A borehole telemetry system including a surface telemetry module, a downhole telemetry module, and a multiplexed datalink between the surface and downhole modules capable of transferring data alternately between an uplink in which date is transferred from the downhole module to the surface module and a downlink in which data is transferred from the surface module to the downhole module; wherein the data link can be switched between a first configuration in which a relatively long time is assigned to the uplink and a relatively short time is assigned to the downlink, and a second configuration in which a relatively long time is assigned to the downlink and a relatively short time is assigned to the uplink. The first configuration can be used to transmit data when logging with downhole tools and the second configuration can be used to program logging tools.