A serial communications device comprises a controller to obtain digital diagnostic data representative of operational characteristics of the serial communications device, memory to store the digital diagnostic data and at least one interface, including an interface to serially communicate data via a serial cable. The serial communications device also comprises a signal controller configured to encode the digital diagnostic data onto a serial data signal for transmission via the serial cable by adjusting signal levels of the serial data signal while preserving original data in the serial data signal. Encoding the digital diagnostic data includes serializing the digital diagnostic data, determining a series of signal levels for the serialized digital diagnostic data based on a signal encoding map, and adjusting signal levels for the serial data signal based on the determined series of signal levels.