A printhead apparatus, an inkjet printer system, and a method of built-in test employ a test flag transmitted through a data channel of a printhead. The apparatus includes a plurality of printheads, a data register at each printhead, and a status channel. Each printhead has a unidirectional data channel (UDC) to provide data to the printhead. The data register detects proper reception of a test flag received by the printhead through the UDC. The status channel reports printhead status information including whether or not proper reception of the test flag was detected by the data register. The system includes the apparatus and a controller to generate the test flag and receive the printhead status information. The method includes sending a test flag, and verifying proper reception thereof.