Transcription of Chapter 7 TIMERS, COUNTERS and T/C APPLICATIONS
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Chapter 7 TIMERS, COUNTERS and T/C APPLICATIONS
Ch 7 Timers, COUNTERS , T/C APPLICATIONS 1 Chapter 7 TIMERS, COUNTERS and T/C APPLICATIONS Introduction Timers and COUNTERS are discussed in the same Chapter since most rules apply to both. Timers and COUNTERS have been in existence for as long as relays and provide an important component in the development of logic. Timers were constructed in the past as an add-on device to relays slowing down the transition of the plunger from immediately opening or closing. The time delay was accomplished with a pneumatic bladder that allowed the air to escape either quickly or slowly depending on the setting of the timer. Quick was usually less than a second and slow was usually between 30 and 60 seconds. Setting this kind of timer was an inexact science and today's traffic lights are an example of the fickle nature of timers that seldom respond in exactly the same from day to day and year to year. For the first time , function blocks are introduced in the rung output position or coil position to provide timer and counter functions.
the main function of all timers, a time delay. While PLC vendors do not need to use the terms of on-delay or off-delay, normally closed, normally open, held closed, or held open, these terms are an important part of design of PLC circuits. Some vendors still use the terms to show linkage between the PLC and the original timer circuits.
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