Interfacing Playbook: Modern ↔ Legacy#

This is the capstone for the Retro & Legacy section. Everything else in this section — era constraints, electrical assumptions, legacy signaling, aging behavior — leads here: actually connecting a modern microcontroller, SBC, or USB device to an older system without destroying either one.

This page is the systematic playbook: how to characterize an unknown interface, identify what can damage modern hardware, select the right protection and conversion topology, and bring it up safely on the bench.

• Domain Identification Checklist — Voltage/current ranges, impedance, grounding model, isolation requirements, and fault scenarios to characterize before designing any interface.
• What Can Kill an MCU? — Overvoltage, negative voltage, ground offset, inductive kick, and backfeeding conditions that destroy CMOS I/O pins.
• Common Adapter Building Blocks — Level shifting, buffering, galvanic isolation, protection components, and protocol conversion patterns.
• Grounding & Shielding Patterns — Single-point grounding, shield termination, and chassis bonding strategies for legacy interfaces.
• Worked Mini-Patterns — Conceptual interface topologies for MCU ↔ RS-232, 4–20 mA, relay/contact, and unbalanced audio.
• Bench Bring-Up Checklist — Step-by-step procedure for first connection, power-up sequence, and functional test of a modern-to-legacy interface.