  Interconnect properties position superconducting digital circuits to build
large, high performance, power efficient digital systems. We report a
board-to-board communication data link, which is a critical technological
component that has not yet been addressed. Synchronous communication on chip
and between chips mounted on a common board is enabled by the superconducting
resonant clock/power network for RQL circuits. The data link is extended to
board-to-board communication using isochronous communication, where there is a
common frequency between boards but the relative phase is unknown. Our link
uses over-sampling and configurable delay at the receiver to synchronize to the
local clock phase. A single-bit isochronous data link has been demonstrated
on-chip through a transmission line, and on a multi-chip module (MCM) through a
superconducting tape between driver and receiver with variable phase offset.
Measured results demonstrated correct functionality with a clock margin of 3 dB
at 3.6 GHz, and with 5 fJ/bit at 4.2 K.
