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State and Memory Review

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State, relays, and memory — review

Sequence to remember:
current inputs → state → feedback → relay holding circuit → SR latch → latch / flip-flop → clocked storage

Relay reading checklist

1. Find the coil reference.
2. Find every contact with the matching reference.
3. Read NO and NC in the normal de-energized condition.
4. Remember that the contacts may be nowhere near the coil on the drawing.
5. Check the drawing legend because reference-letter conventions vary.

Memory checklist

Seal-in relay: feedback through an auxiliary contact keeps the coil energized.
SR latch: cross-coupled logic preserves Q.
Latch: generally level-sensitive.
Flip-flop: generally edge-triggered.
D flip-flop: captures D at a clock edge and stores it at Q.

Engineering checklist

Logic alone is not enough. Real designs must respect fan-out, voltage and current levels, propagation delay, rise/fall time, setup time, hold time, noise margin, power, and other device specifications.

Review problems

A relay contact marked K1 is far from the K1 coil on the schematic. What connects them conceptually?
The reference marking tells us which contacts are operated by which coil.
In a seal-in circuit, what provides the memory after START is released?
Once the relay energizes, its auxiliary contact maintains the current path.
For an SR latch with S=0 and R=0, what happens?
The zero-zero input condition is the hold condition for the active-HIGH NOR SR latch.
Which statement best describes a D flip-flop?
An edge-triggered D flip-flop samples D at the active clock edge and stores that bit at Q.
Which pair of real-world timing limits applies directly to flip-flop inputs around the clock edge?
Setup and hold requirements define the time window around the clock edge in which D must remain stable.
Why can two logically equivalent implementations behave differently during a transition?
Equivalent steady-state logic can still have different transient timing because real devices do not switch instantaneously.

Where this naturally leads

With state and memory understood, the next relay lessons can go deeper into control circuits: poles and throws, contact ratings, interlocks, seal-in variations, timing relays, ladder diagrams, and troubleshooting separated relay contacts on larger schematics.

< D Flip-Flop and Timing   |   Circuit Cookbook >