P ROGRAMMING N O R M A L LY C LOSED I NPUTS

So far in our discussion, we have tried to avoid presenting input device connections that are in the normally closed condition. The reason for this is simple—we did not want to confuse you. Understanding how to program a normally closed input device is a difficult concept to comprehend at first. Once you learn it, try explaining it to someone else and watch their reaction.

To explain how to program normally closed inputs, let’s look at the following example. Suppose we want to implement logic identical to the simple hardwired circuit shown in Figure 9-31. Implementing the same logic means that the pilot light PL1 in the PLC should behave in the same manner as the one in the hardwired circuit—if PB1 is not pushed, PL1 will be ON; if PB1 is pushed, PL1 will be OFF. Figures 9-32 and 9-33 show two possible methods for programming PB1 and implementing the logic. At first glance, you may think that the solution in Figure 9-32 is the answer, but that is not true; Figure 9-33 is the correct implementation.

Figure 9-31. Hardwired logic.

Figure 9-32. Logic implementation with PB1 programmed as a normally closed contact.

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

S ECTION PLC Programming C HAPTER 3 Programming

Figure 9-33. Logic implementation with PB1 programmed as a normally open contact.

In Figure 9-32, the reference address of PB1 (input 10) is programmed as a normally closed contact (examine OFF) that drives output coil 100, which is connected to pilot light PL1. When the PLC starts, it reads the status of the input device connected to input 10 and stores this data in the input table. If PB1 is not pushed (see Figure 9-34a), the processor reads input 10 as logic 1 (power flowing to the module). During the execution of the ladder logic, the PLC will evaluate the examine-OFF instruction, and since the reference (input 10) is ON, it will open the normally closed contact, disrupting continuity. Thus, output 100 will be OFF, and PL1 will not turn ON. Conversely, if PB1 is pushed (see Figure 9-34b), the input module at location

10 will be logic 0 (power not flowing to the module). The processor’s examination for an OFF condition at reference 10 will then be TRUE; therefore, the instruction will provide continuity to the rung and turn output 100 and PL1 ON.

Figure 9-34. Power flow through the circuit shown in Figure 9-32 with (a) PB1 not pushed

and (b) PB1 pushed.

In Figure 9-33, the normally closed input condition has been programmed as an examine-ON instruction. During operation (see Figure 9-35a), if PB1 is not pushed, the input module 10 will read an ON status. When the processor evaluates the ladder rung, its examination for an ON condition at reference 10 will be TRUE. Therefore, contact 10 will close to provide power to the rung, turning output 100 and PL1 ON. On the other hand, if PB1 is pushed (see Figure 9-35b), the input will have an OFF status and the processor will store

a logic 0 in the input table. During the evaluation of the rung, the processor will find its examination for an ON condition at reference 10 to be FALSE (input 10 is OFF), and continuity will not occur because the contacts will remain open. Thus, output 100 and PL1 will be OFF.

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

S ECTION PLC Programming C HAPTER 3 Programming

Figure 9-35. Power flow through the circuit shown in Figure 9-33 with (a) PB1 not pushed

and (b) PB1 pushed.

The programming solution for a normally closed input connection, as shown in Figure 9-33, exemplifies the following: for a normally closed wired input device to behave as a normally closed device when connected, it must be programmed as an examine-ON, or normally open, contact instruction. Discrete inputs to a PLC can be made to act as normally open or normally closed contacts, regardless of their original configuration. This ability to examine a single device for either an open or closed state is the key to the flexibility of PLCs—no matter how a device is wired (normally open or normally closed), the controller can be programmed to perform the desired action without changing the wiring. Remember that the programming state of an input depends not only on how it is wired, but also on the desired control action. The following example shows a case in which the PLC programming of one push button with two contacts differs depending on which contact is wired to the module.

E X AM PLE 9 -4

Show the PLC implementation of the hardwired logic shown in Figure 9-36 for the following scenarios using only one push button connection: (a) with the normally open contact connected to the input module and (b) with the normally closed contact connected to the input module. Describe the operation of each implementation as well.

Use input address 10 for the push button and addresses 30 and 31 for pilot lights PL1 and PL2, respectively. Indicate the lights in the ON condition (without PB1 being pushed) using a shaded PL indicator.

