Using an audio cable (or similar) to create unidirectional communication from a secure server

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http://serverfault.com – I'm interested in exploring how a semi-offline Root CA can be used to update CRLs to the sub CA's. This answer on Security.SE mentions using an audio cable for this purpose. Doe anyone have details on how an Audio cable (or similar) can be used to create a unidirectional path of communication? Since I'm a .Net programmer, I'm also open to code samples, drivers, etc that may enable this scenario. (HowTos)