Improvement of interconnection retimer

A test mode signal is generated, which comprises a test pattern and an error report sequence. The test mode signal is transmitted on a data link that comprises one or more expansion devices and two or more Subdatenverbindungen. The test mode signal to be transmitted on a particular one of Subdatenverbindungen and used by a receiving device to identify mistakes on the determined Subdatenverbindung. The error report sequence is to be encoded with error information to describe error status of Subdatenverbindungen in the plurality of Subdatenverbindungen.