S OLU T I ON

Examining the circuit in Figure 9-36 shows that, if PB1 is not pushed, PL1 should be OFF. PL2 should be ON because the other contact of PB1 (the normally closed one) provides power to PL2. We can wire any

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

S ECTION PLC Programming C HAPTER 3 Programming

Hardwired Logic

Figure 9-36. Hardwired logic for Example 9-4. of PB1’s two connections (A or B) to the input module to satisfy the

required logic. Remember that we can make any contact act as we desire in the PLC program (i.e., as a normally open or normally closed contact).

(a) Figure 9-37 shows the solution for the normally open contact connection. An examine-ON instruction drives PL1, and an examine- OFF instruction drives PL2. When PB1 contact A is not pushed, PL1 is OFF and PL2 is ON. The first rung implements a push button wired as normally open to act as a normally open push button, while the second rung implements a push button wired as normally open to act as a normally closed push button.

B wired to

Figure 9-37. Normally open implementation of Figure 9-36.

(b) Figure 9-38 shows the circuit solution for the normally closed contact connection. In this solution, an examine-OFF instruction drives PL1. During operation, PB1 contact B provides power to the module if it is not pushed; therefore, the reference address (10) is logic 1. The normally closed contact with address 10 will be open as long as PB1 is not depressed, keeping PL1 (output 30) OFF. In the second rung, an examine-ON instruction drives the output for PL2 (31), which is

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

S ECTION PLC Programming C HAPTER 3 Programming

Languages 9

closed as long as PB1 is not pushed. The first rung implements a push button wired as normally closed to act as a normally open push button, while the second rung implements a push button wired as normally closed to act as a normally closed push button.

A wired

to PLC input

10 31 PL2

Figure 9-38. Normally closed implementation of Figure 9-36.

As illustrated in the previous example, a normally open input can be programmed in a PLC to behave like a normally closed device and vice versa. However, for fail-safe reasons, normally closed input devices should

be wired to the input module as normally closed devices and then programmed as examine-ON instructions, so that they behave like normally closed devices. A wired normally open device must not be programmed to act as a normally closed device, especially if it is being used to interrupt continuity when a device is pushed or closed.

Figure 9-39a shows an example of a normally closed stop push button used to stop the power to a motor. During operation, when the start PB has been pressed and sealed by the internal motor contact (100), the motor turns ON (see Figure 9-39b). The normally closed stop PB interrupts the power continuity to the motor output coil contact. The pressing of this stop push button is the only way the motor can be stopped (see Figure 9-39c). However, if the wire connection for the stop PB is accidentally cut, the motor circuit will disengage (see Figure 9-39d).

This same logic operation can also be achieved using a normally open stop PB instead of a normally closed one and implementing it as a normally closed circuit in the PLC program (see Figure 9-40a). When the start button is pushed, the motor turns ON (see Figure 9-40b); if the stop PB is pressed, the motor turns OFF (see Figure 9-40c). However, there is no way to stop the motor from running if the normally open stop PB wire is cut (see Figure 9- 40d). The programmed examine-OFF instruction corresponding to the stop PB will never disrupt continuity in this situation. The only way to stop the motor is to shut down power to the whole PLC system. This type of PLC system configuration is dangerous and should be avoided at all times.

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

S ECTION PLC Programming C HAPTER 3 Programming

Motor M

Stop

(a) The normally closed stop push button is programmed as normally open. Contact 100 is used as an interlock with the start push button after the start is pushed. When the start push button is pressed, the motor turns ON.

Motor M

Stop

(b) After the start push button is pressed and released, the motor remains ON.

Motor M

Stop

Motor M

Stop

(d) If the stop push button connection breaks when the motor is ON, the motor will

turn OFF.

Figure 9-39. Normally closed stop push button programmed as normally open.

Motor M

Stop

(a) The normally open stop push button is programmed as normally closed. When

the start push button is pressed, the motor turns ON.

Motor M

Stop

(b) After the start push button is pressed and released, the motor remains ON.

Motor M

Stop

Motor M

Stop

(d) If the stop push button connection breaks when the motor is ON, pressing the stop push button will not turn the motor OFF. This is a dangerous situation.

Figure 9-40. Normally open stop push button programmed as normally closed.

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

S ECTION PLC Programming C HAPTER 3 Programming

Languages 9

P ROGRAMMING N O R M A L LY C LOSED I NPUTS