This document describes how to connect the 5V PECL outputs of the CY7B951 or CY7B952 to 3.3V PECL inputs, and also how to connect the 5V PECL inputs of the CY7B951 or CY7B952 to 3.3V PECL outputs of some framer chips. CY7B951 and CY7B952 are clock and data recovery chips for 155.52-Mbps and 51.84-Mbps signals.