Section 2 PLC Hardware

Question 1

Is it possible to download a new program to a PLC while the processors key is in the “run”
position? If not, what do you need to do first?

Answer 1

No you must either put the key switch into REM (remote) or PROG (program) and change the mode to program mode in the development PC

Question 2

Explain the scan sequence of a typical PLC? When is the scan sequence executed?

Answer 2

1) Read inputs
2) Solve program logic
3) Write outputs
4) Sequence activates when the PLC is put into RUN mode

Question 3

What is an Alias tag and how is it used?

Answer 3

Alias tags refer to memory locations that are already defined by another existing tag.

Question 4

When would you use an alias tag?

Answer 4

1) Program logic in advance of wiring diagrams
2) Assign a descriptive name to an I/O device
3) Use a descriptive name for an element of an array

Question 5

What is an internal tag and how does it differ from a base tag?

Answer 5

An internal tag is a tag that is not aliased to a base tag address.

Question 6

Explain the difference between a controller tag and a program tag.

Answer 6

A controller tag is global while a program tag is local

Question 7

Describe a Sinking I/O Circuit

Answer 7

Field devices connected to the negative side (DC Common) of the field power supply are considered sinking devices.

Sinking I/O circuits are driven by a current sourcing field device

Question 8

Describe a Sourcing I/O Circuit

Answer 8

Sourcing I/O circuits supply current to sinking field devices