At Lawrence Radiation Laboratory small computers are used on-line with high-energy physics experiments. When the experimenter desires to estimate the relative value of the experimental data, the necessary analysis can only be performed on a large computer. Hence, the need arose for a fast two-way Data Link between one or more small computers and a large computer. The lack of interrupt facilities in the available large computer and the necessity of using twisted-pair telephone lines both posed unusual design problems. The main components of the Data Link are interfaces for the two computers, a buffering and error-correcting system, and a transceiver system using 4-mile-long telephone lines. A unique demand and response system both maintains synchronization and supports a highly accurate errorcorrection system. Control words communicate word count, transfer direction, etc., between the computer programs. Special high-reliability pulse-train signals transmit critical information, such as "error" and "end of record" between the interfaces. The data-link system provides a reliable and satisfactory solution in its present context to the problem of interfacing small data-gathering computers to a large computer for the analysis of a comparatively small quantity of data at infrequent intervals. Should this type of service become more popular in the future, the link can be readily modified to accept changes in the 6600 operating system (such as using a dedicated peripheral processors for all link data) or interfacing to a time-sharing computer devoted to link data handling.