Modbus/TCP PN communication

Hello,

I'm trying to make communication between a 315-2 PN/DP and MSA Gasgard XL.

I want to read out 40010, and 40020 from GasGard, it works sometimes, but most of time the connection does'nt  build up. 

The parameters i using are:

• Start address: 9
• Lenght: 10
• Recv_Time: 5s
• Conn_Time: 5S

I did playing with Recv and Conn Time and it effected the rate of succesful connections, but it still drops too often.

Also i tried to read only 9 and 19, but then it didn't read 19.

Anyone knows how could i optimize the connection?

Thanks