can make it as complex as you want or need.