Delaying outputs on and off

Introduction to delaying outputs on and off

In this post, I’ll explain delaying outputs on and off can be used in the SLC500 processor. For example: We have a motor with a fan. When a main motor starts, a fan needs to start two seconds later. When the motor is shut off, the fan will run for an additional 15 seconds. This document is provided for example only.

We will use both the TON (On Delay) timer, and the TOF (Off Delay) timer. The purpose of the TON is to delay the DN (Done) bit from turning on. The TOF instruction will delay the DN bit from shutting off. Remember that with the TOF instruction, the DN bit will go true immediately when the timer starts.

Our Logic

Here is my first rung:

When the Main Motor starts, the TON instruction will start to time. We will use the DN bit of the timer to turn the fan motor on. The TOF instruction is enabled, so it’s DN bit will go to 1. Later, we will look at when this DN bit shuts off. This will stop our fan motor. I’m setting the time base to the value of 1. In the SLC500 and PLC-5, the total seconds equal the preset times the time base. In ControlLogix, your time base is milliseconds only.

Secondly, we’ll use the DN bit of the timer to turn on our fan motor:

When the Main motor starts, T4:2 will start. After two seconds, the DN Bit will go true. This will then latch on the fan motor.

Next, we need to set the conditions for the fan motor to stop. Remember in the first rung, we energized the TOF instruction. When the TOF is energized, the DN Bit was set. This DN bit will remain on for 15 seconds after the main motor is stopped. When the DN Bit drops out, we’ll unlatch the fan motor’s bit.

Here is our third rung:

Remember the latch is retentive. This means that if the processor goes to program mode, or even looses power, the latch is still on. I’m using S:1/15 to shut off the fan motor when the processor restarts.

Using only TOFs

In training, you may come across projects that are designed to prove your knowledge on the TOF instruction. If this is the case, we need to change the logic up just a little bit.

When the motor starts, we need to trigger the first TOF using an OSR instruction (OR ONS in PLC-5 and ControlLogix). When we place the OSR instruction before the timer, it will begin to time after it has gone true for one scan. The second TOF is used to delay the fan from shutting off later on.

In the second rung, we need to state the conditions under which we need the fan motor to run. Before turning the fan motor on, we’ll make sure the main motor is running, then wait for the first TOF (T4:2). Since this is an Off Delay timer, the DN bit will drop out after 2 seconds, and start the motor.

Finally, we need to condition when the fan motor shuts off. We will do this when T4:3’s DN bit drops out after 15 seconds. In this example, I’m also shutting it off when the processor starts on the first scan.

For more information on the SLC500, visit the SLC-500 Category page!

— Ricky Bryce

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