Topic Finder for Chapter 5
Introduction
Active and Passive Situations
Active and Passive
Situation Signals
A Graphic Convention
for Function Diagrams
Additional Examples
of Graphics Convention
Nomenclature
Convention for Limit Sensor and Limit Switches
Chapter 5 Overview
Review Questions
List of Figures
Figure 5.1 MOC Saw ON/OFF
Control
Figure 5.2 Examples
of Active High and Active Low Inputs and Outputs
Figure 5.3 Complicated
Function Diagram with Active High and Active Low Inputs
and Outputs
Figure 5.4 Active High
Output Pressure Limit Sensors and Limit Switches Examples
Figure 5.5 Active
Low Output Pressure Limit Sensors and Limit Switches Examples
The key aspect of working with digital circuits for control is to appreciate quickly that any situation worth controlling is going to be a situation that will have a reasonable array of digital and analog circuit elements. In addition these digital and analog devices will ultimately interface to a microprocessor. With this fact in mind, it is important to develop a method of graphically representing a control situation that will provide maximum information transfer with minimal mental memory manipulations. One approach to this task is to use the idea of active and passive situations together with devices that have inputs and outputs that can be labeled as active high or active low.
Table 14 provides some examples of active and passive situations. Although these examples are straight forward, other scenarios may not be as clear. From a general perspective, an active situation is one that once engaged demands the implementation of an action that makes the situation different than it was before. Thus the fire alarm bell is passive when it is not ringing but active when it is ringing. The fact that the bell is ringing demands that a new action be taken, i.e leave the building.
From a control perspective, the passive situation might be the case of a motor always running. For example, a pump that is supposed to be pulling oil from your only oil well in your back yard! I suspect that in this scenario, you will be "fat and happy" as long as the pump keeps pumping. However, if the pump should stop, that indeed will put you into an active mode with respect to getting that "black gold" flowing again. Thus an active or passive situation is determined by the general expected behavior of the equipment under control and the action that is to be taken.
Although it may seem uncomfortable at first, the normal state of affairs is usually viewed as passive. This despite the fact that the equipment might be running and things might be happening. The key is to identify what the unusual operating conditions are. The steady state, i.e. "normal", operations that stay with in boundaries defined for that process are, for control purposes, the passive situation. The active situation is when a process action is called for to bring the process back into this normal steady state operational mode.
Active and Passive Situation Signals
Once the general active and passive assignment for a control action has been established the next step is to establish what type of digital signal cause the desired action. Figure 5.1 illustrates a circuit some of which was presented in Figure 1.2. The focus of attention at this point is to establish the active and passive state of process motor #0, a saw for this example, and to assign the input to the MOC 3011 solid state switch, pin 2, as an active high or active low input. The active or passive nature of the saw assignment itself is defined by the saw's role in the process.
In the example under consideration, the saw will be defined as active when it is running. The assignment of pin 2 is now straight forward. Determine what actual digital signal at pin 2 is required to make the saw operate. If it is the low signal, i.e. 0 volts, 3 psi or 4 milliamps, then pin 2 is identified as an active low input. If the high digital signal, 5 volts, 15 psi, or 20 milliamps, is required, then the input is labeled an active high input. In this example, a 5 volt signal applied to pin 2 will eventually start the saw. Therefore, pin 2 is an active high input. Once 5 volts appears on pin 2 of the MOC 3011, the saw will run and remain on until the 5 volts is removed from pin 2.
A Graphic Convention for Function Diagrams
Identifying the active high input in Figure 5.1 was easy this time. What is needed now is a uniform way to graphically indicate that the input in question is indeed active high. Figure 5.2 illustrates the convention that has been adopted. Before exploring the figure, it is prudent to consider two important warnings.
The first caution that can be offered is to state again that this convention is just a visual trick to help identify if a specific input or output is active high or active low. No other inference or definition of the circles is intended nor should any other inference or definition be imposed on the symbols in the figure.
The second caution is more frustrating. It is important to remember that this graphic convention is not always used by everyone and it does not apply to every situation. It will require a bit of experience to recognize when these two constraints are true. Fortunately, these two situations are quickly recognized once the convention is used a few times.
The pertinent observation for the graphic convention shown in Figure 5.2 is to associate the idea of active low with any input or output that has a circle attached to it. If the circle is not present then the terminal is assigned an active high status. Figure 5.2A is a diagram of the 74138 decoder with a different pin appearance and arrangement. The pin information in Figure 5.2A is the same as the pin information provided in Figure 4.4. The circles in Figure 5.2A do not represent any new devices in the diagram. They do not represent inverters that someone wanted to add to the diagram. They are just circles that were added to indicate the input and output pins on the 74138 that are active low outputs.
