USCG Exam questions related to PLCs by Frank Owen, Maine Maritime Academy, 23 October 2018
Timers The USCG questions reference three different types of timers: 1. On-delay timer (TON) 2. Off-delay timer (TOF) 3. Pulse timer (TP) Let s look at how these are defined in a reference book on the exam. We did not study a pulse timer in ET401 (so far). These timers are not standard timers. What we learned about were more standard than these. But you must learn these definitions to be able to answer these questions correctly. None of the timers shows Preset and Accum, as we studied. USCG-TON Obviously the Input is our TGR (trigger). When TRG goes high, USCG-TON starts counting up. The Timer Output is what we ve called DN (done). But different from the standard TON, the timer keeps timing even if TGR goes low. There is no
information on how to reset the timer. Perhaps there is a separate reset input. What characterizes USCG-TON is that one turns the Input on, but it is only after some delay (the elapsed time), that the Timer Output goes high. USCG-TOFF Like a standard TOF, the counting starts when Input goes low. The Timer Output is a standard TOF s DN and TT ORed together: DN TT ( stands for OR; && stands for AND). USCG-TOFF s Timer Output goes high when Input goes high, but it stays high also when Input goes low and starts the timing. It remains high while the timing is in progress and only goes low when the timing has stopped. Again, there is no information on how to reset the timer. What characterizes USCG-TOFF is that one turns the Input off, but it is only after some delay that the Timer Output goes low. USCG-PT The pulse timer is characterized by the fact that the Timer Output goes high when Input goes high, but then it automatically goes low after the elapsed time is over. So this Timer Output turns on, then off after a prescribed period (the length of the pulse).
Figure A: The output coils represent two TONs. But when energized, TI and T2 do not turn on until after their delay is exhausted. For an example, assume In1 is a retentive start switch. Assume both timers have 5-second delays. When we start, T2 is low. So when the In1 is switched on, T1 starts counting. After 5 seconds, T1 goes high. This initiates the timing cycle of T2. After 5 seconds, T2 goes high. This opens the break in the first rung, resetting T1 i.e. making it go low. This also resets T2 i.e. making it go low. This closes the break on the first rung, and the cycle starts over again. Out1 is simply an indicator light that shows when T1 is high. Thus this light would be on only when T2 is counting, not as shown. T1 stays high only when T2 is counting. It goes out once T2 is finished counting and it s been reset. That is, only when T1 is counting and has not produced a high output yet will the light be off. This is a bit confusing, but in the diagram, the 5-second blocks where T1 and Out1 are high are the very periods after T1 s timing has been done and T2 is actively timing. The diagram in C is simply a different way to write the ladder logic in A. It does not, however, include Out1 as an output. In terms of standard TONs, T1 and T2 are the DNs for the two timers. The indication that the two timers are TONs and not TOFs is given in the question, not in the figure. An explanation from a study guide is given below.
If we assume that Figure B goes with Figure A, the momentary input In1 turns the TON on. The input is latched on by Out1, which is an indicator that the timer is actively timing, like the standard TON s TT variable. Once the timing completed, Timer goes high. But the break, identified by TON timer, resets the timer. Why Timer stays high after that is not clear. But, if we look at Figure B, we see that an input went high, and then after a period the output went high. So just looking at Figure B, what is represented here is a TON. Figure D shows In1 going high, which sets Out1 high for a certain period of time. Then when the timing is finished, Out1 goes low. Thus Figure D represents a pulse timer output. If Figure D goes with the ladder diagram in Figure C, the input is a retentive switch (In1 goes on and stays on). This starts the timer. Timer is the timer output, so it remains off until the time has elapsed. Out1 seems to be like a standard TON s TT output. It goes high and stays high as long as the timing is active. The ladder in Figure E seems to perform the same function as the ladder in Figure C. The Timer output is the internal relay, which is off until the timing is finished. So Out1 will be high during the timing. When the timing is done, the internal relay goes high, which makes Out1 turn off. Thus, this timer generates a pulse.
I disagree with the first conclusion. You push a button and then, after a delay, the timer output goes high (Timer). So for 1 we have, instead, an On-delay timer.
