I had successfully read LT400 through Modbus potocol. The TTL to RS485 coverter I was use showed in the attached photo.
I am using an external 12V battery for the testing setup.
I burnt out one of the logic pin somehow, I could not find out the reason. maybe the 12V meet the logic pin accidentally.
I sent a private message to you with the link to download my LT400 Modbus test programme, hope that it can help you.
My 3G datalogger read LT400 through SDI12. I directly connect 3.7 Li battery to the LT400 power. It worked very well so far.
I remember I tried once connect 3.7 Li battery to LT400 and read it through Modbus. It worked, but I am not testing it hardly so I am not sure of it.
So far my SDI12 datalogger working well and I am not putting time modify it to Modbus.(I am now trying to read Soil Instrument TLT6 MEMS tilt sensor by Mayfly datalogger)