Excuse me if I'm pedantic but in ModBus TCP there is no master and no slave but clients and servers. That's not that unimportant because in standard ModBus (RTU) you really can have just one master that controls all it's slaves but in ModBus TCP you can have several clients asking the same server although usually you have the same setup with just one client and several servers.
The Link to the Kepware is: https://www.kepware.com/en-us/products/kepserverex/
Why do you want to communicate with this device in ModBus? I wasn't even able to find ModBus in the supported protocols but it speaks MQTT, SNMP and a lot of other protocols, all of them easier to use for string transmission than ModBus TCP.
#include "Mudbus.h"
Unfortunately your choice of library (you forgot the link to it: GitHub - luizcantoni/mudbus: Automatically exported from code.google.com/p/mudbus) is not the best. The quality is quite weak, no limit checks and the like, don't use this in a productive environment.
My goal is share one string of 10 char like "ABCDEFGHIJ" with the Kepware.
What will be the real content of this string? I'm quite sure you won't send the start of the alphabet to the KepserverEX.
As you are saying, if i use one register i can share one char, but nothing more... it works good if i share 1 int.
No, we say one register can hold two characters. But that doesn't help you much if the transported data is a real string.
My test is to load in Hex one char in each register Mb.R[0...9] and then in the Kepware i say to read from 400001.10 (1 to 10) and i just read the first register..
If you try to read register 40001 (register number 400001 does not exister, register numbers are 16bit) the mudbus library will probably freeze your Arduino because it will access any memory far outside the area that was allocated by the sketch. As I said, the library is quite low quality.
Please specify what you want to achieve and not how you think it should be achieved. What kind of data is the Arduino sensing? What will the KepserverEX do with that data?