It appears your interfaces are 5-volt logic rather than 3.3-volt logic. This is good information, and means the FTDI cable is proper for communication. More recent panels use a voltage closer to 3.3 V.
Your measurements all look OK except the Vcc for the PowerMax. Either the pin you think is Vcc is not that, or you made an error with your measurement. There is an exposed plated-through hole under the RS that looks like ground. You can probe this point for your ground reference (i.e. black probe) and make your measurement again. The measurement is only useful if you intend to power the USR module from the panel versus an external power supply. In either case, you’ll have to identify the proper way of powering the module for a 5-volt logic interface. I’ve quickly looked at the documentation, and the English translation leaves some room for misinterpretation. I’ll see if I can find better info on the 'net.