Modbus communication with S7-400

We have here one application using S7-400 communicating with RKI INSTRUMENTS in Modbus mode. We have 7 instruments connected in this net. The problem is that some times some of this instruments lost the communication. I would like to know what could be causing this problem. The communication is lost for around 4 or 5 s.

I think the problem might be synchronization. We used here the blocks SFB 12 and 13 in S7-400 to make this communication. If someone have similar problem please tell what you recommend to solve this problem.

Antonio Luiz
antonio.lreis [at] arcelormittal.com.br