6 Basic Memory Circuits
In this chapter, we take a look at the memory circuits used in programs in the US and in Europe. There are differences. The Allen-Bradley programming language definitely encourages the use of seal circuits while Siemens’ programming allows the Set/Reset circuit or the seal circuit. Both will be discussed in depth and their use in programming applications follow. Also, this chapter introduces the one-shot or edge trigger. It is a highly used addition to most circuits and its use should be well understood.
The memory circuit, while either the US or the European type, is the basic building block for all programs involving automation and control of a process. Its use is foundational to all logic that follows. In Ch. 7, we discuss the memory circuit (seal circuit) with a timer. In Ch. 8, we discuss the memory circuit with comparison statements. Ch. 8 also introduces the idea of numbers instead of bits to set memory steps or states. Ch. 11 introduces the concept of states and discusses various ways of programming them. The first method proposed in most state diagrams is the use of the basic memory circuit (either seal or S/R). Later chapters use the memory circuit to build a foundation for control of a particular process – either batching (Ch. 13), mode (Ch 15), motion (Ch. 17) or PID (Ch. 19).
After a discussion of the one-shot and introduction of some simple one-shot circuits, we discuss some early design of memory circuits. These examples form the foundation for more sophisticated applications in later chapters.
Developing the Memory Circuit
We look first at a very simple system. This circuit gives a look at how memory circuits have been a part of digital logic and how the digital concepts have links to the Ladder diagram circuits used later in the chapter.
The Acme Company has a problem flow of molasses in a storage tank. In the winter, viscosity of the molasses is so high that the molasses run too slow to exit the flow valve. A present system allows for flow out of the tank on request but no sensing of temperature. A second model is to be put in place to allow for a heater to be turned on while the molasses are cool and then flow out of the valve.
The first model does not have a temperature sensor or heater. Sensors consist of two level sensors, LH, and LL. The tank outlet valve turns on to empty the tank when the upper level is reached. After opening, the outlet valve was closed when only when LL has been reached.
The system as designed is shown below.
Signal assignment must be made of the level switches LH and LL. These switches must be assigned a value of 1 or 0 when the switch is covered (level exceeds the switch). A switch must be assigned a value that is safe, that is, that limits bad consequences if the switch fails. The most likely fault is for the switch to lose a wire (wire fall off and open the circuit). If this happens, would the circuit allow some bad event to occur? In the event of LH, the switch is used to st