To appreciate the distinctive difference among devices with open collector, 3 state and normal TTL outputs, and the impact these devices have on a circuit consider the three simplified applications illustrated in Figure 13.1, Figure 13.2 and Figure 13.3. In all three cases, the status of two stations with an identical group of sensors is to be monitored. This is not an uncommon scenario. Perhaps the process has two identical two phase, liquid and gas, storage tanks. At this point, the focus is not the application itself, but how the selection of a normal, an open collector or a 3 state device alters the implied importance of a station's sensors.
A 7405 Open Collector Circuit Example
Consider the 7405 open collector circuit shown in Figure 13.1. There is an interesting physical and mental point to be made. The physical observation is very apparent. It is presented first.
Figure 13.1 indicates the presence of a wire that connects the two Q1 output pins together, i.e point (e) is electrically connected to point (a). The other corresponding Q output pin pairs are connected together at points (b), (c), and (d) as well. This, at first glance, seems to violate the second law of thermodynamics . Oh sorry, I meant to say second law of TTL circuits! (I bet I scared you there for a second),
i.e. NEVER CONNECT THE OUTPUTS OF 2 TTL DEVICES TOGETHER. (The first law is ALWAYS CHECK THE POWER AND GROUND CONNECTIONS.)
However, since the danger in connecting TTL outputs together develops as the output of one the connected devices goes to 0 volts when the other's output pin is at 5 volts, the output pins of two separate 7405 open collector devices can be connected together. This is true for the very simple reason that neither of the two connected 7405s can internally produce a 5 volt signal and therefore neither will ever be able to draw too much current from the other.
The mental point associated with the connected 7405 open collector devices in Figure 13.1 is simple but may not be so obvious. The 4 LED's on the control panel will only show their respective alarm status when both corresponding group sensors are in alarm. There is no way for the circuit to indicate when only one station's corresponding sensor is in alarm.
A detailed investigation of this example may be necessary to make this point clear. Suppose there is no unusual flow condition at either monitoring station. The FSL signal from both sensors would be 5 volts. This means the Q1 outputs of 7405 #1 and 7405 #2 are both at 0 volts. It also means that the voltage at point (e) and point (a) are both at 0 volts. Since the anode of the control panel's top LED is connected to point (a), that control panel LED is OFF.
Now suppose that the flow at station #1 has gone into an alarm condition. FSL 1-1 is now at 0 volts but FSL 2-1 remains at 5 volts. The Q1 output of 7405 #2 remains at 0 volts but since D1 of 7405 #1 has changed from logic 1 to logic 0, the corresponding Q1 output pin of 7405 #1 has now changed to its logic "nothing" state. This really means that there is no electrical connection between point (e) and the Q1 output of the 7405 #1. Therefore, the voltage at point (a) only depends on the Q1 output of 7405 #2. Since this output is still at 0 volts, point (a) remains at 0 volts and the LED remains OFF.
If the situation were reversed, i.e the flow at station #1 was normal but the flow at station #2 went into alarm, then there would be no electrical connection between point (a) and Q1 of 7405 #2. This would mean that the voltage at point (a) would depend on the voltage at point (e). Since the flow at station 1 is normal, FSL 1-1 is at 5 volts and Q1 of 7405 #1 is at 0V. Therefore point (e) is also at 0V and the flow alarm LED remains OFF. In summary, the only time the flow alarm LED on the control panel is light is when both flow sensors go into alarm.
Figure 13.2 show the 3 state device circuit for the same process scenario. An interesting physical and mental point can be made about this diagram as well. From a physical perspective the basic wiring to both station's respective sensors remains the same and therefore the basic concern remains the same. Points (a), (b), (c) and (d) correspond to the places where the corresponding output pins for devices in the two packages are still connected together. This time the second law of TTL,
(i.e NEVER CONNECT THE OUTPUTS OF TWO TTL DEVICES TOGETHER)
is not violated as long as the enable pins of the corresponding connected 3 state devices are not enabled at the same time.
Figure 13.2 shows the enable pins of the 74126 3 state device are all connected together and delivered to the output pin of a status push button. Like wise, the enable pins of the 74125 3 state device are also delivered to the same status push button. Thus both set of enable pins respond to the same push button output signal.
The combination of two facts keep the outputs of these two 3 state devices from being enabled at the same time. First, the enable pins on the 74126 are active high while the enable pins on the 74125 are active low. Second, a push button can only be in one active state at a time.
The mental point about the 3 state device circuit in Figure 13.2 is straight forward. The group of sensors at station #1 is more important to the operation of the process than the corresponding group of sensors at station #2. An analysis of the circuit will emphasis this point.
