A wearable computing device comprising: a band configured to couple the wearable computing device to a wrist of a wearer; an electro-optical sensor of a communication cable, the electro-optical sensor arranged to translate received light into an electrical signal; an optical emitter of the communication cable, the optical emitter arranged to emit a diagnostic reference light, wherein the electro-optical sensor is configured to translate the diagnostic reference light and a communication light into the electrical signal; an alignment mechanism arranged to align the electro-optical sensor with the optical emitter of the communication cable and align the optical emitter with the electro-optical sensor of the communication cable; a logic machine; and a storage machine arranged to hold instructions executable by the logic machine to: during a first mode, assess a physical parameter of the wearer of the wearable computing device from the electrical signal; during the first mode, instruct the optical emitter to emit the diagnostic reference light; during the first mode, recognize the electrical signal translated from the diagnostic reference light; during a second mode, instruct the electro-optical sensor to receive the received light via the communication cable and recognize the electrical signal translated from the communication light; and during the second mode, extract communication data encoded in the electrical signal, wherein the wearable computing device is configured to sense connection of the communication cable, and automatically switch from the first mode to the second mode based on sensing connection of the communication cable to the wearable computing device. a band configured to couple the wearable computing device to a wrist of a wearer; an electro-optical sensor of a communication cable, the electro-optical sensor arranged to translate received light into an electrical signal; an optical emitter of the communication cable, the optical emitter arranged to emit a diagnostic reference light, wherein the electro-optical sensor is configured to translate the diagnostic reference light and a communication light into the electrical signal; an alignment mechanism arranged to align the electro-optical sensor with the optical emitter of the communication cable and align the optical emitter with the electro-optical sensor of the communication cable; a logic machine; and a storage machine arranged to hold instructions executable by the logic machine to: during a first mode, assess a physical parameter of the wearer of the wearable computing device from the electrical signal; during the first mode, instruct the optical emitter to emit the diagnostic reference light; during the first mode, recognize the electrical signal translated from the diagnostic reference light; during a second mode, instruct the electro-optical sensor to receive the received light via the communication cable and recognize the electrical signal translated from the communication light; and during the second mode, extract communication data encoded in the electrical signal, during a first mode, assess a physical parameter of the wearer of the wearable computing device from the electrical signal; during the first mode, instruct the optical emitter to emit the diagnostic reference light; during the first mode, recognize the electrical signal translated from the diagnostic reference light; during a second mode, instruct the electro-optical sensor to receive the received light via the communication cable and recognize the electrical signal translated from the communication light; and during the second mode, extract communication data encoded in the electrical signal, wherein the wearable computing device is configured to sense connection of the communication cable, and automatically switch from the first mode to the second mode based on sensing connection of the communication cable to the wearable computing device.