Timer

Let's now see how a timer works. What is a timer? It's exactly what the word says: an instruction that waits a set amount of time before doing something.

As always, different types of timers are available with different manufacturers. Here are brief descriptions of the most common:

• On-Delay timer-This type of timer simply "delays turning on." In other words, after our sensor (input) turns on, we wait x-seconds before activating a solenoid valve (output). This is the most common timer.

• Off-Delay timer-This type of timer is the opposite of the on-delay timer listed above. This timer simply "delays turning off." We hold the solenoid on for x-seconds before turning it off. It is less common than the on-delay type listed above.

• Retentive or Accumulating timer-This type of timer needs 2 inputs. One input starts the timing event (i.e., the clock starts ticking) and the other resets it. The on/off delay timers above would be reset if the input sensor wasn't on/off for the complete timer duration. This timer, however, holds or retains the current elapsed time when the sensor turns off in mid-stream. For example, we want to know how long a sensor is on during a 1 hour period. If we use one of the above timers they will keep resetting when the sensor turns off/on. This timer, however, will give us a total or accumulated time.

Let's now see how to use them. We typically need to know 2 things:

1.     What will enable the timer. Typically this is one of the inputs (a sensor connected to input 0000, for example).

2.     How long we want to delay before we react. Let's wait 5 seconds before we turn on a solenoid, for example.

When the instructions before the timer symbol are true, the timer starts "ticking." When the time elapses, the timer will automatically close its contacts. When the program is running on the PLC the program typically displays the elapsed or "accumulated" time for us so we can see the current value. Typically, timers tick from 0 to 9999 in 10 and 100 msec increments.

Shown below is a typical timer instruction symbol we will encounter (depending on which manufacturer we choose) and how to use it. Remember that, although they may look different, they are all used basically the same way. If we can setup one, we can setup any of them.

Figure 19. Timer Symbol

This timer is the on-delay type and is named T. When the enable input is on the timer starts to tick. When it ticks Y (the preset value) times, it will turn on its contacts that we will use later in the program. Remember that the duration of a tick (increment) varies with the vendor and the time-base used (i.e., a tick might be 1ms or 1 second etc.).

It's important to note that, in most PLCs, counters and timers can't have the same name because they typically use the same registers.

Acounter

A counter is a simple device intended to do one simple thing: count. Using them, however, can sometimes be a challenge because every manufacturer seems to use them a different way.

What kinds of counters are there? Well, there are up-counters (they only count up 1,2,3...). There are down counters (they only count down 9,8,7,...). There are also up-down counters (they count up and/or down 1,2,3,4,3,2,3,4,5,...)

Typically a high-speed counter is a "hardware" device. The normal counters listed above are typically "software" counters. In other words, they don't physically exist in the PLC but instead are simulated in software. Hardware counters do exist in the PLC and are not dependent on scan time.

To use them we must know 3 things:

1.     Where the pulses that we want to count are coming from. Typically this is from one of the inputs (a sensor connected to input 0000, for example).

2.     How many pulses we want to count before we react. Let's count 5 widgets before we box them, for example.

3.     When/how we will reset the counter so it can count again. After we count 5 widgets let's reset the counter, for example.

When the program is running on the PLC, the program typically displays the current or "accumulated" value for us so we can see the current count value.

Typically, counters can count from 0 to 9999, -32,768 to +32,767 or 0 to 65535. Why the weird numbers? Because most PLCs have 16-bit counters. 0-9999 is 16-bit BCD (binary coded decimal) and -32,768 to 32767 and 0 to 65535 is 16-bit binary.

In this counter we need 2 inputs. One goes before the reset line. When this input turns on the current, (Accumulated) count value will return to zero. The second input is the address of the pulses we are counting.

Figure 18. Counter Symbol

For example, if we are counting how many widgets pass in front of the sensor that is physically connected to input 0001 then we would put normally open contacts with the address 0001 in front of the pulse line.

C is the name of the counter. If we want to call it counter 000 then we would put "C000" here. V is the number of pulses we want to count before doing something. If we want to count 5 widgets before turning on a physical output to box them we would put 5 here. If we wanted to count 100 widgets then we would put 100 here, etc. When the counter is finished, it will turn on a separate set of contacts that we also label C.

Note that the counter-Accumulated value ONLY-changes at the off to on transition of the pulse input.

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