Consider the two most likely physical actions that can be taken on the circuit's only push button. If the active low Status PB push button is not pushed two things will happen. First, the 74125 3 state device is not enabled and all of its outputs go to the logic "nothing" state. Second, the 74126 3 state device is enabled and the sensors at station 1 report their status to the control panel LEDs. If the button is pushed, the 74126 goes passive while the 74125 goes active. This button action prohibited transmission of station #1 sensor status but enables 3 state device #2 and allows the sensors at station #2 to report their status to the control panel LED's. The end result of these two possible push button action options is clear. If nothing is done, then the current status of the station #1 sensor group will always appear on the control panel LED's. Taking the time and energy to push and hold the Status PB down will allow the status of the station #2 sensor group to be displayed.
Using any control room display arrangement philosophy you can conceive, it still follows that if the status of a group of sensors is to be continuously displayed the most important sensors should be in that group. For the situation shown in Figure 13.2, the information from station #1 is more important than the same type of data from station #2. The assignment of the 74126 3 state device to station # 1 assured that status to data from station #1.
Figure 13.3 shows the WORST way to wire the process sensor display circuit being discussed. The diagram is shown to emphasize what not to do. It is shown in small print so you will not easily be able to use it. If the two 7404's are wired as shown then the second law of TTL,
NEVER WIRE THE OUTPUTS OF 2 TTL DEVICES TOGETHER,
has been violated. The result of this violation will become sensually detectable when the output pin Q1 of 7404 #1 goes to 5 volts at the same time pin Q1 of 7404 #2 is at 0 volts. Under this set of conditions, the amount of current that flows from 7404 #2 Q1 output pin into the 7404 #1 Q1 output pin will be higher than the transistors inside 7404 #1 can handle. Heat will be generated in 7404 #1 and eventually the device will, as they say, "fry", i.e. the smoke will escape and the chip will not work any more.
There is another but subtle way that the second law of TTL can be violated. If a Stop button or a control switch is to be connected to a TTL interface circuit, attention as to where this external electrical device is connected to the circuit is required. It may be tempting to connect it to the input of a TTL device that is connected between two other TTL devices.
For example, examine the connection to the Clear pin on the 74161 in Figure 3.4. Perhaps it is desired to have a Clear push button also attached to pin 1 of that counter. Avoid the temptation to just connect the output wire of a button wired as shown in part (A) of Figure 6.2 to pin 1 of the 74161 in Figure 3.4. If this were done, the button would certainly be connected to the Clear input pin of the 74161 and supply it with 0 volts or 5 volts depending on the status of the button. However, the button would also be supplying these same voltage possibilities to the output pin of the 7400 in Figure 3.4 as well.
It is this duel wire connection complication that represents the problem. It is important to remember that buttons and switches are voltage sources for TTL circuits and must be considered as output signals for those circuits. Thus in this example, the button wire connection is actually an output signal that is attached to an output pin of a 7400 at the same time it is attached to an input of the 74161. Thus the button wire and the 7400 output are two output wires connected together. This is bad news for the 7400 and the 7400 will soon be joining other defunct devices in the great chip graveyard in the sky.
Current Use for Open Collector Devices
After reading this information about open collector, 3 state and normal output TTL devices, it may be tempting to state that open collector devices have out lived their usefulness. For applications that involve transmission of bit patterns along a data bus this may very well be the case. However, Figure 13.4 illustrates one very important control application where an open collector TTL device is the perfect choice.
The figure provides the solution to the following controls problem.
Suppose you wish to activate a 24 volt, 300 mamp relay if TWO different 5 volt process sensors are in alarm at one monitoring point in the process AND either of two additional sensors are in alarm at a different monitoring point in the process.
The logic for this task is simple and it is easy to construct with normal TTL devices. Just connect the two sensors from station number one to the inputs of a 7408 AND device and the two sensors from station number two to the inputs of a 7432 OR device. Now connect the output from the station number one AND device and the station number two OR device to separate inputs of a second AND in the 7408 package. The output signal from this second AND device will perform the logic as stated in the controls problem above. However, the electrical signals associated with this logic are 5 volts and zero volts. The technical issue here is not the complexity of the control scheme, but the complication the 24 volt relay requirement introduces.
There are only two ways to wire a normal TTL type device output signal to the coil of a relay in this problem. One way is to attach the 24 volt 300 mamp power supply directly to the coil's "+" terminal and the TTL output pin from the second AND device to the coil's "-" terminal. With this arrangement, the 0 volt signal from the output pin of the second 7408 AND logic device will energize the coil if 300 mamps can flow from the ground connection, pin 7, on the 7408 through the "-" terminal of the relay coil to the positive terminal of the 24 volt power supply.
Unfortunately this current flow will not happen using a 7408. Although the 0 volt output will provide the 24 volt signal across the two coil terminals, a normal TTL package can only handle up to 16 mamp current flow. The 300 mamp current the 24 volt power supply will try to draw from the 0 volt connection on the 7408 would fry the chip!
