The bus error LED lights on the interface module in this case because the DP master expects other modules. In the case of defective modules or incorrect parameters this behavior might be welcome
The reserve modules are implemented when specific functions are to be added to a system. For this reason, the parameters for the final configuration of the system are assigned, but reserve modules are slotted instead of the input/output modules. The "Option Handling" function is provide for the Hardware Configuration to reserve the addresses required.
Proceed as follows to configure option handling:
If, when you remove one I/O module, the ET 200S station is to maintain cyclic data communication, you must select the option "Operate with setpoint <> actual configuration" in the Object Properties of the DP station. If you do not select this option, then when you remove one module, the whole station fails (including call of OB 86 in the DP master).
More information on option handling is available in the manual "ET 200S Distributed I/O System". The manual is available in Entry ID: 1144348 .