MODB4H Working

Hi,

can any one Explain me the how MODB4H Block work,
Basically I am using the S7-400H System As a Server & Client is one Modem which talk with GPRS service, My problem is the Client side is only one port of TCP/IP they will assign only one I/P address for taking & my Side there is a 2 CP 443-1 Which is compatible with this MODB4H Block, Now my client as no facility to change the I/P address in there system Because if My 1 CP get Fail it will Switch over to Other CP 443-1, & work with modem

Please Explain, as this Support is only available in Germany it was difficult to contact then From -06.00 GMT time

In this System both CP are Sending the Information to Client from CP Simultaneously or It will send one by one when one CP get fail?

& if My Client is not able to Change the I/P, is there any other way to work this by doing some modification in Logic or Hardware without any extra cost.

& How to read this by using the Modbus Simulator, so I can test the my Block is working & Modbus Simulator getting the same data.

Which setting I have to do in this Simulator Like Device ID ? & other Setting also