The top figure is a sort of Function-Block Diagram, another way to represent ON/OFF logic. It is interesting to develop the truth table for it. When you do, you will see that this represents an XOR function. Note that the Output is high only when one of the inputs is high and the other is low. The relationship between this FBD and the ladder in Figure B is quite close too. Regarding the ladder at lower left, obviously we have Lamp1 on whenever SW-0 is on and Lamp2 on only when both switches are closed. The time sequence at right is an illustration of this. Function-block diagrams The representation of the function block shown above in EL-0231 Figure A is not standard. Here is a little about them, because you are liable to run across them in automation work in industry. Below is how a function block is normally represented: The flow is from left to right. The two lines on the left are inputs. OUT is the function block s output, determined in some way by the input. The little title at the top indicates what type of function the block performs. An example makes this clearer.
This is the standard representation for the XOR function shown at the very top of EL-0231 Figure A. Note that there are no NOT blocks here. Negation is indicated by the little circle on the inputs to the AND blocks at left. Thus, going into IN1 of the top AND block is NOT A. Negation can also be indicated on an output, as shown below. This block is AND, but negated. So it performs the NAND function. There is also an assignment block: What this block does is store the input somewhere in memory. That memory location is specified when the block is set up. But the block does not change the value of the input, so OUT has the same value as IN1 and can be used further downstream. IN1 may contain either a digital value (0/1) or an analog value. If IN1 is an analog value, then its sign is flipped. For the case of a digital input, the assignment block allows us to create a NOT block: The first two blocks negate the input in one way or another, but they are subtly different. (a) does negate the input, stores that negation, then passes it on to the output. (b) does not negate the input, in fact stores it in the designated memory location, but it passes the negation of the input on downstream. (c) is actually a negation block (NOT). Often in computer programming the not function is indicated by <>. Set/Reset blocks: There are two of these blocks SR (set/reset) and RS (reset/set). These are somewhat different blocks from those already covered. They are designed to latch current values, unless other conditions require a change. The blocks are similar to how they function. With both blocks, for example
SET, if high, attempts to set OUT high RESET, if high, attempts to set OUT low maintain OUT at its previous value if neither input is high. This is the latching function. With SR, if SET is high, it blocks RESET. SET high always sets OUT high, regardless of the value of RESET. You might say that SET overrides RESET in an SR block. RS behaves much like SR, but here RESET has priority. SET cannot override RESET. In fact, SET is disabled if RESET is high. SET is only effective if RESET is low. RS has the same maintain feature of SR: if both inputs are low, OUT stays right where it was on the last scan. Examples of using SR and RS are shown below. Function-block timers are shown below. We have studied these timers, and they function as previously described. All three have the same two inputs and outputs. IN is the timer trigger. PT is the Preset time. ET is the elapsed time (Accum). Q is the timer output (DN on a ladder timer). Thus for the TON, the input is triggered on, the timing starts, and Q goes high after PT is finished. TOF is triggered by IN going low. A period of time elapses, and then Q (initially high) goes low. TP produces a pulse. Q goes high for the designated time, then goes low. Thus there is really nothing new here, just the format. There are, of course, function-block counters too: The up counter and down counter count when CU and CD are pulsed. CV is the counter value. The CTU starts counting at 0 and counts up to PV, the preset value. When CV PV, Q goes high. When R is pulsed on the CTU, CV is set once again to 0. The CTD works similarly. But this counter starts with CV = PV and counts down, once for each pulse of CD. When CV reaches 0 or falls below it, Q goes high. The CTU and the CTD only count one way. The up/down counter, CTUD, counts both ways. If CU is pulsed, CV increases by 1. If CD is pulsed, CV decreases by 1. QU goes high if CV PV. QD goes high if CV falls to 0 or below. This counter has two resets. If RU is pulsed, CV is set to 0. If RD is pulsed, CV is set to PV. Example: Motor latch circuit
