THE S-R LATCH

Bi-stable multi-vibrator contains two stable states, as indicated by the prefix “bi” in its name. Typically, one state is referred as set and the other as reset. The simplest bi-stable device, therefore, is known as a set-reset, or S-R, latch. In order to create an S-R latch, wire two NOR gates in such a way that the output of one feeds back to the input of another  and vice versa, like this: The Q and not-Q outputs are supposed to be in opposite states. It is "supposed to" because making both the Sand R inputs equal to 1 results in both Q and not-Q being O. Due to this reason, having both S and R equal to 1 is called an invalid or illegal state for the 5-R multi-vibrator. Otherwise, making S=1 and R=0 "sets" the multi vibrator in such a way that Q=1 and not-Q=O. Conversely, making R=1 and s=0 "resets" the multi-vibrator in the opposite state. When S and R are both equal to 0, the multi-vibrator's outputs "latch" in their prior states. Note here that how the same multi-vibrator function can be implemented in ladder logic, with the identical results:

By definition, a condition of Q=1 and not-Q=0 I ~ set. A condition of Q=O and not-Q=1 is reset. These terms are universal in describing the output states of all multi vibrator circuits.

The smart observer will find that the initial power-up condition of either the gate or ladder variety of S-R latch is such that both gates (coils) start in the de-energized mode.  One would expect that the circuit will start up in an invalid condition, that is, with both Q and not-Q outputs being in the same states. In fact, this is true! However, the invalid condition is unstable with both S and R inputs inactive, and the circuit will quickly stabilize in either the set or reset condition because one gate or relay is bound to react a little faster than the other. If both gates or coils were precisely identical, they would oscillate between high and low like an astable multivibrator upon power-up without ever reaching a point of stability! Fortunately for cases like this, match like this of components is a rare possibility.

It is important to be noted that although an astable (continually oscillating) condition would be exceptionally rare, there will most likely be a cycle or two of oscillation in the above discussed circuit, and the final state of the circuit (set or reset) after power-up would be un-predictble. The main cause of the problem is  race condition between the two relays CR1 and CR2. This race condition occurs when two mutually-exclusive events are initiated at the same time through different circuit elements by a single cause. In this scenario, the circuit elements are relays CR1 and CR2 and their de-energized states are mutually exclusive due to the normally closed inter-locking contacts. If one relay coil is de-energized, it `s normally losed contact will keep the other coil energized, thus maintaining the circuit in one of two states either set or reset. Interlocking effectively prevents both relays from latching.

However, these 2 relay coils start in their de-energized states ( after the whole circuit has been de-energized and then powered up) both relays will ‘race’ to become latched on as soon as they receive power (this is  "single cause") through the normally-closed contact of the other relay. The one of those relays will unavoidably reach that condition before the other relay, thus opening its normally-closed inter-locking contact and de-energizing the other relay coil. Which of the relays ‘wins’ this race depends on the physical characteristics of the relays and not on the circuit design, therefore, the designer cannot ensure which state the circuit will get into after it is powered up.