Introduction

This article provides a brief introduction into documenting software programs for Programmable Logic Controllers (PLC). Its aim is to provide clear guidance in the program documentation for PLC's.

This article is based on the Ladder Logic format of PLC Programming, but many of the points also related to Instruction List (IL) and Function Block Diagram (FBD).

PLC Programs should be written and documented so that they can be easily maintained and modified by others than those who originally wrote the program. All PLC Programming Editors have increased documentation capabilities including Siemens, Modicon, Allen Bradley and Mitsubishi.

Documenting PLC Software
Early Programmable Logic Controllers had little or no capacity for the documentation of programs. For technicians to understand the logic the program had to be followed from inputs to outputs. This made fault finding or modifications to programs difficult.

As PLC's have developed the programming packages have also improved, allowing for the software to be clearly annotated.

The programming tools available for most PLC's allow for the following documentation.

Rung Title
Rung Comments
Symbols and Symbol Comments.
This article provides a practical guide to using these documentation techniques.

Symbols and Symbol Comments
The purpose of symbolic addressing is to allow the PLC programmer to write the software using meaningful symbols rather than having to remember addresses.

General rules for Input and Output symbols should be to use the tags from Process and Instrumentation Diagrams (P&ID) or electrical drawing references. Where these are not available use of a simple description should be used.

For the above the following could be used.

Symbol Address Description
VAC01_STPB I 0.0 Vacuum Pump Start Pushbutton
VAC01_SPPB I 0.1 Vacuum Pump Stop Pushbutton
VAC01 Q 0.0 Vacuum Pump Run
VAC01_TRAL M 1.1 Vacuum Pump Trip Alarm (Latched)
VAC01_AL M 1.1 Vacuum Pump Alarm Latch (Failed to Start/Stop)

Although many program editors can allow for more characters within the symbols it is recommended to keep the symbol short and use the comments to provide further details. Our recommendation is to build the symbols from the blocks of two characters to represent the the meaning and to keep the length less than 10.

The numbers tell a tale of programmable logic controller ( PLCs ) firm entrenchment in control applications. For instance, hardware sales figures for 2001 show $80 million spent for open control (PCbased control), with $1.1 billion being spent on programmable logic controller ( PLCs ) solutions. Even as the trend is for both figures to grow by 2004, programmable logic controllers ( PLCs ) still have a sizable advantage over PCs.

Engineers these days are calling for networking capabilities. A programmable logic controllers ( PLCs ) are now low-cost dedicated controllers. They offer more processing power and have added networking capabilities with other programmable logic controllers ( PLCs ) through serial connections. A micro programmable logic controller ( PLCs ) has an Ethernet connection with optional DeviceNet or Profibus modules. So the total system is not just a stand-alone controller, but one that can network to bigger programmable logic controllers ( PLCs ). Also, increased processing speeds allow for any given programmable logic controller ( PLCs ) to do more. Because of these changes, the programmable logic controller ( PLCs ) remains a viable control option.

One trend is towards a so-called hybrid programmable logic controller ( PLCs ). These more common hybrid applications are challenging the capabilities of each control approach. Their number is growing because fewer applications are pure discrete or pure process installations. One example is a programmable logic controller ( PLCs ) on a board with sequence logic-controller functions. It plugs into a PC ISA slot and has the same programming and instruction set as a stand-alone programmable logic controller ( PLCs ). So if you already have a PC on the factory floor, you don't need to buy a programmable logic controller ( PLCs ) in a box, just the board that plugs into the PC. It has a separate power supply so that if the PC crashes, the programmable logic controller ( PLCs ) keeps running. The board also has its own I/O connectors.

Another type of programmable logic controller ( PLCs ) is an industrially hardened gateway device. It comes with a number of ports and can be loaded with a soft programmable logic controller ( PLCs ). It has Ethernet, DeviceNet, and serial communications and serves as a bridge between protocols on the factory floor between legacy systems and current systems. For instance, data from serial devices such as bar code scanners can be connected to the gateway and through the soft programmable logic controller ( PLCs ) program.

After more than a decade of articles touting the benefits of PC-based control over a programmable logic controller and predictions of the PLC's demise, this much is clear: both PCs and programmable logic controllers ( PLCs ) have their place in contemporary control systems. In many cases, they will work as a team.

In lower-end process industries such as food plants where the programmable logic controller ( PLCs ) is strongly entrenched, there is little interest in switching to PC-based control. The reason is the programmable logic controller's ( PLCs ) suitability. Sequential logic control provided by a programmable logic controller ( PLCs ) is best suited for round-the-clock operations common in those industries. In particular, a programmable logic controller ( PLCs ) can offer repeatable, reliable control conducive to high-speed, repetitive processes. Dedicated performance and a high level of security are also benefits of a programmable logic controller (PLCs ). The packaging side of a food plant lends itself well to sequential logic control which makes a programmable logic controller ( PLCs ) the best choice for conveying and sorting applications.

Five years ago, the prediction was that PCs were going to take traditional programmable logic controller ( PLCs ) applications by storm. The reason they didn't is that the programmable logic controller ( PLCs ) has continued to be a good, low-cost control option. For instance, a programmable logic controller ( PLCs ) with highspeed counter inputs, loop controls, analog points, and basic math calculations are available in the $200 to $400 range with 100 or more I/O and with options for expansion. On the other hand, even buying an inexpensive PC requires loading software, setting it up, and making sure it's robust enough to not crash.

However, PC-based control has its merits. Specifically, it has greater number-crunching capability, more communication options, and is geared more toward data logging and trending, plant monitoring, report generation, and documentation. The key reasons to switch to PC-based control include networking the control system to higher-level applications such as ERP, handling complex mathematical applications such as vision inspection systems, and connecting to bar-code scanners, in-motion weigh scales, and other devices. In short, most users believe that when data must be transferred from the controller into a database for analysis, the transfer goes more smoothly when the data is already on a PC-based system.