Standard: two screw terminals labeled + and -. See www.Modbus.org The problem with your idea is that often RS-485 requires termination resistors to prevent communication problems from reflections. So with screw terminals we can just insert the resistor. If anything make it plug-in. That way it is a “plug” but one I can easily wire up. With DB-9, RS-485, etc., we can’t do any of that. We are back to making goofy connectors by hand. The pins require special tools and often fall out. DB9 was used because DB25 was huge. DB25 existed for mouse immunity with every other pin grounded back in the teletype days with massive noise problems if the day. Connections were still soldered so a hollow pin made lots of sense at that time. Plus DB9 is already associated with RS-232. RJ-45 is Ethernet. Reusing these (which some vendors do) just creates confusion. If I see DB9, there’s a 95% chance it’s RS-232. There are vendors who do this (Allen Bradley) and it causes nothing but problems. People are constantly trying to plug an Ethernet device into the RS-485 “DPI” ports. Now there are exceptions and this is also why AB and others do it. A common configuration is where you have say an RJ-45 or RJ-11 or RJ-12 6P6C connector bug two pins carry 24 V and ground abc 2 carry RS-485/Modbus. This is a “remote keypad” port used on a lot of VFDs on the market. It has the convenience that the port can double as a programming or networking port and that if the factory cable breaks you can get a new one anywhere. Answer from PaulEngineer-89 on reddit.com