PLC-2 On Delay Timer

Introduction to the PLC-2 On Delay Timer.

Despite being obsolete and out of date, I still love to program the PLC-2. In this post, we’ll discuss the PLC-2 On Delay Timer. We use the TON instruction to delay the DN bit from energizing. You will see this on conveyors, so a horn will blow for a few seconds before actually starting. In addition, you will see TON on alarms. We might prove the alarm condition exists for a certain amount of time, before setting an alarm for the operator. The TON in the PLC-2 works in a similar way the TON works in the PLC-5, SLC-500, and ControlLogix. Basically, the addressing and limitations are just different.

Without delay, let’s take a look at the timer instruction.

Memory Layout

Notice the timer address typically starts at “030”. The PLC-2 had three possible timer bases: 1.0, .1, and .01. The preset value is valid between 0 to 999. It’s important to realize the processor stores the preset at memory location 130 We store the accumulator at memory location 030 (in the first 12 bits (BCD). These are the 12 right-most bits when viewing the data table. Bit #17 (octal) is the EN (Enable) bit, and Bit #15 (octal) is the DN (Done) bit for the timer at memory location 030.

Here, we have a timer that is running:

Notice the timer enables as soon as the rung goes true. Meanwhile, let’s take a look at memory location 130, which is the preset. We’ll look at this in binary.

As you can see, memory location 131 in binary shows the following:

0000 1001 1001 1001 Which is 0999 in Decimal.

While waiting on the timer to finish, let’s take a look at memory location 030 in binary. Remember the first 12 bits (right-most bits) is our accumulated value.

As you can see, bit 17 is true, meaning the timer is enabled. 0110 = 4, 0011 = 3, and 1001 = 9. This means the timer is enabled, and the accumulated value is currently 439 seconds.

Done (DN) bit for the PLC-2 On Delay Timer

Once the timer has timed out, we’ll take a look at memory location 030 again. This time, you will see that bit #15 is true. Bit #17 is still true because the timer is still true. The EN bit follows the rung condition for timers.

For Timers in ControlLogix, please view the ControlLogix Timers page!

— Ricky Bryce

For Customized automation training, visit my employer's website at!

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