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This article presents a modular approach for testing multigigahertz, multilane digital devices with source-synchronous I/O buses. This approach is suitable for integration with existing ATE and can provide more than 100 independent differential-pair signals. We describe a specific application with 32 lanes of PCI Express, running at 2.5 gigabits per second (Gbps) per lane, and 32 data channels of HyperTransport, at 1.6 Gbps per channel. The differential source-synchronous nature of these buses presents difficulties for traditional (single-ended, synchronous) ATE. We solve these problems by using true-differential driver and receiver test modules tailored for the specific I/O protocols. We satisfy a further requirement for jitter tolerance testing by incorporating a novel digitally synthesized jitter injection technique in the driver modules. The modular nature of our approach permits customization of the test system hardware and optimization for specific DUT test requirements.