Interfacing digital logic families: Which electrical quantities must be mutually compatible when connecting the output of one logic family to the input of another (e.g., TTL to CMOS)?

Introduction / Context:
Digital interfacing requires that a driver's output electrical levels properly satisfy a receiver's input requirements. Two key dimensions define this compatibility: voltage thresholds for logic HIGH/LOW and current capabilities for sourcing/sinking without violating those voltages.

Given Data / Assumptions:

• A source device (driver) and a destination device (receiver) from potentially different logic families.
• Receiver HIGH/LOW input thresholds (VIH, VIL) and input currents (IIH, IIL) are specified.
• Driver output levels (VOH, VOL) and output currents (IOH, IOL) are specified.

Concept / Approach:
Valid interfacing requires VOH ≥ VIH with the receiver's input current IIH applied, and VOL ≤ VIL with IIL applied. The driver must supply or sink the required current while keeping its output within valid logic voltages. Both parameters matter simultaneously; meeting only voltage levels at zero current is insufficient, and meeting only current without guaranteed levels risks indeterminate logic.

Step-by-Step Solution:
Check driver VOH against receiver VIH at IIH.Check driver VOL against receiver VIL at IIL.Ensure driver IOH/IOL ratings exceed total load currents (including fanout).Confirm no exceedance of absolute maximums or rise/fall time constraints.

Verification / Alternative check:
Simulate or measure with worst-case corners (temperature, supply tolerance) to verify margins for VOH/VOL and IOH/IOL.

Why Other Options Are Wrong:
“Only currents” or “only voltages” ignores the coupled nature of the specifications.

Power and impedance by themselves do not guarantee logic-level integrity.

Propagation delay affects timing, not static compatibility.

Common Pitfalls:
Ignoring fanout and assuming one output can drive unlimited inputs; neglecting series resistors or level shifters when families differ in supply voltage.

Final Answer:
both the voltages and the currents