Sorry for sticking my nose in again but I think you are missing the obvious - the comms link is working ok else you wouldn't be getting the output from EQMOD as I beleive the Serial code for the Open command is the same. Its the handshaking protocol (as KNRO is hinting at I think) that's failing (sorry if I am wrong) for the Synscan legacy code. Have you tried the AZGTI driver (non wedge) ?

If you Logic Analyser I would have thought thats a voltage problem - as I stated earlier the AZSWGOTO wont work with 3.3v has to be 5v - unlike my AZEQ6 which is ok wih 3.3 and 5v.

I have used the FDTI USB with success never a problem (so long as its the correct voltage version) - the Prolific works but is very "iffy" weak signals ? - suppose thats why its cheap.

If you can please try your cable set up in a Windows Environment using Synscan App Pro and see if that is able to control the mount. Unfortunately the last time I looked there is no Python API for Indi for Windows (might be wrong again) but I believe you can use SWIG to convert the C code but KNRO would know if thats true. As I said , and this was true for an Arduino WiFi ESP32 project I did, SWAZ GoTo doesn't like being started up first . Any how just a thought !

I will get back under my rock now