  Superconducting digital circuits are a promising approach to build
packaged-level integrated systems with high energy-efficiency and computational
density. In such systems, performance of the data link between chips mounted on
a multi-chip module (MCM) is a critical driver of performance. In this work we
report a synchronous data link using Reciprocal Quantum Logic (RQL) enabled by
resonant clock distribution on the chip and on the MCM carrier. The simple
physical link has only four Josephson junctions and 3 fJ/bit dissipation,
including a 300 W/W cooling overhead. The driver produces a signal with 35\,GHz
analog bandwidth and connects to a single-ended receiver via 20 $\Omega$ Nb
Passive Transmission Line (PTL). To validate this link, we have designed,
fabricated and tested two 32$\times$32 mm$^2$ MCMs with eight 5$\times$5 mm$^2$
chips connected serially and powered with a meander clock, and with four
10$\times$10 mm$^2$ chips powered with a 2 GHz resonant clock. The meander
clock MCM validates performance of the data link components, and achieved 5.4
dB AC bias margin with no degradation relative to individual chip test. The
resonator MCM validates synchronization between chips, with a measured AC bias
margin up to 4.8 dB between two chips. The resonator MCM is capable of powering
circuits of 4 million Josephson junctions across the four chips with a
projected 10 Gbps serial data rate.
