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programming environment is like the Java Virtual Machine. The programs are interpreted within the ladder logic model. Some features are not available, while others are enhanced.

•Each PLC vendor has developed one or more different PLC models that bare some similarity at the level of basic inputs and outputs. To unite these models there has been recent work to develop a common standard so that all PLCs are similar (IEC 1131) but this is not to the point where it will free the user from a single vendor.

•At the present time inputs and outputs are becoming more flexible, but the central computation controller (PLC CPU) is still vendor specific. Consider that you can write a program in Java that will compile on dozens of java compilers.

•There are two possible directions that this might go,

-some third party companies are developing products that are based on other companies models, such as SoftPLC being based on the PLC-5

-a standards group may be able to develop a standard addressing and instruction set. And a common execution model may also be developed that will allow vendors to develop a common engine.

19.6 PRACTICE PROBLEMS

1.PLC’s typically contain:

a)EPROM

b)Battery

c)Both a) and b)

d)None of the above

2.A PLC can only go through the ladder logic a few times per second. Why?

3.Indicator lights are used on PLC’s for,

a)maintenance

b)diagnostics

c)a) and b)

d)none of the above.

(ans. c)

8. What will happen if the scan time for a PLC is greater than the time for an input pulse? Why? (ans. the pulse may be missed if it occurs between the input scans)

11. What is the difference between a PLC and a desktop computer?

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19. Why do PLCs do a self check every scan?

4. TRUE or FALSE -- The test time for a PLC will be long compared to the time required for a simple program.

(ans. true)