The trigger is the simplest device, which is a digital machine. It has two states of stability. One of these states is assigned the value “1”, and the other - “0”. The state of the trigger, as well as the value of the binary information that is stored in it, is determined by the output signals: direct and inverse. In the case when a potential that corresponds to a logical unit is established at the direct output, the state of the trigger is called unity (the potential at the inverse output is zero). If there is no potential at the direct output, then the state of the trigger is called zero.
Triggers are classified according to the following criteria:
1. By the method of recorded information (asynchronous and synchronous).
2. By the method of information management (statistical, dynamic, single-stage, multi-stage).
3. By the method of implementing logical connections (JK-trigger, RS-triggers, T-trigger, D-trigger and other types).
The main parameters of all types of triggers are the largest value of the duration of the input signal, the delay time required to switch the trigger, and also allowing the response time.
In this article, we’ll talk about a type of device such as an RS trigger. They are of two types: synchronous and asynchronous.
Asynchronous RS-trigger structurally has two direct (R and S) inputs. This device operates according to the conversion table.
Forbidden for such a trigger is a combination of signals at the inputs of the device, causing a state of uncertainty. This combination can be expressed by the requirement RtSt = 0. When minimizing the Carnot map, the trigger operation law is derived, which is called the characteristic equation: Q (t + 1) = St V R'tQt. In this case, RtSt will be equal to zero.
The functional diagram shows the RS-trigger of an asynchronous type on AND-NOT elements and in the second version on OR-NOT elements.
The second type is a synchronous RS-trigger. Such a device structurally has three direct inputs S, R, and C. The difference between a synchronous type trigger and an asynchronous type is the presence of a synchronization input (C). It is necessary for the following reasons: after all, the inputs of a device (logic element) do not always receive signals simultaneously. This is due to the fact that they go through different types and numbers of nodes that have different latencies. This phenomenon is called "competition." As a result of such “competitions”, the obtained signal values will be superimposed on the previous values of other signals. All this leads to a false response of the device.
This phenomenon can be eliminated by applying temporary gating signals to the input of the device. Namely: at the input of the logic element, in addition to the information signals themselves, key synchronizing pulses are supplied, by this moment the information input signals will have time to fix on the inputs.
The main condition for the correct operation of the operation of logic cascades in the RS-trigger and the logic circuits controlled by them is the inadmissibility of the simultaneous action of the Rt or St signal switching the device and the removal of information from the output Q (t + 1) of the trigger. In this regard, potential series of elements contain only synchronous ones.
The synchronous type RS-trigger is represented by the characteristic equation: Q (t + 1) = StCt V R'tQt V QtC't.
The photo shows a synchronous RS-type trigger on AND-NOT elements.
The input logical elements AND NOT transmit a switching logical unit from the information input S or R to the necessary inputs of an asynchronous trigger of type RS with inverse inputs only if there is a signal at the synchronous input (C) with a level of a logical unit.