The PLC gets small: the physically smaller PLCs get, the more control applications they fit in. Now comes one that's a single chip.

Its name tells it all. "PLC on a Chip" is a programmable logic controller (PLC) on a single microprocessor chip. Sure, it needs to be mounted on something. Sure, as with other microprocessors, a static charge will fry it. Sure, it requires a power supply and a bus for inputs/outputs (I/O) and some type of programming interface and some other external, off-chip items. But just as assuredly, it is an honest-to-goodness PLC that can be embedded into devices, machines, and systems. At an affordable price.

The goal of the chip, explains Terry Divelbiss, president of Divelbiss Corp. (Fredericktown, OH), is to "offer an alternative to OEMs manufacturing front-end equipment, tail-end equipment, or the equipment in the middle." He points out that there's a need to go directly from the logic level of the sensor, machine tool, or whatever to a PLC. An embedded PLC does exactly that. "To me, a PLC is a combination of the right programming capability with everything else ready to go--communications, digital and analog I/O, and high-speed counting capability," says Divelbiss.

The PLC chip he's selling measures 21.5-mm (0.8465-in.) square. It comes in four types that are mostly differentiated by on-board flash memory (128 KB to 512 KB) and random access memory (8 KB to 14 KB). The chips are similar to the CPU in a desktop computer in that they require 5 VDC. They can operate in temperatures between -40[degrees]C to 85[degrees]C (one is ruggedized for temperatures up to 125[degrees]C). All the chips have asynchronous serial communications (up to 11.5 kbaud) and eight analog ports (0 to 5 VDC input, 10 bit). Two of the chips offer up to five CAN ports. (These are proprietary CAN ports that can communicate to CANopen.) They also have some pulse-width-modulation ports: up to eight 8-bit channels or four 16-bit channels. For digital I/O, the chips support 26 direct I/O or 24 inputs, 15 outputs, plus 256 external I/O points. All I/O and integrated functions are preassigned.

The chip's kernel is programmed through conventional PLC ladder logic or function block languages, versus C or some other chip-level programming language. (You program the chip using Divelbiss' own PC-based EZ Ladder software, which conforms to the IEC-61131 standard plus extensions.) The chip can hold up to 51 instructions per function block. These are typical PLC instructions, including contacts, counters, times, drum sequencers, and functions for math, bit manipulation, closed loop control, and communications. Function block programming provides additional decision process capabilities, such as comparisons so that a counter, when reaching some pre-specified value, triggers an event. In test operations, PLC on a Chip can scan through 500 rungs of ladder logic in 2.5 msec. Not surprisingly, scan times are longer in control programs with a lot of math in the function blocks.

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WHAT YOU GET IN A SMALL BOX

You could get these same PLC capabilities through a custom control chip. That's not only expensive, but dedicated chips and "one-offs" don't often conform to any sort of standard. "The ability for the OEM's customer to make changes doesn't exist, unless it's a simple menu thing," says Divelbiss. Another option is to go the "soft PLC" route. This is where you have PLC functionality in an industrialized personal computer. The debate over conventional PLCs and soft PLCs is legion.

Then there's the tried-and-true approach: Buy and install a box of industrial control. "The marketplace is going for small controllers. It's growing and it's diversifying," says Sy Stevens, product marketing manager for Rockwell Automation (Milwaukee, WI). "And as technology increases, obviously the size (of PLCs) decreases."

According to Divelbiss, the PLC on a Chip is much more than a smart relay, and much more than a smart relay, and much more than a relay replacer that basically consists of a timer and a counter. This chip, says Divelbiss, is more like a micro PLC.

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Okay, let's look at that. Micro PLCs are targeted for small machine control, where space for additional electronics (read "control system") is at a premium and the operating environment (read "harsh") demands reliable performance. For instance, the 16 I/O, DC-powered Allen-Bradley MicroLogix 1000 measures 120 X 80 X 40 mm (4.72 X 3.15 X 1.57 in.). Execution speed for a typical 500-instruction program is 1.56 ms (throughput 1.85 ms). For communications, the PLC offers direct connections to programming devices or operator interfaces, as well as options for DH-485 networking, DeviceNet, and EtherNet/IP.

If PLC size is the overriding factor, the next step in terms of compactness is the nano controller. The Allen-Bradley Pico GFX-70 announced in January, for example, is aimed at simple logic, timing, counting, and real-time clock operations. Unlike other Allen-Bradley Pico controllers, this one includes a 70-mm (2.75-in.) backlit monochrome LCD display (a.k.a. human/machine interface, HMI) and keypad buttons for both control programming and control monitoring. The unit measures (with keys) 86.5 X 86.5 X 43 mm (3.41 X 3.41 X 1.69 in.) and weighs 130 g (0.287 lb). Mounting requires two 22.5-mm (0.886-in.) holes spaced 30 mm (1.18 in.) apart. The components for the power supply and I/O snap onto the back of the display. The processor can also be mounted to a panel or DIN rail for use without the HMI. The Pico communicates through GFX proprietary Pico-Link, a proprietary protocol based on CANopen, that lets up to eight Pico units be connected from up to 1,000 m (3,280 ft) away, providing up to 272 I/O points through Pico expansion I/O modules. However, admits Stevens, "If you're looking to get information from basically the shop floor to the top floor, you're not going to want to use a Pico. You want something like a MicroLogix, which can jump onto a major network."

ONE WORD: "EMBEDDEDNESS"

The cost of the PLC on a Chip ranges from about $20 (low-end chip; 5,000 quantity) to just over $40 (high-end chip; in trays of 60). Modules, separate boards with features such as connectors and critical circuits that facilitate implementation of the chips, cost more, but they're simpler to use. Low quantities of the 128 KB flash-memory version of the module, without the real-time clock, can go for as much as about $70. With CAN ports, the module can cost as much as $90. Compare this to the cost of the Pico GFX-70. These start at $525, while a standard Allen-Bradley Pico (without the HMI) can be had for $150 in large quantities.

The comparison is not exactly apples-to-apples. The Pico PLCs are handy, small, inexpensive, powerful, packaged units. However, they satisfy a completely different niche on the factory floor than the PLC on a Chip, which can be embedded into sensors, motor drives, machine and engine controls, remote control and monitoring system (such as SCADA systems)--you name it. Divelbiss envisions sprinkling these PLC chips all over a conveyor system. This way, you can have sensor capabilities and motor control--intelligent decision/control points--all over a materials handling system. The alternative approach would be to physically wire individual points or local controllers to whatever flavor of PLC out on the shop floor or that can be hung nearby.

By Lawrence S. Gould, Contributing Editor