Mercury PCM-200 B - 78-8M0127879. The Mercury PCM-200 B (Part #78-8M0127879) is an essential power and data communication module engineered for advanced Mercury Marine propulsion systems. Designed to deliver precise, real-time data control between your helm and engine, this module helps ensure optimal system responsiveness and integration across your onboard electronics.. Key Features. OEM Precision Communication Module - Enables accurate signal transmission across Mercury propulsion systems. Pl