A host device and an accessory exchange information (e.g., commands and data) via an intermediate device. The host device and accessory can each connect to the intermediate device. The host device can exchange commands and data with the intermediate device, while the accessory device can serially exchange data with the intermediate device. The host device and the accessory can also “tunnel” information to each other through the intermediate device, by packaging the tunneled information as a payload of a command recognizable by the intermediate device; the intermediate device can repackage and forward the payload. In some embodiments, the intermediate device can control serial communication parameters between the intermediate device and the accessory in response to commands received from the host.