i have 1 im 153-2 and three fdc 157-0 in series

IM 153 FD C 157 fdc  157 fdc 157 and i have 3 pa bus comming from field

the problem is 1 and 2nd FDC are working alternately , one active at a time and the third one is idle 

but when i connect all  3 pa bus on 1st FDC 157-0 it work or if i power off the 1st fdc second works 

third remain idle every time   

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Split from DP PA COUPLER.