dchartier

Hello Quangdc;
Basically, the difference between different serial communication protocols used on PLCs is mostly a matter of history and cutommer preference.
In the late 70s industrial communications were starting to be used in plants, and Siemens amongst others started to provide communication processors that permitted users to program these. The basic implementation in S5 PLCs was called RK512, and was a point-to-point connection; a secondary layer named 3964R was added to control the data flow and ensure more robust comms between partners. At that time, Siemens used S5 PLCs;at present, the CP 340/341 in the S7-300 family still support these comm protocols, mainly used to exchange data between S5 and S7 processors.
In chapter 2.5 of the following manual you will find a description of the 3964R procedure:
http://support.automation.siemens.com/WW/view/en/1137332

At about the same time, Modicon developped a serial protocol they named Modbus, that allowed (if needed) multi-point connections between a single master and several slaves. The electronics behind it was simple and relatively inexpensive, the programming was simple and uses a small list of Modbus Functions (each one is dedicated to a specific register type of the Modicon PLC addresses); it became popular with many manufacturers of PLCs and instruments, and with users. It developped into a de facto standard for serial comms in an industrial environment.

To satisfy the demands of their clients, Siemens developped a driver for Modbus on their equipment, so 3rd-party PLCs and instruments could be connected to their system. To make it work, the code has to convert Modbus addresses (based on Modicon PLC addressing) to Siemens standard DBs addtressing. Since the Modicon CPUs of that era did not support floating-point registers, Modbus only uses 16-bit registers, and the programmer has to specify the number and position of registers used for a value that needs a 32-bit representation ( a DWord or Real value). Which explains the detailed requirements for the user when programming a Modbus exchange over Siemens hardware.
In the following link you will find the CP 341 Modbus slave manual, with a short description of the Siemens implementation of the Modbus protocol:
http://support.automation.siemens.com/WW/view/en/1218007

Hope this helps,
Daniel Chartier