Below are shown two motor-start latching FBDs. The MTR is started by pressing a momentary NO pushbutton. It is stopped by pushing a NO push button. The FBD on the left used an SR block; the FBD on the right uses a RS block. Let s explore the differences between how these two FBDs function. With both, if the STP button is not pushed, when the STRT button is pressed, the motor starts. With the SR block, OUT is always set high when SET is pushed, regardless of the value of RESET. With the RS block, OUT is set high when STRT is pushed, but only if RESET is low. Thus, if STP is not pushed, so RESET is not active, in both cases the OUTPUT goes high. Also, in both cases, if STRT is not held high and STP is pressed, then OUT will be set low. In the case of SR, RESET works, but only if STRT is not high. In the case of RS, RESET going high sets OUT low, regardless of the state of SET. In both cases, OUT retains its previous value if both SET and RESET are low; so in both cases STRT will be latched so long as STP is not pressed. Thus, the only difference between the functioning of these two FBDs is in the case of both buttons being pushed simultaneously. For safety s sake, the STP button should disable the starting; i.e. STP should have priority over STRT. So the choice of RS to start and latch the pump is best. Example: Motor start with indicator lights Add RED and GRN lights to the FBD above with RS to indicate not running (RED) and running (GRN). The GRN light is directly connected to the MTR; when the MTR is on, GRN is on. The little circle before RED flips the 0 to 1 to make RED illuminate. If MTR is low, RED receives a high command because of the inversion. Example: Motor start with lock-out Take the above example and add a lock-out (LKOUT). For some reason, not specified, MTR can be locked out from starting. Modify the above FBD for this case, with LKOUT as an input. Also, if LKOUT is active, a YLW light comes on. The command from RS to start the motor only is valid if the motor is not locked-out. Thus LKOUT high means the motor is locked out. So if LKOUT is low, the AND block allows the start signal from RS to go through and start the motor. If LKOUT is high, the inverter before IN2 in the AND block flips the high signal to low. With one low signal, the AND block outputs 0, so MTR doesn t start. Example: Motor start after a delay
Start the motor, like above, but with a 10-second delay. During the wait phase, let a YLW light be illuminated and the RED light go off. Obviously for this we need a 10-second timer. Easiest is a TON. The key here is that if the timer has been started but is not yet finished, then IN2 of the XOR will be high, and IN1 will still be low. This means turn the YLW light on. Another thing you can add, if you wish, is to keep the RED light out if YLW is lit. Example: Motor start locked out after a specified number of starts Make a motor-start FBD like above, but let the motor start only 10 times before it is locked out. After 10 starts, some type of other operation/inspection needs to be performed before the motor can be started again. Use the variable names above. Add another input, RST, a button that resets the system after 10 starts. If the motor is locked out, illuminate a YLW light to indicate this. Obviously here we need a counter. Use a CTU unless there is a compelling reason to use CTD or CTUD. For the STRT command to get through to MTR, the output (Q) of the counter must be low. If it is high, the STRT signal does not make it through the AND because Q is negated at the input IN2 of the AND block. Only after RST is pushed does Q go back to low, thus allowing the passage of RS s OUT to MTR. Example: Pump start We want to start a pump. But certain conditions have to exist prior to starting the pump. Liquid has to be present at the pump suction before allowing the pump to start. If a command to stop the pump is issued, the pump will stop or it will disable the start. And the pump start can t have been locked out. If the pump is ever locked out, the locked-out
state must be cleared before the pump can be started again. The decision-making surrounding a pump start is to set the pump state (OFF or ON) and also to determine whether for the next cycle of the controller, the pump should be locked out or not. If the pump is to be started, it is only started after a 2-minute delay. What could cause the pump to be locked out from starting? If a command to stop the pump has been issued, then the pump is locked out from starting until that lock-out is reset. We start off simple. If we have a legitimate Start command and the pump is not LockedOut, then we set PumpRun high. Here LockedOut is an output to be determined by other conditions, but it will also be used as an input to determine whether to allow the pump to run. Recall from the problem statement, that if a legitimate PumpRun command is issued, the pump does not start for 2 minutes. Thus we need a TON too. But the pump is not to start unless there is liquid present. Thus the output of the TON needs to check this condition before the pump is started. Here we ve also moved PumpRun downstream, after the decision about the liquid being present is made. Now, if the pump has been locked out (by pushing the Stop button), it is to remain locked out until the lock-out has been reset. Until this is done, then the pump remains locked out. Thus, here we need to use an SR block. The pump lock-out can only be set if there is no command to start the pump. Thus the SR block would be
The SR block works as follows. If the pump has been locked out, the SR block will keep that state maintained (1 on the OUT of the SR block). The SR block will only allow OUT to be set low if SET is also low, i.e. there is no command to start the pump. Still to be added to the FBD is the Stop command. When it is given (stop button is pushed), the pump stops (PumpRun should go low), and the pump should be locked out (LockedOut goes high). An SR block is a retentive block. So we should use one on the left side of the FBD with Start feeding SET. Then as long as the block is not reset, the output will stay high. If the stop button is hit, OUT is set low. This happen also if Start goes high simultaneously. I.e., Stop takes priority of Start. Thus, maybe we need an RS block instead of a SR block.
