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pH Meter Sensor Modbus Table: https://ibb.co/g2Zdna 

Siemens S7 1200 Mb_comm_load: https://ibb.co/nO7QYF

Siemens S7 1200 Mb_master: https://ibb.co/dKJona

Siemens S7 1200 Database Table: https://ibb.co/dcw8na

Hi, It could be a problem of endian type of readed data and how they are then stored in your DB.

I suggest to swap bytes in every word you get from ModBus and then reassemble your DB.

I also suggest to use a PC based modbus master application with a USB/RS485 converter and perform some try to see how data is formatted when you perform reading.

Usually, when I Use Com module, I use a Word Array to store raw data and then I deserialize on my DB or on the destinations the data should go. This way I can operate some operations (like swap) before putting data in the final place.

In my experience, in many application I needed to swap every data >8bit.

Hope this was helpful.

1)  When you say that the process value doesn't make sense, it could be a word/byte swap issue with floating point, or it could be that it isn't the pH process value you expect, it could mV (or maybe something else?).  

Apparently the process value is in registers (4)2092 and (4)2093, but there are multiple process values, depending on what the unit value in registers (4)2090 and (4)2091 is.   In your 1st example, the process value is pH, the 2nd example is mV.

Since these are not writeable registers, the unit value in registers (4)2090 and (4)2091 must be set by some other means than Modbus, because a Modbus master can only read those registers.    That means of configuring the process value must be resident on the pH meter. That could be a firmware/keypad setting or DIP switch or a jumper.   Or it could be the model where a pH meter is pH, but an ORP meter is mV.

You need to pay attention to what the unit value in register (4)2091 is.  Is it hex 1000, the unit value for pH?

2) With memory costing almost nothing nowadays, why vendors of Modbus slaves do not offer different number formats for any given value in input or holding registers amazes me.

One would think that for pH meter that the extra dozen bytes necessary to show the pH value in both floating point formats, short integer and long integer would be trivial, and you could poll the register with the format most easily handled by your master.