The Network is the PLC
Once upon a time, when your programmable logic controller (PLC) applications grew—that is, when the input/output (I/O) increased beyond some number—you had to buy a bigger, more powerful PLC. The bummer is that “larger” PLCs were usually deeper, taller, wider, more expensive, and not exactly compatible with what you had. Which raises the question, “Why does performance have to coincide with an increase in size?” Jon Giardina, Group Marketing Manager, Omron Electronics LLC (Schaumburg, IL), asks rhetorically.
Well, it doesn’t anymore. PLC vendors are offering a whole gamut of conventional and “soft” PLCs with different processor speeds, program and data memory, and such—all based on the same PLC architecture, plug-in I/O and networking components, programming tools, and associated software tools.
This isn’t necessarily new. What is new is the depth and breadth.
Playing cards & harmonicas
PLCs come in a variety of sizes: large, medium, small, nano, and micro. A “large” PLC is something like an Allen-Bradley PLC-5 from Rockwell Automation (Mayfield Heights, OH). At the other end of the spectrum are self-contained PLCs about the size of a deck of cards. The PLC sizing convention tends to be a function of the number of I/O. Or used to. “I/O capacity no longer seems to apply to form factor,” points out Giardina.
For example, Omron’s CJ1 series of controllers is a micro-sized version of Omron’s CS1 series. Instead of having a backplane, the CJ1 series has a through plane; you build the backplane by stacking together various modules for power supply, CPU, I/O control and interface, serial communications, temperature, and so on. This helps make the CJ1 require only 40% to 60% of the panel space of traditional mid-size PLCs. The CJ1 features processor speeds ranging from 80 nanoseconds to as low as 20 nanoseconds per basic instruction, and up to 2,560 I/O points.
You want smaller? How about the size of a harmonica? Omron’s SRM1 has all of its processing power and control in the master unit, which supports up to 256 networked I/O (128 inputs/128 outputs) across a twisted pair network up to 500 meters away. The PLC’s execution time for basic instructions is 0.97 microseconds.
PLC “downsizing” has a number of benefits. First, specifying PLCs becomes easier because you can buy a lot of power in a small package. Second, smaller PLCs are inexpensive. Figure 30% to 40% less than rack-mount systems. At that price, you don’t have to think twice about sprinkling PLCs all over a machining center, transfer line, entire plant, or whatever. This is even more true now that built-in, integrated communications capabilities are typically standard in these smaller sized PLCs. In short, points out Giardina, the smaller PLCs let you apply controls in places where you couldn’t before because the existing space wasn’t big enough or the networking capabilities didn’t exist easily.
In fact, a lot of logic is being crammed in these micro PLCs. The Simatic S7-200 from Siemens Energy & Automation, Inc. (Alpharetta, GA) has over 200 instructions, including floating-point math, PID, for/next loops, subroutines, and sequence control for both simple discrete control and analog control. Combined with the device’s 20-KHz high-speed counters, interrupts, and 20-KHz pulse outputs, the S7-200 series provides real-time control with Boolean processing speeds of 0.375 microseconds per instruction.
Useful & meaningful
Gone are the days when you have to buy for each family of PLCs different software for PLC programming, troubleshooting, human/machine interface (HMI), and so on—even for families of PLCs by the same vendor. WinPCL, for instance, from Bosch Rexroth Corporation (Hoffman Estates, IL) started out as “an answer to the Windows platform,” explains Karl Rapp, Branch Manager, Machine Tool Industry, for Bosch Rexroth, “but at the same time we revamped the software in the PLC itself.” WinPCL provides a common programming environment for Rexroth controllers independent of application or hardware—soft, PC-based, and rack-based Rexroth CNCs and PLCs. It includes editors, compiler, and various tools.
One of those tools is IndraStep, which provides standard operating modes, system flags, and interlock methods to sequence programming for uniform control implementations across different controllers. Another tool, ProVIi, lets users assign properties to each message within a PLC network. These properties include message type (status, fault, startup, setup, warning), language, module number, and associated additional information (text, pictures, HTML). A third tool is contact-level diagnostics, which displays the conditions that triggered an error. Other information displayed includes trigger status, current on-line status of conditions, and a direct display of logic (ladder or instruction list). The overall end result is better, faster troubleshooting.
WinPCL also lets users select the address or computer name for the PC-based controllers out in the plant. Then, if the local PC on the machine permits remote programming, the user can upload programs, modify programs, download collected data, and keep everything on that local machine.
