plc外文文献及翻译 第7页

All PLCs use RAM for the CPU and ROM to store the basic operating system for the PLC. When the power is on the contents of the RAM will be kept, but the issue is what happens when power to the memory is lost. Originally PLC vendors used RAM with a battery so that the memory contents would not be lost if the power was lost. This method is still in use, but is losing favor. EPROMs have also been a popular choice for programming PLCs. The EPROM is programmed out of the PLC, and then placed in the PLC. When the PLC is turned on the ladder logic program on the EPROM is loaded into the PLC and run. This method can be very reliable, but the erasing and programming technique can be time consuming. EEPROM memories are a permanent part of the PLC, and programs can be stored in them like EPROM. Memory costs continue to drop, and newer types (such as flash memory) are becoming available, and these changes will continue to impact PLCs.plc外文文献及翻译 第7页

6 。 SOFTWARE BASED PLCS

The dropping cost of personal computers is increasing their use in control, including the replacement of PLCs. Software is installed that allows the 本文来自辣,文.论#文,网，加7位QQ324'9114找源文
www.751com.cn to solve ladder logic, read inputs from sensors and update outputs to actuators. These are important to mention here because they don’t obey the previous timing model. For example, if the computer is running a game it may slow or halt the computer. This issue and others are currently being investigated and good solutions should be expected soon.

7 。 SUMMARY

• A PLC and computer are similar with inputs, outputs, memory, etc.
• The PLC continuously goes through a cycle including a sanity check, input scan, logic scan, and output scan.
• While the logic is being scanned, changes in the inputs are not detected, and the outputs are not updated.
• PLCs use RAM, and sometime EPROMs are used for permanent programs.

8 。 PRACTICE PROBLEMS

1. Does a PLC normally contain RAM, ROM, EPROM and/or batteries?
2. What are the indicator lights on a PLC used for?
3. A PLC can only go through the ladder logic a few times per second. Why?
4. What will happen if the scan time for a PLC is greater than the time for an input pulse? Why?
5. What is the difference between a PLC and a desktop computer?
6. Why do PLCs do a self check every scan?
7. Will the test time for a PLC be long compared to the time required for a simple program?
8. What is wrong with the following ladder logic? What will happen if it is used?

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