In this article, an asynchronous serial transceiver that is capable of transmitting and receiving an auxiliary data stream concurrently with the primary data stream is described. The transceiver instantiates the auxiliary data stream by modulating the phase of the primary data without affecting the primary channel transmission and recovery mechanisms. Standard receiver interoperability is maintained since the auxiliary data appear as primary data jitter. Analysis of the proposed transceiver and considerations of the system parameters are included and can be used to determine how such an auxiliary channel is implemented. The proposed transceiver with the auxiliary channel can be widely used in many data communication applications such as for transmitting signatures for authentication or other control information, steganography, or additional data in an existing serial link. A prototype transceiver, implemented in a 65-nm CMOS process, demonstrates the proposed concept with an 80-Mb/s auxiliary channel in a 2.56-Gb/s asynchronous serial link.