That naming convention—symbolic, IEC 1131-3 compliant addressing—is also available with Allen-Bradley PLCs (and others). The convention lets you name your data based on its use in an application. This approach makes programming easier. It also, explains John Nesi, Strategic Marketing Manager for the Automation Control and Information Group at Rockwell Automation, eliminates about 25% of the network traffic management that typically resides in a traditional PLC. Why? Because you are fundamentally programming objects using alphanumeric symbols instead of register locations. Therefore, your PLC no longer has to “translate from a memory mapped register to a structure that actually means something to somebody,” continues Nesi.
Hardening soft PLCs
An interesting thing has happened since the advent of soft PLCs: They gave conventional PLCs a run for their money. However, the PLC has proven “very adaptable,” says Giardina. Yet in the same breath, he admits that if the PLC hadn’t adapted and adopted powerful communications, control, HMI, and programming capabilities, then it may have been “swept away a little bit by PC solutions.”
To satisfy both camps, most of the conventional PLC vendors provide soft PLCs. Egon Hillermann, Manager, Product Marketing, Siemens, says it best: “The ultimate Siemens’ view is that we’re not too concerned whether it’s a hard PLC or a soft PLC.”
Same at Rockwell Automation. Its PC-based control system, Logix, does all the things that PLCs do. However, adds Nesi, “We also like to position it as a good deal more than just a PLC.” In particular, Logix-based controllers have more horsepower, more memory, and more networking options (ODBA supported networks, DeviceNet, ControlNet, Ethernet I/P, and SERCOS). They can also handle multi-tasking with a multi-tasking operating system, and they can have multiple processors in the same rack or cards. And there’s no limit to expanding their motion capabilities.
The converse is also true: The soft PLC vendors are offering “hard” PLCs. SoftPLC in Tealware from SoftPLC Corporation (Spicewood, TX), is a PLC with power supply, rack, backplane, I/O modules, plus a 486-based CPU. The standard unit contains 16-MB (expandable) DRAM, 8-MB (expandable) flash disk, and 128-KB battery-backed RAM. It also has built-in serial ports, a 10BaseT Ethernet port, and a remote I/O port. Plus it has hot backup capability. With a PC-104 magazine, SoftPLC in Tealware can have a CPU module, power supply module, four I/O modules, and up to five PC-104 cards. These cards can be for alternate I/O systems (Profibus, DeviceNet, PC-board I/O), communications cards (additional serial ports, Data Highway Plus card, Ethernet card), device interfaces (motion cards, proprietary interfaces), or other cards.
SoftPLC Corp. also sells a line of hardware called a SoftPLC Processor for about the same price as any small PLC. These are cable-ready, lunch box-sized blocks (6 in. x 6 in. x 4 in.) with a mess of electronics inside and slots for plugging in Profibus, DeviceNet, motion, and other capabilities. There is no hard drive, no CD drive, no monitor, no keyboard. It’s a solid-state box that sits in a cabinet. The new series is equipped with a Celeron 500-MHz CPU (with 32MB base memory standard).
Unlike PC-based control systems that first boot an operating system, then the application for control, both these SoftPLC systems are PLCs, with a PLC operating system. Upon bootup, SoftPLC embeds a 32-bit, real-time, multi-tasking kernel into RAM, creating a hard real-time deterministic controller. According to the company, SoftPLC’s instruction execution times on a 486-based system are generally 2 to 10 times faster than conventional PLCs. On Pentium systems, scan times are 50 to 100 times faster than a conventional PLC.
Memories. . .
For about $200, you can add up to 72 MB of memory to SoftPLC in Tealware. Additional memory for conventional PLCs can run you a couple of thousand dollars. Rexroth, back in 1990, used to sell PLCs with 500 KB RAM; about 20% of that was battery-backed for permanent data. Today, Rexroth ships all of its controllers with 2 MB of RAM (4 MB optional) for user data. This supports PLC programs equal to about 300,000 instructions.
What to do with that memory? Cindy Hollenbeck, SoftPLC’s vice president, says people are storing documents in a PLC. By documents, she’s including PDF files, AutoCad drawings, control program documentation (with comments!), archive PLC source code, bills of material, Web pages with menu access to documents, GIF/JPB/MBP graphics, Java applets/servlets, and even video clips. No longer do users need to worry about losing the books, auxiliary programs, and user manuals. Nor do they need to worry about having the proper manual when troubleshooting. It’s all in the PLC’s memory. And it’s all available through the SoftPLC embedded FTP server, which lets users upload, download, and view the documentation just like any other Web page on the Internet.
What’s all this leading to? Siemens’ Hillermann says it best. “You’ve heard `the network is the PC’? Well, that’s where the programmable control is going. In totally integrated automation, the network is the PLC. I no longer have a central PLC controlling everything. Instead, I’ve got intelligent devices scattered all over the place. They can be drives, I/O, weigh scales, anything.”