EDI message not created because ALE_SERIAL_CHANNEL_GET fails / table TBD56

Dear experts,

I am trying to create an Idoc in SAP ECC 6.0. To my knowledge, all EDI/Idoc settings have been done correctly but for some reason the Idoc is not being created.

Functionmodule ALE_SERIAL_CHANNEL_GET returns an error; it can not find an entry in TBD56.

Where (which transaction) do I have to fill settings so this table is being filled?

Thanks in advance,

William