9ZXL1231E & 9ZXL1251E Clock Buffers Renesas Electronics 9ZXL1231E and 9ZXL1251E Clock Buffers are 2nd generation enhanced-performance 12-output DB1200ZL differential buffers for PCIe Gen1-4 and UPI. These Renesas Electronics 9ZXL1231E / 9ZXL1251E buffers are pin-compatible upgrades to the 9ZXL1231A and 9ZLX1251A clock buffers offer a much-improved phase jitter performance and increased system security features. The 9ZXL1231E / 9ZXL1251E buffers offer fixed external feedback, which maintains low drift for critical QPI/UPI applications. Typical applications include servers, storage, networking, and Solid State Drives (SSD).