I'm not familiar with OnStep, but I am a bit (unfortunately) familiar with ttyUSB0 issues. Whenever you have more than on serial device, I believe it's wise to use symbolic links rather than the specific device filenames that linux seems to randomly assign.
So, first, if you setup, and you're having this issue, figure out which device your OnStep was assigned. E.g. in a terminal type:
ls /dev/ttyUSB*
and perhaps you'll see /dev/ttyUSB0 and /dev/ttyUSB1.
Then you know your device is one of the two, and you can change the port (in your picture above) to the right device (test the both) and see if things are working.
If this doesn't work, sorry, I guess I misunderstand and ignore the rest of this post. If it does, continue.
The above is a short-term solution, though, because you probably don't want to do this every time you boot up your RPi.
Assuming though that this solved your problem in the short term, the next step is to permanently assign symbolic names to your OnStep device.
I have only done this on Raspbian, so I can't say whether the advice below will work on your OS (not sure what you're using) but this is what I do on Raspbian:
I have two serial devices (focuser and mount) that are randomly assigned, one to /dev/ttyUSB0 and one to /dev/ttyUSB1. To avoid this randomness,
I create symbolic device names for them by adding the following four lines to /lib/udev/rules.d/99-observatory.rules
# MoonLite Focuser
SUBSYSTEMS=="usb", ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6001", ATTRS{serial}=="A601WGWD", MODE="0666", SYMLINK+="focuser"
# EQMod Mount
SUBSYSTEMS=="usb", ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6001", ATTRS{serial}=="FTA663FB", MODE="0666", SYMLINK+="mount"
You'll always want to use "usb" and "0666" above, but the other magic strings in quotes (after idVendor, idProduct, and serial) come from the commands below:
(a) the output of the 'lsusb' command gives you vendor and product:
> lsusb
...
Bus 001 Device 008: ID 0403:6001 Future Technology Devices International, Ltd FT232 USB-Serial (UART) IC
...
Bus 001 Device 006: ID 0403:6001 Future Technology Devices International, Ltd FT232 USB-Serial (UART) IC
...
(b) you can get the value needed for "serial" from the below command:
# I'm sure the moonlite is connected to USB1 right now
udevadm info -a -n /dev/ttyUSB1 | grep '{serial}'
ATTRS{serial}=="A601WGWD"
ATTRS{serial}=="3f980000.usb"
(c) Of course, the SYMLINK name (in my case "mount" or "focuser" is arbitrary, but once you have chosen it, you
set your port for your OnStep to that, e.g. /dev/focuser