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Disclaimer: This document is written in the hope that you can utilize for your own education to gain knowledge of PLC systems (should you decide to utilize this document). Although I believe the information in this document to be accurate, it is YOUR responsibility to verify this information before implementing it in any way, especially when damage to personnel or equipment could result. By continuing to read this document, you agree to hold no one who writes, modifies, or distributes this document liable in any way (even negligence). Due to the wide variety of plant applications, some of the examples in this document may be prohibited at your location, or could cause damage to equipment, or harm personnel.

Author: Ricky Bryce (Montgomery County, IL)

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Data Tables

Data Tables store data. In this lesson, we will take a look at various data tables in the SLC-500. You can think of data tables as variables, such as X, Y, and Z in algebraic formulas. Program files automate the process of reading the values of these variables, and also have the ability to change the value of a variable. In the PLC, these variables are called Addresses.

First, let's look at the first two data tables in the SLC 500. These are the Output Table (O0) and the Input Table (I1).

The processor has a SCAN CYCLE that it performs continuously. During the first part of the scan cycle, the processor reads the value of inputs, and stores the status of the input modules into the input data table. The processor will then execute the ladder logic based on the value of the inputs, and writes to the output table (among other tables). During the last part of the scan cycle, the processor will take the output table, and update the output modules.

Open the Input Data table while you are online. Toggle some switches that are wired to your local input module. You will see the data table respond to changes on the input modules. Remember this takes place during the first part of the scan cycle. Ladder logic normally does not write to the input table, but reads from the input table to make decisions based on the current status of the inputs on your equipment.

Your Potentiometer is wired to channel 0 in slot 4. I:4.0. Since it is an analog signal, it's going to require the whole word (all 16 bits) The reading doesn't make much sense looking at the bit level. Change your radix to decimal, and you will notice a difference in the way the data is displayed for the potentiometer. Changing the RADIX changes the numbering system by which you display data.

Now, let's go to the output table. Remember the ladder logic normally writes to the output table, but we don't have any logic. Therefore, we can manually turn on bits in the output table. When the processor updates the outputs, you will get a status light for any address that has the value of “1”.

Now that you understand how inputs get into the input table, and how outputs are updated, you have a good understanding of how the processor works. Ladder logic just automates this process of looking at the input table, and based on the values of certain inputs, writes values to the output table to control devices in the real world.

Let's look at some of the other data tables: First open the Status S2 file. The S2 file contains information about the condition of the processor. The addresses in the Status file can be used in or ladder logic. For example, we could turn on an output if the battery becomes low, or if forces have been placed in the processor. At this point, you don't need to understand what all the functions are in the status file. Just be aware that this is the file that contains information about the processor.

The next file is the B3 file. The B3 file is internal bit storage. For example: The programmer might turn a bit on stating that the machine is in safe condition and ready to run. This bit can then be used repeatedly throughout ladder logic where this condition is required before movement can take place. The B3 file, when used properly can greatly reduce the amount of time and code required to write a program.

The next file is the T4 file. This file is used to store Timers. Each timer consists of a PRESET value, and ACCUMULATED value, the DN (Done) Bit, TT (Timer Timing), and EN (Enable Bit). Timers can be used to delay events from turning on or off.

The C5 Data file stores Counters. This is similar to the timer file. Each counter will have an ACCUMULATED value, a PRESET value, CU (Count Up bit), CD (Count Down bit), DN (Done bit), OV (Overflow), and UN (Underflow). Counter are used in ladder logic to count rung transitions. For example: A counter can be used to count the number of machine cycles, or how many boxes or parts come off a line.

The R6 (Control) file is just a special workspace for some advanced instructions to operate. We will not be covering this file for this course.

The N7 (Integer) file stores numbers. These are whole numbers (without decimal places). These numbers can range from -32768 to 32767 (0 is considered positive). For example if we need to divide a number by 10, you would need some place to store the result so it can later be used in ladder logic. N7:0 would be an example of a memory location where you could store this value.

The F8 (Floating point file) holds numbers that contain decimal points. For example: If you need to divide 2 by 5, you would store the result into a floating point file. Each element of the floating point file uses twice the memory of an integer element.

Note: You can have 256 data files each of these files containing 256 elements (or until your run out of processor memory) For example: If you need more than 256 timers, you could create another timer data file such as T9. You would then have 256 more timers available for your project.