Help wanted: MESA board selection, parallel vs ethernet and THCAD understanding.

So I’ve done some poking around and I just want to make sure I’m heading in the right direction as I never used mesa or Linux before. I’ve used mach4 and been advised that this isn’t not a good choice for the CNC plasma table that I am building.
The machine I am building is using closed loop stepper drivers; it will have up two Z axes 1 being a machine plasma torch the other being a (spindle/other) I would like to add a rotary axis at some point bring my total axis up to 6 however I’m happy to swap out axis control so I really only need a 5 axis board.

I have after some investigation determined that a 7I76 driven via a 6I25 (sold as a plug and go kit). Is a reasonable choice; however I am very ignorant and am making assumptions which may be very misinformed and are largely based on information from people who had a seller’s bias. These assumptions are as follows:

1) Parallel port is faster possesses less latency.
2) Parallel port is more HF/noise resistant

That’s it, that’s my whole justification for opting for parallel port instead of Ethernet. If this information is in accurate are there some advantages I should consider for using Ethernet?


Some THCAD questions:

The other thing I am unsure about is the need for a THCAD-5 High isolation A-D accessory. My inverter has a 50/1 voltage divider output (although I can change this to 20 or 30) with my typical cutting arc voltage of 110v I think. I assume the whole purpose of the THCAD is to isolate the board from the high voltage supply. I presume this is necessary as the divide voltage output is probably not isolated and we do not wish to release trapped smoke of the mesa board.

A.) Assuming it was isolated or we were for some reason 100% certain it would not cause trapped smoked to be released we could simply run our divided signal into an analogue input on our control board? Or is there another reason that a the THCAD should also be used in this situation? (This question is framed to better understand the THCAD, I fully intend to use it regardless).

B.) I also figure I would improve the voltage accuracy/resolution of my THCAD by dividing my voltage by 30/1. As 110/30=3.6v out of 5v instead of 110/50 = 2.2v, or is it really not going to make that much difference given the resolution of the THCAD anyway?

C.) The THCAD documentation mentions “When used for plasma voltage monitoring, the 5V
and 10V versions require an external high voltage resistor” is this just referring to the voltage divider I already have in my machine or is it wanting a current limiting resistor as well? Or is this just for extending the input range? Reading the data sheet it seems that the THCAD-5 has an internal 100k voltage divider and is measuring the current expressed across that (50MA) for 5v max. Or it could be referring to the fact that in theTHCAD300 it already possesses the resistors but that they might not be suitable for HF torches and so buy getting a THCAD5 or 10 you can select a resistor that is HF suitable while also dividing to the correct voltage range? :S

Either way my machine has blowback start in both torches instead of HF start, i.e. it doesn’t even have HF capability.

Well I think that covers everything I’m currently wrestling with any insight would be greatly appreciated.
-Gruntled Raptor

1 parallel port is not faster, well it is faster than an RS232 serial port for sure, but not in any way shape or form faster than Mesa boards. Parallel can do 20000 pulses per second if you are lucky up to 50000 if you are extremely lucky to find a PC with such low latency, while Mesa can do easily 10000000 (yes 10 million) pulses per second.
2 It is at the same level of noise immunity with a Mesa 5i25 or 6i25 if they are used as a parallel port (but much faster), but it is nowhere near as immune as an 7i92 wired through Ethernet and used as a parallel port. All mentioned Mesa can do 2 parallel ports at the same time.
Your calculations for THCAD are correct, and it can withstand 500V indefinitely, if i am not mistaken.
A. the only analogue input on a PC are two audio inputs that can handle 1VPP, so pretty much useless. And yes if it is isolated you can use a voltage to frequency converter for 3$ and it will work, or you can add an opto isolator to that VF converter and have it fully isolated form the plasma, everything for under 10$ in parts and it uses a digital input, not analog, so directly to parallel works.
B. answered above.
C. No need for a resistor if it has the voltage divider.
On the topic of blowback start, does your torch have a moving copper or bras part inside, when you shake the torch can you feel something moving up and down ?
That is absolutely not your fault or issue, it is the manufacturers that slap new marketing names to everything. So far i have seen only one plasma source using blowback start from over 30 or 40 or 50 of them that i used.
But that does not matter for using it, it will work no matter what type of start it uses, even scratch start ones can be made to work with Linuxcnc with some added programming.

1. Parallel ports do have low I/O latency, but parallel port based CNC systems also require
low latency since the individual step pulses are generated by the host computer. Ethernet
connected devices have higher latency but also don't require low latency since the step
pulses are generated by hardware or software on the connected device, not the host.

2. Ethernet devices tend to be resistant to EMI due to the use of differential signalling and transformer coupling with common mode chokes at both cable ends. Parallel port based
systems with completely isolated BOBs should also be EMI resistant

A. Isolation of the plasma voltage is important because plasma sources have major issues with ground conducted EMI. The low side of the plasma voltage divider is nominally at ground (table) potential but can have short transients in the hundred of volt region. This is because at arc initiation and quench, the current step is almost instantaneous (in the picosecond region)

B. If you can set you divider ratio to 1:50 or 1:30 then either THCAD model
should work fine. If you only have a 50:1 ratio, a THCAD-5 is a better choice.

C. Yes you only need external resistors for the THCAD-10 or THCAD-5 if you are monitoring the full plasma voltage (your plasma supply has no built-in divider)
For HF torches you need external voltage divider that is protected against high voltage,
high frequency EMI so the internal divider on the THCAD-300 cannot be used, but the
THCAD-5 or THCAD-10 can be used with an external divider.

thank you for you response. yes it is definitely blowback start, the electrode is initially compressed against the nozzle (with a spring, so no shaking), the compressed air forces it back before it exits through said nozzle. it's actually very well made i was quite surprised. it is a IPT100 torch then i have a PTM-100 for the machine both are blowback. it's a jasic 80 plasma cutter. which turned out to be the cheapest bang for your buck where i am, i managed to get an 80amp machine with a 25mm clean (35mm severance) all integrated CNC control/dividers air regulator, filter, centrifugal dehumidifier hand and machine blowback torch for £1275 (a deal so good it made the other suppliers i called angry "thats a stupid price he has given you" they told me) so i tore his arm of for it... and then spent all my savings on airlines a 5+0.01 micron air filter for the system.

hmm well after reading a little more now realising that ethernet is a fine choice i decided to go and try to order a 7I95 as it has 6 axis out the box, it is however out of stock and for some reason MESA does not permit back ordering through their store?

Gruntled Raptor wrote: hmm well after reading a little more now realising that ethernet is a fine choice i decided to go and try to order a 7I95 as it has 6 axis out the box, it is however out of stock and for some reason MESA does not permit back ordering through their store?

I think the 7i95 is a new board and might have supply issues due to the covid thing.
An alternative might be the 7i76e and a 7i84 daughter card for the encoders if they are required.

We are building more 7I95s (and 7I96s and 7I76Es ) but things are a bit
slower since we have a lockdown here and are only working 1/2 time.

rodw wrote: An alternative might be the 7i76e and a 7i84 daughter card for the encoders if they are required.

7i85
7i84 is IO serial card.

PCW i don't suppose there is an approximate ETA? i would call and ask but im a about 12 hours out of sync with the timezone. im very patient and i still need to build the table so im only really concerned if its months vs weeks, as i will need to get something else to tide me over. i have encoders on the stepper and i would like to have 6 axis control so 7i95 really seems the best bet.

Its probably about a month away.

Note that unless your drive supplies feedback to the controller, (like many servo drives)
its not generally a good idea to connect to the encoders of closed loop step drives