Hi I have V570 PLC [ET1]Master and [ET1]21 Slaves [10xV570-11 V280] as the ET1 cards are no more available. If I upgrade my V570 PLC Master to Unistream (unistream communicates with ET1 and ET2 cards) then slave replacement of ET1 I can use ET2 card [ ET1 cards don't communicate with ET2] hence the unistream master which communicates with ET1 or ET2 . As I am using UDP in Master V570 -then replacement to unistream I will not have to re-program slaves. Kind regards Vic