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Here is my latest EtherCAT card, the EaserCAT 6000, with 12 digital inputs, 4 digital outputs, and one frequency counter input for THCAD output.
 
It's the one down in the corner. There are three RTelligent ECT60 EtherCAT stepper motor drivers as well.
The inputs are limit switches for X, Y, Z, e-stop, floating, break-away and arc-OK.
Plasma-on is controlled by output 1.
There is one frequency counter input. Come to think of it it wouldn't be difficult to add another frequency counter for ohmic sensing, but I don't think my plasma cutter supports that, a Herocut 55X.
More details are here, including all schematics and source code.
github.com/MetalMusings/MyOwnEtherCATDevice
I haven't been cutting a whole lot with it yet. The worry has been the frequency counter that had some interference, seems to have been due to poor cable choice. With that sorted, voltage reading is now stable and THC works just fine.

Hello everyone,
As it's my first message here, let me first introduce myself.
Feel free to skip this part

So... I'm a 40+ years old electronics engineer. I'm currently self-employed, and in long term partnership with a French CNC/3D-printer vendor. I'm using Linux since kernel 2.4.18 (~20+ years), and currently using Linux exclusively.
Most of the time, I develop custom solutions to specific problems, that includes board design, firmware development, computer software, web interfaces... I also worked on few CNC-based 3D printers. (few keywords for the curious: PIC, AVR, C, C++, python, MySQL, php)
I'm quite familiar with Beckhoff and Bosch-Rexroth, as these are the most common in my area (there's Siemens too, but Siemens being Siemens, I just can't...).

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Read this as a quite general discussion about PLC synchronization, I'm currently working on a project to eject and replace any closed-source Windows-based software for CNCs (i.e. no more TwinCAT and IndraWorks, for runtime and development), and my feelings are telling me that I can easily replace existing PLC tasks (that are synchronized) with some python script (that won't be synchronized). Note, you may also think of C as a replacement for python, it's not the point here.

I'm planning to use python as a base to write PLC programs, but as I see it, it won't be synchronized with servo-thread.
Does anyone has any arguments on the critical conditions that any PLC program running has to be synchronized with servo-thread, or the opposite ?
As far as I'm concerned, I don't see anything that has any real impact of both the servo-thread and PLC task are not synchronized (read it as: I has no side-effect for the servo-task to get an action from the PLC on next cycle), but I may miss some cases.

Ofc, I don't talk about critical procedures like homing or emergency stop. I'm talking about procedures like, let's say T3 M6, this has no obligation to be synchronized, so having a python task that run independently is fine, as long as it has the correct checks and waits, and acknowledge the procedure after everything is done.

I build a small pyvcp side panel and did some gcode coding to build a test program. Not finished yet, but I think it will be handy for testing.

Random values and one can change the limits on the go via the panel.

Sorry for the bad video and picture quality. 
Video