Deck 7: Relay Type Instructions

Question 1

Explain the purpose and functionality of the EXAMINE OFF instruction.

Answer 1

When the N.C. symbol is programmed in a ladder diagram, the processor views it as a request to examine the address of the contact for an OFF condition. Any address that is actually OFF becomes logically true and power can flow. If an N.C. contact has an input address and the real-world input device is open, or OFF, the processor sets the bit to 0, or OFF. As the EXAMINE OFF instruction is looking for an OFF condition, a bit set to 0, or OFF, is a true condition and there would be logic continuity, so power can flow through the contact. If the input device had been closed, or ON, the bit would be set to 1, or ON. The EXAMINE OFF instruction can only be logically true when an OFF condition exists. Any bit set to 1 is viewed as an ON condition which makes an EXAMINE OFF (N.C.) contact false and no power can flow.

Question 2

Explain the letters shown in a PLC-5 word or bit address.

Answer 2

An "I" preceding a word and bit number indicates an input whereas an "O" preceding a word and bit address indicates an output.

Question 3

Because the processor views the N.O. contact instruction as a request to examine a given address for an ON condition, it is referred to as a(n) ___________________ instruction.

Answer 3

EXAMINE ON

Question 4

Discuss the PLC N.O. contact symbol.

Question 5

___________________ contacts in a motor control circuit control the path for current flow to the output.

Question 6

Ladder logic is a good choice for a programming language because it closely resembles the way circuits are hard-wired.

Question 7

What do the terms current flow and power flow indicate?

Question 8

When the N.C. symbol is programmed in a ladder diagram, the processor views it as a request to examine the address of the contact for a(n) ___________________ condition.

Question 9

Whenever the processor sees an N.O. contact in the user program, it views the contact symbol as a request to examine the address of the contact for a(n) ___________________ condition.

Question 10

The EXAMINE OFF instruction is only logically true when the device referenced is ON, or open.

Question 11

There are many more applications and circuits that normally require two- or three-pole devices that now only require single-pole devices when programmed for a PLC.

Question 12

The number of times an input device can be addressed and used in a programmed circuit is limited to eight.

Question 13

When the N.C. symbol is programmed in a ladder diagram, any address that is actually ON becomes logically true and power can flow.

Question 14

Explain the purpose and functionality of the EXAMINE ON instruction.

Question 15

A PLC is normally programmed using a(n) ___________________ logic-type language.