Programming Logical Control

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.

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Greatest Super Bowls

There have been a number of great games over the years which makes finding one of the greatest super bowls out of this list a difficult task. Over the nearly 40 year history of this big game there have been a number of blowouts, but the close ones have given us something to remember for years to come.

5. Super Bowl XXIII (1989, 49ers 20, Bengals 16)

This was San Fran's 3rd Super Bowl of the 80s and was won in thrilling fashion. The 49ers led in net yards 452 to 229 but still was down 16-13 after Jim Breech kicked a 40-yard field goal with a little over three minutes remaining.

In comes Joe Montana, driving 92 yards and ending with a 10 yard TD pass to John Taylor with 34 seconds left.

Rice was named the Super Bowl MVP after gaining a Super Bowl record 215 yards on 11 catches. Montana had a Super Bowl record 357 yards passing.

4. Super Bowl XXV (1991, Giants 20, Bills 19)

The Giants won their 2nd Super Bowl in 5 years by controlling the ball for 40 minutes in this game. The Bills scored 95 points in their two other playoff games that year, but only had their offense on the field for 8 minutes in the 2nd half.

But this game is known for only one thing, and that's Scott Norwood's missed field goal with seconds remaining, as it sailed wide right. That missed field goal basically set the tone for the Bills over the next four years, as they only found disappointment in the Super Bowl.

3. Super Bowl XXXVI (2002, Patriots 20, Rams 17)

As Norwood's miss set the tone for disappointment with the Bills, Adam Vinatieri's 48 yard FG to win the game help guide the Pats to Super Bowl wins in 3 of the next 4 years.

The Pats were 14 point underdogs who were outgained 427-267 in total yards. There were several turnovers in this game that let the Pats stay in it, the first being Ty Law's return for a TD, then Terrell Buckley's fumble recovery, and an Otis Smith interception. These three big plays let New England jump to a 17-3 lead.

After the Rams struck back to tie it up at 17, Brady moved the ball easily with 1:30 left in the game to help Vinatieri drill a 48-yarder as time expired. Brady ended up the MVP despite going 16 of 27 for 145 yards.

2. Super Bowl III (1969, Jets 16, Colts 7)

The guarantee by Joe Namath basically makes this game great. The AFL hadn't even been competitive in the Super Bowl before this and they beat a Baltimore team that had only lost once in 16 games.

Namath backed up his claim by completing 17 of 28 for 206 yards and earning the MVP honors. This game put the AFL on the map.

1. Super Bowl XXXIV (2000, Rams 23, Titans 16)

This game ended with Mike Jones being tackled by Kevin Dyson at the 1-yard line as time expired, giving the Rams their first Super Bowl title.

The Titans defense really stepped up as the Rams took it into the red zone on their first 6 possessions but were held to 3 field goals and a TD.

The Titans finally got their offense going by scoring a pair of touchdowns on two seven minute drives, but a missed 2-point conversion made the score 16-13. Then with only 2:12 left in the game Al Del Greco tied the game up with a 43-yard field goal. It didn't take long for the Rams to strike back as on the first play Isaac Bruce went deep for a quick TD with 1:54 left. The Titans wouldn't quit and drove downfield to Tennessee to the 10 with only six seconds remaining. Without a timeout left, McNair attempted a quick pass to Dyson, who caught it at the 3 but was stopped at the 1 by Jones as time expired. Warner was the MVP with a Super Bowl record 414 yards and 2 TD. Holt and Bruce both had big games, the former with 7 catches and 109 yards with the latter going for 162 yards on 6 grabs.

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