TK850 Cable, TB850 Terminator, and BC810s
1 - Redundant Controllers, but NOT redundant CEX Modules.
2 - Redundant Controllers and Redundant CEX Modules.
3 - Redundant Controllers and have remote Profibus I/O. BC810 is connected to each controller and then a CI854A is connected to each controller. Each CI854 is connected to the remote IO through CI840. Should the CI854s be connected together using a TK850 cable or should each CI854 have a TB850 connected to it? What is the difference between the two configurations?
The advantage of using BC810 is the BC810 offers a way to section the CEX-bus into two independent segments. This improves availability in systems with redundant communication interfaces.
Practically it means primary (active) CPU can work with primary CI854 from opposite CEX-bus segment if CI854, which is in the same CEX segment as primary CPU, is not operational e.g. if it has both profibus connectors disconnected.
Voltage to communication interfaces is supplied from BC810 but not from CPU therefore CPU baseplate may be replaced in redundant systems with BC810 used.
If BC810 pair is used, CEX-bus shall be terminated with CEX terminators. From practical experience CEX bus may work temporary without CEX terminator however I cannot recommend that.
In addition to Kamil's answer - all above configurations are possible without BC810.
Anyhow BC810 improves availability of the system, when you have redundant CPU's - it is already explained.
In case you don't use BC810, you have to use TK850 cables to connect the CEX buses of the redundant CPU's. No TB850 is required in that case.
In case you use BC810, TK850 are not used, you have to terminate each CEX bus end by TB850.
2 pcs TB850 + TK851 are included with each BC810.
TK850 + TK851 are included with each redundant pair of processor.
So principally you don't have to care about that.
Source: 3BSE036351-600 A