This is a bad question. As we have learned, make instructions can go with either NO or NC pushbuttons. Often NO is confused with make and NC is confused with break. This goes back to the days of relay-logic ladder diagrams. So if we assume that this is the mistake made, then Input A stands for a normally open, momentary contact switch. I selected this an confirmed it on the USCG website.
The power supply s two lights show that there is AC power, from which the DC power is produced. The AC power is 120/240 VAC, while the DC power is conventionally 24 VDC. The Processor Module is the PLC itself. The Run, Scan, Fault lights are: Run means that the PLC is in run mode, i.e. actively scanning the ladder program in it. Scan, not sure what that means. My PLC has a Stop button, which means that the program scanning is suspended and the PLC is in stand-by mode. Fault, when lit, indicates that some software fault has been detected by the processor. The DC and AC inputs and outputs are all digital inputs and outputs. The digital DC I/O senses or actuates DC devices while the digital AC I/O senses or actuates AC devices. Often the analog inputs and outputs use 4-20 ma signals. So, for instance, to position a valve at 50%, a signal of 12 ma would be sent out to the valve positioner. Current loops are used because the current is the same throughout the loop, while a voltage signal experiences losses in transmission. Also the 4 ma as a minimum signal is a built-in fault detector. If there is a break in the circuit, the current goes to 0 to signal that break.
Temperature switch have the little zig-zag symbol attached to the switch shown in 8 and 9 above. Thus the NC thermostat is #8. When the temperature rises above a certain level, the switch opens. It would be interesting to know what all these symbols mean. At the top, the pictures, A is a limit switch. You can see the little lever with the wheel on it that something runs along until a protrusion causes the switch to be actuated. Whether it is normally open or normally closed is unclear. Often this is determined by how the switch is wired. There will be three contacts on it, so that one could wire it as normally open or normally closed. In B is shown a proximity switch, I believe. They look like that. C seems to be a pressure switch, since a fluid fitting can be seen on its face. D is, I believe, a temperature switch, judging just from its appearance. E is an electrically actuated valve. F is a float switch. The float on the end of a lever is evident.
Homework problems: 1. Write the truth tables and ladder-logic diagrams for these functions: OR, AND, XOR, NOR, NAND, NXOR. Do not use anything but makes and breaks for each operation (i.e. don t create NAND and NXOR simply by negating AND and XOR). 2. Use our standard TON and TOF to emulate the USCG-TON, USCG-TOF, USCG-TP timers. 3. Create the truth table for the temperature controller shown below. 4. Identify all of the items in the graphic below. 5. Using the information and examples shown for timers, how can you initiate and complete timing with a standard TON using a NOPB? Draw the ladder to do this below. A standard TON has as outputs DN and TT. 6. Write a ladder diagram to make a standard TON behave like a USCG-TON. The difference is that an output will remain high, even when the timer trigger (TGR) goes low.
7. Write a ladder diagram to make a standard TOF behave like a USCG-TOF. The TOF (TGR/DN/TT) should in the ladder should behave as in the time sequence given below. 8. From the figure below develop the truth table and FBD for this case. 9. Take the example shown below. Replace the ANDs with ORs and the OR with AND. Work out the truth table and ladder logic for that configuration. Also answer the question: What type of logical function is this? 10. Work out the truth tables for the SR and the RS blocks. You will need a column OUT-PREV also to determine the results for OUT.