No, that’s not a PC, it’s a PLC

There is much to be written, and much to be done, in the quest for a solid, reliable, real-time control engine to run in the Windows NT environment. Quite a few good people in quite a few good companies are hard at work on the problem—and some may actually have solved it. I’ll be exploring some of their work in this column over the coming months. For those who have made up their minds on a PC-based PLC-like control solution but still shudder at the thought of the dreaded “blue screen,” there may be a reasonable solution already available. In fact, it’s been available since 1988.

Runtime engine is the OS

SoftPLC Corp. (Humble, Tex.) offers a package that includes a proven, mature runtime engine and a ladder-logic programming environment. Where it differs from most is in the fact that the two components run in separate computers. The runtime engine does not run “under” some other operating system; it is a 32-bit real-time operating system for the machine on which it runs. No question about multitasking or multithreading or whether the control engine gets a time slice; the SoftPLC OS kernel makes sure it’s the only game in town.

The PC platform you choose, actually, then becomes a PLC. The instruction set, I/O ad-dressing, program and data file structures are almost identical to a PLC-5. If you’ve ever worked with one of those Allen-Bradley machines you won’t need training to figure this one out. Custom functions and user instructions can be written in C, C[++], or Java. SoftPLC has drivers to run several standard PLC I/O families including A-B’s 1771 and SLC-500 remote, GE Fanuc’s 90/30, Profibus, Interbus-S, Square D Seriplex, DeviceNet, ComputerBoards, ADAM, and others.

Ladder logic 101

The second computer is really a standard personal computer with Microsoft Windows 95 or NT and the programming software from SoftPLC to code and debug the ladder-logic application. That package has been available for years and is called TopDoc. Glitzy or fancy TopDoc is not; it has no pretense other than being a ladder-logic editor and debug tool.

Although it runs under Windows 95 or NT, TopDoc still looks and feels like a DOS console application. Don’t bother clicking the mouse; everything happens by way of function keys and hot-key combinations. Readers that were used to A-B programming in the good-old days will feel right at home again. SoftPLC Corp. plans to add true Windows graphic user interface functionality to TopDoc some time in 1998, and already offers an option to add Java virtual-machine and web-server capability in the run-time computer.

The SoftPLC runtime engine can run in a very minimal (and inexpensive) machine configuration: an 80386 or better, 4MB RAM, 4MB flash disk, and the appropriate I/O interface card. No keyboard, monitor or rotating memory is required.

The programming environment for TopDoc runs under Windows 95 or NT. It requires at least an 80386 processor with 4 MB of RAM, 4 to 6 MB of available hard-disk space and a 1.44 MB floppy. Single-user pricing for the SoftPLC runtime is $995. A single-node license for TopDoc is $995 as well.

For more information on TopDoc, visit www.controleng.com/info .

Author Information

Contributing Editor Jay R. Jeffreys, P.E. is a senior systems engineer at Industrial Systems Design, Johnson City, Tenn.

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