The other way to attach the relay to the second AND output pin is to permanently connection the "-" terminal of the coil to 0 volts. With this arrangement, the output pin of the second AND is connected to the "+" terminal of the coil. Now when this AND output is at logic one, 5 volts will be delivered across the coil. Unfortunately this will not energize the relay contacts. When a 7408 AND output is at 5 volts it can only draw 40 micro amps before the chip is damaged.
In summary, it is important to realize that normal TTL device output pin current flows are 16 mamps at 0 volts or 40 microamps at 5 volts. If these current flow values are exceeded a normal chip will malfunction. Therefore, for the application under discussion, a normal TTL device will not provide enough current at logic 0 nor the voltage or current at logic 1 to satisfy the needs of the relay.
The open collector TTL device is the perfect answer for the application problem presented above. The function diagram for this application is shown in Figure 13.4. The O.C. label on the respective AND and OR devices indicate that these devices are not the normal 7408 and the 7432 you might have expected but their corresponding open collector counterparts. The pinouts for the normal and the open collector packages are usually the same. For example, the 7404 is a TTL device with normal outputs while the 7405 is a TTL device with open collector outputs. Both packages must have 5 volts connected to pin 14 and 0 volts connected to pin 7 if they are to operate correctly. Both the 7404 and the 7405 have six devices inside the package and the pin assignment for all of the devices are identical in both packages.
The only way to tell the difference between the 7409 open collector TTL four AND device package and the 7408 TTL four AND device package is to remember two important facts. First, open collector devices allow larger amounts of current to flow when in the logic 0 output state. Second, open collector devices do not provide a logic 1 output state. Open collector devices provide a logic "nothing" state instead. With this in mind, the rest of the circuit in Figure 13.4 is easy to interpret.
To begin an analysis of Figure 13.4 it is useful to remember that the a N.O., normally open, type relay will close it contacts when there is a 24 volt drop across the "+" and the "-" coil terminals of the relay. If either pressure sensor PSH 1-2 or flow sensor FSL 1-1 at station #1 is not in alarm, the AND process condition is not met at station #1. The AND output pin will be at logic zero and the relay coil "+" terminal will be at zero volts. The values of the resistor shown is selected to limit the flow of current from pin 7 of the open collector AND device through the two power supplies. In this situation the voltage across the coil will then be close to zero volts and the coil will not have the 24 volts required to drive 300 mamps through the coil. Therefore, the coil contacts will remain open.
If neither temperature sensor TSH 2-4 nor level sensor 2-3 at station #2 is in alarm, the OR process condition is not met at station #2. The OR output pin shown in Figure 13.4 will be at logic zero and the "+" terminal of the relay coil will be at zero volts. Again the voltage across the coil will be near zero volts and the coil driven contacts will remain open.
There is only way for the relay coil to sense the 24 volts it needs to close its contacts. If both sensors at station number 1 and one of the sensor in at station number 2 go into alarm. Consider this situation in steps.
First, PSH 1-2 and FSH 1-1 to both go into alarm. This will put logic 1 signals on the AND inputs and force the AND output to the logic "nothing" state. This means that there is no electrical connection between point (a) in Figure 13.4 and pin 7, the ground pin, of the open collector AND package. Now there is the possibility of a volt drop across the plus and negative terminals of the coil if the open collector OR output is also in its logic "nothing" state.
The open collector OR output will be at the logic "nothing" state if either of the station #2 sensors supplies a logic 1 signal to its input pins. Thus the required electron current can flow from ground, the "-" terminal of the coil, to the 24 volt power supply only when both station #1 sensors are in alarm and one of the station 2 sensors is in alarm. This 300 mamp current will generate a magnetic field strong enough to close the relay contacts and allow the final control element (FCE) to be energized.
13.1 Examine Figure 13.1. What is
a) the voltage at D1 when station #1 sensor FSL
1-1 is in alarm?
b) the voltage at point (e) if TSH 1-4 at station
#1 is in alarm?
c) the general name for the resistors in this circuit?
d) the voltage at point (d) when TSH 1-4 is active?
13.2 What is the fundamental rule to remember when creating normal TTL circuits?
13.3 Examine Figure 13.2. What is
a) the voltage at point (e) when station #1 sensor
FLS 1-1 is not in alarm?
b) the voltage at point (a) if station #2 sensor
FSL 2-1 is in alarm?
13.4 What is wrong with the circuit shown in Figure 13.3?
13.4 Where is the "-" terminal for the coil shown in Figure 13.4
13.5 What is the logic state of the open collector AND device in Figure 13.4 if PSH 1-2 is active and FSL 1-1 is passive?
13.6 What is the logic state of the open collector OR device in Figure 13.4 if TSH 2-4 is active and LSL 2-3 is passive?
13.7 What is the maximum current that can flow through the coil shown
in Figure 13.4?
a) What is the value R3?
13.8 Draw a function diagram to shows the correct connection of a push button as wired in part (A) of Figure 6.2 to the Clear pin of the 74161 counter shown in Figure 3.4