RS flip-flop. Scheme, description

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RS flip-flop. Radio - for beginners

Radio - for beginners

RS flip-flops are usually composed of two-input NAND gates. You can see a diagram of such a variant of the RS flip-flop in Fig. 1, a. It is formed by two elements of the 2I-NOT of the K155LAZ microcircuit with cross-feedback between their inputs and outputs.

RS trigger
Rice. 1 RS flip-flop

The trigger has two independent inputs and the same number of outputs. The first input-input S-output 1 element DD1.1, the second input-input R-output 5 element DD1.2. Outputs: direct output 3 element DDl.1, inverse - output 6 element DD1.2.

To better understand the operation of the RS flip-flop, mount the parts shown in the diagram on a breadboard and conduct a few experiments. Instead of LEDs indicating trigger states, you can use the familiar transistor indicators with incandescent lamps. It is not difficult to indicate the states of the trigger elements using a DC voltmeter, connecting it alternately to the output of one or the other element. Instead of push-button switches without fixation, you can use pieces of mounting wire with bare ends, which will simulate the supply of low-level voltage to the trigger inputs.

After checking the installation of an experimental trigger with its circuit and making sure that there are no errors, that the soldering is reliable, turn on the power. One of the LEDs should turn on immediately. Suppose it will be the HL1 LED. This means that the element DD1.1 turned out to be the first in a single state, which will also be confirmed by a voltmeter connected to its output - there should be a high level voltage here.

Measure the voltage at the output of the element DD1.2 - there will be a low level, so the HL2 LED does not light.

After recording the measurement results, short-circuit the contacts of the SB1 button. What changed? Nothing! Only the HL1 LED is still on. And if you briefly press the button SR2? The HL1 LED will immediately turn off and HL2 will turn on. Now the element DD1.1 will be in the zero state, and DD1.2 will be in the single state. In this state, the elements can be indefinitely until the power is turned off. But it is now worth pressing the SB1 button again and the elements will switch to the opposite state.

Let's analyze the work of an experienced trigger. We believe that when the power was turned on, the element DD1.1 turned out to be in a single state. At this moment, therefore, at the top input of the DD1.2 element, connected to the output of the DD1.1 element, a high-level voltage appeared, which set the DD1.2 element to the zero state. Applying a low-level pulse to the upper input of the DD1.1 element according to the circuit (by pressing the SB1 button) could not change its state, since at that time there was already a low voltage level at its lower input.

At the moment of pressing the SB2 button, a low-level pulse was received at the lower input of the element DD1.2. Switching to a single state, this element switched the DD1.1 element to the zero state with a high-level output voltage. Switching element DD1.1 turned out to be possible because at that moment its upper input was free, which is equivalent to applying a high level voltage to it.

So by pressing the buttons in turn, you can switch the trigger from one stable state to another and thereby control various devices and digital devices connected to its outputs.

The logical state of the RS flip-flop is characterized by the signal level at its so-called direct output. If the voltage level is high here, then the trigger as a whole is in a single state, and if the voltage level is low, it is in a zero state. Sometimes the direct output of the trigger and the signal itself at the direct output are denoted by the letter Q.

With a single state of the trigger, its second output will have a low level voltage, and with a zero state - a high level. Therefore, this output is called inverse and it is designated (and the signal on it) with the same letter, but with a dash at the top - Q, which means inversion.

The input through which the trigger is set to a single state is denoted by the letter S (this is the initial letter of the English word set). The other input, through which the trigger is switched to the zero state, is denoted by the letter R (from the word reset-return). Therefore, in the experimental trigger, pin 1 of the microcircuit is the S input, and pin 5 is the R input.

Strictly speaking, the designations of the inputs S and R of the experimental trigger should be written with dashes at the top, since the level of pulses applied to them to switch the trigger from one state to another is low. They are therefore inverse, i.e., S and R. The flip-flop described here is usually called an asynchronous RS flip-flop with setting inputs.

The trigger states depending on the input signals are illustrated in the table in fig. 1b. What can she tell about? If a low level voltage is applied to both inputs of the trigger, for example, by simultaneously pressing both buttons, a high level voltage will appear at both its outputs. Such a state of the trigger contradicts the logic of its action, therefore, such a combination of input signals is considered to be unacceptable.

A combination of low-level signals at input S and high-level signals at input R leads the trigger to a single state, and the opposite combination of voltage levels to zero. When a high level voltage (logical 1) appears on both inputs, the state of the trigger does not change - this is indicated by crosses in the table.

Check the validity of the table. Simulate the supply of pulses corresponding to a high voltage level by opening the contacts of the buttons SB1, SB2.

RS flip-flops are most widely used as digital information storage cells, i.e. as memory elements. They are used in various amateur radio devices, electronic machines.

See other articles Section Beginner radio amateur.

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