I just tested with ZWO 120mm. Did not perform actual guiding as I have to mount it into a guide scope first, but I used manual pulses and ALL pulses behaved exactly as expected. So on my side, it looks like it's working fine.

OK, I think I know where I can find a USB2 version of the ASI120MM. I'll check that one after I get back from my trip. If that also works on my system then the problem would be related to the USB3 version.
Thanks for checking this at your end.
Jo

I am back home for one day and repeated the test with the ZWO ASI120MM (USB2 version) after firmware update to the newest version. I can confirm your results, that USB2 version of the ASI120MM works as you describe using ST4 signal input. All deflections are proportional and prompt in all 4 directions. However, the camera is useless for guiding, as it does not acquire an image, not when using Capture, Loop or live preview. Either nothing happens, or it crashes my Pi3 to the point where I need to restart. That pretty much matches what the ZWO website has to say about that USB2 version of the ASI120MM. By contrast, the USB3 version of the same camera (ASI120MM-S) acquires images just fine in still capture or live stream mode and it will guide if guide pulses are sent through the mount, but not through ST4.
Looks like these are separate problems affecting the USB2 and USB3 versions of that camera.

I have been using the ZWO ASI 120 MM (USB 3 version) as my guide camera in KStars/INDI on my raspberry PI for the past year or so. For the last couple of months I have been using PHD2 running on the PI with an ST4 cable connected to the mount. It has been giving excellent results. Before that I used the internal guider in KStars. The only issue that I had when I did it that way was that the lag time in sending it over wifi was making the guiding worse. It now works very well with PHD2 running on the PI, it keeps the guiding down typically to an arc second or two max error with an rms of less than 1 arc sec on my Losmandy mount. I should probably also note that I have always used a powered USB3 hub attached to my PI to connect to my devices.

I have the 120MM USB2 version and I can confirm that it's working just fine including guiding, capture & live view from RPI3/StellarMate. Yesterday, I updated the ZWO ASI SDK. Not sure if it will help or not, but you can give it a try.

I can confirm the behaviour reported here. I have an ASI120MC-S (USB3) and use it on an HEQ5-Pro that is connected to Ekos with an EQDir cable (EQMod driver).

When using the EQMod guiding within Ekos, guiding works fine although the guiding errors are rather large, around 5” either way on both axes and more. Quite consistently.

When using the camera for guiding, calibration never stops. The mount just starts drifting and will never revert direction. I need to physically shut it down and restart it (not just Ekos/KStars, the mount as well) before it starts responding again.

When using PHD2 for guiding with on-camera, guiding goes very well. The errors generally remain within 2.5” though occasionally (every 5 to 10 frames) a large step (7.5” or more) is taken in RA followed with a reverse step tow or three frames later.

All guiding is done with 4.0 sec frames.

Thanks, wvreeven. Exactly what I am seeing in all respects, except I don't need to shut down the mount to recover, just Ekos and KStars. However, I think I have found a way to make the guiding through the mount, using the internal guider, more consistent. Can you try playing with the proportional gain in the Control Parameter settings and set proportional gain for the RA axis to 200% and DEC axis to 100%. In my case, that brought the deviations down to <2" on an Atlas Pro, 70% of the time <1" with guide pulse lengths rarely exceeding 400 ms, 200-250 ms most of the time. If you can improve further, please let me know. But with those settings, it is quite workable at the focal lengths I am using (~800mm and at the abysmal seeing I had in my light polluted and smog-filled inner city area. In my case, I used 2" frames, not ideal, so I will change to 4" frames next time. I thought I had set the guider to 4" frames, but it reverted on its own without me noticing. Thanks for pointing that out.

Jasem, can you post instructions on how to perform that update on the Pi3? I installed the latest indi-asi update today, but the USB2 ASI120MM still won't capture any images, although the camera is recognized by Ekos. However, when I try to capture an image, the timer either counts down to 0.01" and then stops, freezing Ekos to the point where I cannot disconnect the camera or stop Indi, I need to force quit Ekos and restart it, or it will loop endlessly without ever rendering a frame.
I am running all services directly on the mount, nothing remotely on my desktop, I merely connect to the Pi3 via VNC.

I set the Proportional Gain as you indicated but to no avail as you can see in the screenshot. I'm not sure if my guide scope is focused well enough though. I ordered a new one with a better focuser so maybe that'll help.

Nevermind, I figured it out.
indilib.org/devices/ccds/zwo-optics-asi-cameras.html
I had flashed the firmware, but I had a) not selected to OVERWRITE the old firmware and b) used the ASI120MM.icc file, not the ASI120MM-compatible.icc file to change the firmware.
Now that I flashed the firmware with the -compatible version, the USB2 version of the ASI120MM captures and loops images just fine.
That does not resolve the original problem for which I opened this topic, though, i.e. that ST4 guiding through the ASI120MM-S (USB3 version) does not work.

Give PHD2 or LinGuider a go and see if the results are different.

Did you try sending pulses manually via the INDI Control Panel like you did with the USB2 version?