The intent of Figure 5.2A was to transmit the operation of the 74138 to the reader in a simple visual way that would eliminate the need to study the operational information provided in the chip's function table, i.e. Table 13. Actually Figure 5.2A does an excellent job at this task as long as the reader is familiar with the general idea of a decoder. ( A decoder is a device that examines the digital pattern presented to its input pins and selects one of its output pins to change state.) The diagram indicates what the new signal is on the selected channel. The decoder examines the digital pattern presented to its input pins and selects one of its output pins to go active. This output pin selected goes low and corresponds to the channel number as indicated by the binary value of the digital pattern present at the inputs. To repeat, the circles on the outputs only indicate that if an output channel is selected the pin associated with that channel will go low.
Figure 5.2B illustrates an alternate view of the 74161 counter. There are two circles on this diagram neither of which are present in the 74161 pinout shown in Figure 3.1 or Figure 3.2. Although the meanings of the circles is specific, they do not mean that someone has attached inverters or any other device to the pins where the circles are located. The circles just identify active low inputs such as the clear pin, pin 1, or the load pin, pin 9. The interpretation of the circles in Figure 5.2B is directly related to the assignment of active low status to the pins in question. For example, the clear pin, pin 1, on the 74161 is an active low input. If 0 volts is applied to that pin then all of the counter's output pins will go to zero volts and remain at zero volts until the clear pin is returned to 5 volts. At that time the next clock pulse at pin 2, the clock pin, will produce 0 0 0 1 on the output pins. In other words, the clear function is active when pin 1 is at 0 volts . Once clear is active the output pattern remains at 0 0 0 0 even if clock pulses arrive at the counter's clock pin, pin 2. The 74161 output pins will not respond to an incoming clock pulse until the clear function goes passive. Once the clear pin is passive, i.e. it is returned to logic 1, the counter will respond to clock pulses and begin to count from zero again.
Additional Examples of Graphics Convention
Figure 5.3 provides two more diagrams that illustrate the usefulness of identifying active high and active low terminals on a process function diagram. Each diagram introduces more complications but also demonstrates the power of this visual labeling convention. For a starter, even though in Figure 5.3A there appears to be an AND and OR and a undefined device shown in a big rectangle, the illustration represents only one device as represented by the big rectangle by itself. Three conceptual parts are separated out to help the reader figure out when the device will performs its function. At this point, since no knowledge of the device or its function has been provided, Figure 5.3A is only an example of how to interpret active high and active low inputs.
The device shown in Figure 5.3A has three input pins, pin 3, pin 4 and pin 5. The package also has two output pins, pin 6 and pin 1. Pins 3 and 4 are active low inputs. Pin 5 is an active high input. Pin 6 is an active high output and pin 1 is an active low output. Since both outputs are labeled with the same letter, Q, they both respond to the same set of input pin stimuli. The only difference in the two outputs is that one is the complement of the other. For example, if pin 6 is at 5 volts then pin 1 is at 0 volts and if pin 1 is high then pin 6 will be low. The issue at this point is to be able to predict what conditions on pins 3, 4, and 5 will force the output at pin 6 to go high, i.e. the output at pin 1 to go low. This is where the active high and active low connotations become useful. For this example using the limited knowledge provided about the chip's overall function, it is safe to state that if pin 3 OR pin 4 is low AND pin 5 is high then pin 6 will be high. In other words, if pin 3 OR pin 4 is active AND pin 5 is active then output pins 6 and 7 will be active. Since pin 6 is active high and pin 1 is active low then pin 6 will be at 5 volts while pin 1 is at 0 volts. The way to make pin 3 or pin 4 active is to bring them to 0 volts. The way to make pin 5 active is to bring it to 5 volts. The actual function of the device as well as the role of the capacitor and resistor shown in Figure 5.3A will be discussed in the Pulse Generation section of these notes. It is a 74121 monostable device. However, even with out that information some valuable operation knowledge of this device was available from the diagram just by identifying the active high and active low inputs and outputs it possesses.
The function of the 74138 shown in Figure 5.3B has also been explored. The new educational value of this diagram lies in the visual difference between it and Figure 4.4. The extra information provided in Figure 5.3B is related to the signal combinations applied to the enable pins to make the decoder function. As suggested by the additional logic symbols, the 74138 will be enabled if pin 6 is high AND pin 5 OR pin 4 low. The beauty of the diagram is that this information was provided in an efficient, elegant and visual manor. It was not necessary to look up in any reference book or rely on your memory for the required operational information. As always with this type of diagram, Figure 5.3B is not suggesting that you will find a 7408, 7432 and a 7404 with the 74138 on a circuit somewhere. Actually the only chip you would find is the 74138. The other symbols do not represent real devices in this diagram. They are added to the diagram as a visual aid only.
Nomenclature Convention for Limit Sensor and Limit Switches
Just when it seems safe to come in from the confusion, there is one more active high active low image that must be introduced. This one as a built-in possible confusion path because it also involves the N.O. ( normally open) and N.C. (normally closed) concepts associated with mechanical contacts as well as the distinction between a limit sensor and a limit set of contacts. Perhaps the quickest way to jump into this swirling mix is to examine Figure 5.4.
