1. The circuit diagram of FIGURE 1 is of a bell indicator circuit.
(a) Convert this diagram into industrial ladder diagram format (retain all devices).
(b) (i) Convert the diagram of FIGURE 1 into a suitable PLC ladder diagram having the simplest form.
(ii) Provide I/O designation charts to show your choice of inputs and outputs.
(iii) State three benefits obtained by the PLC implementation.
2. The diagram of FIGURE 2 is an extract from an industrial control panel wiring diagram.
(i) Convert the diagram into PLC format.
(ii) Identify all inputs and outputs by the use of I/O designation charts.
(iii) With respect to the operation of the circuit:
(a) explain the purpose of rung 1
(b) explain fully the condition of the circuit elements when indicator IND2 is illuminated
(c) If indicator IND3 is on, explain what will happen if P3 is operated.
3. Provide a block diagram of a PLC system which contains the following:
(i) (a) a CPU
(b) a system clock
(c) a power supply
(d) input and output interfaces
(e) a keyboard
(f) a display device
(g) necessary types of memory.
Describe very briefly the function and essential features of each block.
(ii) Use the diagram shown in FIGURE 3 to briefly explain the fetch/execute cycle of a microprocessor for the instruction 32 00 BC. The instruction is stored in memory locations 1661 to 1663. The op code means 'store the contents of the accumulator in memory address location'. The address given is in the reverese order.
The contents of the accumulator are 78H. Indicate the contents of the PC, AS, registers, accumulator and address BOO after the cycle is completed.
4. The circuit of FIGURE 4 shows a 3-phase, star/delta motor starting circuit. The motor is initially connected in star (contactors C1 and C3 close to form a star with the three motor windings) and run up to speed, then the circuit is switched to run in delta (i.e. a triangular connection of the windings) for normal running (contactor C3 opens and C2 closes, contactor C1 remaining closed). FIGURE 5 shows the control circuit for the motor. The 3 amp fuse can be regarded as a normally closed switch (N/C), and a contact from the fused isolator as a normally open (N/O) switch. The overload current (OIL) can be regarded as a N/C switch and the start button as a N/O switch. The CENT N/C contacts are operated by a centrifugal switch on the motor and open when the motor has reached a run-up speed connected in star.
The contact positions in FIGURE 5 are the positions of the de-energised control circuit (prior to the start button being pressed).
(a) Describe briefly how you think the motor control circuit of FIGURE 5 sequence of events proceeds after pressing the start button. (The start button returns to the open position on release.)
(b) Convert the circuit to an industrial ladder diagram using conventional plc designations for ladder input components (X000, X001, etc.) for switches, contacts, etc. and output designations Y001, Y002, Y003 for output contactor coils C1, C2, C3. Output contactor contacts in the control circuit should be designated as Y001(1_1), Y002(2_0, Y003(3_1) and Y003(3_2). Input and output designation tables may be drawn up or, alternatively, FIGURE 5 component designations can be indicated alongside the chosen plc designations. Memory relays may be used if desired.
5. Explain the operating features of an I/O data image table.
