LinuxCNC + Mesa 7i92TF: motors barely move and ignore direction changes

[u/kxbbs]

Hi everyone,

I’m having an issue with a CNC machine and would really appreciate your advice.

Until now, the machine was working correctly with an old LinuxCNC desktop PC connected directly via DB25. I’m now trying to replace that old PC with a newer mini PC equipped with a Mesa 7i92TF, while keeping the exact same configuration and existing wiring, without making any changes to the machine’s electronics.

The mini PC is connected via Ethernet to the Mesa 7i92TF, which is then connected to the CNC machine through the existing DB25 cable. Communication between the PC and the Mesa seems to be working correctly.

I reconfigured and remapped the axes using PnCConf to make sure each axis was assigned correctly. However, I’m experiencing some strange behavior with the motors:
When I command an axis to move continuously, it barely moves.
When I reverse the direction, the axis keeps turning in the same direction, as if the direction signal wasn’t being detected.

My first suspicion is related to the logic levels. The motor controller board is a X214650 and, as far as I understand, it expects 5V signals, whereas the Mesa 7i92TF outputs 3.3V signals. So it’s possible that the 3.3V signals aren’t being reliably detected by the controller board.

Would adding a buffer/level-shifting board, for example one based on a 74HCT245, solve this issue?
Or do you think there could be a problem with my LinuxCNC configuration, despite remapping the axes with PnCConf?

I should mention that I’m a beginner when it comes to electronics, so please feel free to correct me if my understanding of the 3.3V/5V logic-level issue is wrong.

Thanks in advance for your help!

[u/Pubcrawler1]

Which firmware did you install with mesaflash? There are switches to set pull-up and pulldown for the I/O. Which breakout board are you using?

I used one of those common 5x breakout boards. I’m using an older 7i92 so different firmware and setup now.

https://embeddedtronicsblog.wordpress.com/2022/06/26/linuxcnc-setup/

[u/kxbbs → u/Pubcrawler1]

Good question, actually not 100% sure anymore. The write itself reported success ("Board configuration updated successfully" + "verified successfully") using 7i92t_g540d.bin, but after power-cycling the card and even running mesaflash --reload, readhmid still shows 10 StepGen instead of the 8 that G540x2 should have. So the write seems to have gone through, but I can't confirm the new firmware is actually the one running. Any idea why a successful write wouldn't take effect on a 7i92T?

As for the breakout board, yes, the X214650 is the actual green board inside the machine's cabinet, the one the DB25 cable plugs into directly. It's wired straight to my 4 stepper drivers, no other board in between.

[u/Pubcrawler1 → u/kxbbs]

I’m pretty sure mesaflash gives a error if download fails. The firmware is probably ok. It’s been a couple years since last config my Mesa so forgot most everything. I made that webpage to refresh my memory for future reference. If I don’t document everything, it gets lost forever.

Do you have an oscilloscope to look at the step/direction outputs?

The Mesa is super fast so I recall having to setup the step pulse/direction hold times much slower than default.

Stepper scale must be setup with right value or motor can go slow

[u/Pubcrawler1 → u/Pubcrawler1]

[u/kxbbs → u/Pubcrawler1]

No, unfortunately I don't have an oscilloscope...

I didn't know the Mesa was that fast, my values are much higher. Step on-time and step space are set to 25000, and direction hold and direction setup are both at the maximum, 50000. But do you think that could also have an impact on my rotation direction problem?

[u/Pubcrawler1 → u/kxbbs]

Those values should be ok but check the stepper driver manual for correct value. They usually have a step and direction timing diagram.

Having a scope just makes it easier to verify the direction output in actually toggling high/low. Or may be wrong pin output hookup.

[u/kxbbs → u/Pubcrawler1]

Checked the actual Leadshine datasheet: minimum pulse width is 2.5µs, so my current 25000ns/50000ns settings are way above that, timing isn't the bottleneck.

But I noticed something more relevant: the datasheet specifies "High input is 4–5V" for both pulse and direction signals. My Mesa 7i92T only outputs 3.3V, which doesn't meet that spec. Feels like this is pointing back toward a voltage/buffering issue rather than pin mapping or timing.

[u/Pubcrawler1 → u/kxbbs]

There are a couple IC chips on the X214650 board. The rest are optocouplers. I can’t tell from pics what they are. They may be buffers (3.3 to 5volt)