Figure 5.4 provides two vertically parallel paths that lead from the idea of a limit sensor or a limit switch to the same identification symbol, PSH. The three letter code, PSH, can be translated as Pressure Sense High and refers to the fact that both the high pressure limit sensor and either the N.O. or N.C. high pressure limit switch provide a high signal when either is active. Naturally, they are only active when the pressure that is being monitored is higher than the predetermined limit pressure value. Equally as obvious at this point is the fact that when the process pressure value is below the predetermined limit value, the pressure sensor and either N.O. or N.C. limit switch is passive. In the passive state these devices will provide low signal.
Although the meaning and significance of the PSH code is the primary message of Figure 5.4, there are other important images in the figure. First, the limit sensor works because of the well understood relationship between the resistance of a strain gage and the amount of strain that gage must endure. As the pressure applied to the strain gage is varied, the strain on the gage also varies and the resistance of the gage changes proportionally. thus the high pressure limit sensor based on the operation of a strain gage requires an accompanying electronic circuit to measure the change in resistance and determine when that changing resistance value exceeds the value associated with the predetermined process pressure limit value. Note that Figure 5.4 shows this additional circuitry as part of the package that includes the strain gage. Not also that electrical engineers might identify this as a three port device. Two of the ports are the wires where the pressure sensor essentially plugs into the wall socket to obtain the power it needs to function, while the third port is the output wire that will carry the low or high signal generated by the sensor to the controller. ( Actually, electrical engineers tend to also identify this sensor as an active sensor, but that a story for a different day)
By contrast, either contact based pressure limit switch works because of a mechanical action that either closes a N.O. set of contacts or opens a N.C. set of contacts. This idea is shown in the middle right portion of Figure 5.4 and incorporated into limit switches with its appropriate wiring in the bottom right part of the same figure. Study that section of the diagram to be prepared to answer question 5.5 below. In the mean time, notice that both the N.O. and the N.C. contacts are examples of two port devices with the control signal present on one of these two wires. Actually, it does not make any difference which of the two wires is also used to send the alarm signal to the controller.
A pre-apology might be in order for the following statement, since it is provided just to play a bit with your mind. This statement deals with the contents of Figure 5.4 and Figure 5.5. Figure 5.5 provides the identical summary of the properties of a high pressure limit sensor and either a N.O. or N.C. high pressure limit switch but the results are just the opposite of those shown in Figure 5.4.
Hopefully, upon review of Figure 5.4 and Figure 5.5, the meaning of above statement was quickly ascertained. The point of the contradiction is in the fact that in both figures the alarm devices, either the high pressure limit sensor or the high pressure limit switches, go active when the measured pressure exceeds the predetermined limit pressure value. The difference in the devices in Figure 5.4 vs Figure 5.5 is the signal that is provided to the controller when the devices are in the alarm condition. Thus for the devices shown in Figure 5.4, each will provide the high signal when the pressure exceeds its limit high pressure value. By contrast, for the devices shown in Figure 5.5, each will provide the low signal when the pressure exceeds that very same limit high pressure value. Review Figure 5.5 again and be prepared to answer question 5.6. Notice specifically, the line that is included over the PSH identification for the active low pressure limit devices shown in Figure 5.5. this line or bar is used in function diagrams as a common way to identify a device that has an active low output. Note also that the line or bar is not present over the PSH identification in Figure 5.4.
Chaper 5 provides background information as to the meaning and use of the terms active and passive. It also introduces the concept of an active high or an active low input or output. Examples of active hgih and active low inputs and outputs are provided. The chapter concludes with a discussion of the differences and similarities among limit sensors and limit switches.
5.1 Why are pins 1,2 and 3 on the top function diagram in Figure 5.2. not labeled as active high or active low?
5.2 Examine the bottom function diagram in Figure
5.2.
a) What happens if pin one becomes active?
b) What signals make pin one active, makes the Load pin active?
5.3 With the Pinout in Figure
4.4 and the 6 input pins in the bottom function diagram of Figure
5.3 in mind,
(a) What voltage on each
input pin makes pin # 10 active?
Pin 1 Pin 2
Pin 3 Pin 4
Pin 5 Pin 6
b) What voltage will be present on Pin #10 when it is active?
5.4 Draw a new function diagram for the same circuit shown in Figure 4.3A but this time try to incorporate as much of this active high or active low symbolism as possible.
5.5 Figure 5.4 shows the high presure
limit sensor and limit switches in there passive state.
a) What is the value of
the signal ( high or low) that is currently on the output wire of the N.O.
high pressure limit switch?
b) What is the value of
the signal ( high or low) that is currently on the output wire of the N.C.
high pressure limit switch?
c) What is the value of
the signal ( high or low) that is currently on the output wire of the strain
gage high pressure sensor?
5.6 Figure 5.5 shows the high presure
limit sensor and limit switches in there passive state.
a) What is the value of
the signal ( high or low) that is currently on the output wire of the N.O.
high pressure limit switch?
b) What is the value of
the signal ( high or low) that is currently on the output wire of the N.C.
high pressure limit switch?
c) What is the value of
the signal ( high or low) that is currently on the output wire of the strain
gage high pressure sensor?