Pass it on: Relaying PLC messages

This month's TechFlash column reviews the latest advances in relays, and why the programmable varieties in particular retain popularity with control system designers.

By Jim Montague, Executive Editor, and Joe Feeley, Editor in Chief

WHAT'RE YOU lookin’ at? These days, that can be a very good question. For example, the line between relays, which are growing more capable and flexible, and PLCs, which are becoming smaller, less-complicated and downright inexpensive, is really blurring. At this transition zone, it appears that even calling one or the other a PLC or a relay can be a tossup.

“We have smart programmable relays that incorporate many features that smaller PLCs have,” says Tim Roberts, staff product specialist at Schneider Electric. He describes some of those functions as:

  • Expansion capability up to 40 I/O, with optional analog inputs;
  • Programming in both function block diagram (FBD) and ladder logic;
  • Electrically erasable, programmable read-only memory (EEPROM) for easy and fast transfer of a program from one relay to another;
  • Programming capabilities on the front face of the module or via a PC; and
  • Remote control from a communication module (RTU) or Modbus via an RS485 port, not to mention an Ethernet communications option.

Omron Electronics has small programmable relays that combine relays, timers, counters, time switches and some advanced control functions. In addition, Omron says the CPU units can be extended with up to three expansion units, and there are up to 44 I/O, including expansion modules. These basic units are available as a C1 type, which have a real-time clock, an LCD screen and buttons, while theC2 type has LED indication and no buttons.


"You now can expect more from relays in discrete control applications. They give flexibility to perform multiple logic sequences from one source, instead of relying on multiple control relays to do the same functions."

In fact, a compact series of programmable relays from KB Electronics can replace PLCs in many machine automation and process control applications. Several models offer a choice of input power (AC or DC), number of input/outputs (10, 12, and 20), and input type (digital DC or AC and 0-10 VDC analog). All models contain high-current independent output relays.

There are many more examples, and many sound pretty much like PLCs. “By incorporating many features of smaller PLCs, such as more I/O count and simple programming logic coupled with true industrial control relay rated contacts, a relay can be a better option than a PLC,” argues Roberts. "Where a PLC once was required to replace a group of control relays, a smart relay now can do the same, save the customer time, and reduce wiring costs.”

Most of these new relay/controller devices provide programming capability, some downloadable, to individually program each relay to be energized above or below a setpoint, or be disabled. Devices supplied by companies such as Laurel Electronics can be individually programmed to operate in a latching or non-latching mode. In latching mode, when an alarm or shutdown condition is reached, the output remains in the alarm condition until reset by front-panel pushbuttons, via the serial interface, or via the rear connector. In non-latching mode, the output is automatically reset when the alarm condition no longer exists.

Similarly, relays can be set to operate in a band-deviation mode, in which an alarm is generated whenever the reading is a specified number of counts above or below the setpoint. In noisy environments, a programmable time delay and reduction of relay chatter can be achieved too.

Everyone seems to agree that you now can expect more of relays in discrete control applications, since they give users greater flexibility to perform multiple logic sequences from one source, instead of relying on multiple control relays to do the same functions. That adds up to savings in components, wiring time, set-up and commissioning time.

Similar influences appear in the safety relay/safety controller/safety PLC realm, and we’ll look at that area soon. For now, however, it’s enough to say that safety controllers have come a long way since their early beginnings.

“It would seem like all of the needed products are in place, from safety relays for small systems to safety PLCs for larger systems,” says Tina Hull, application engineer at Pilz Automation Safety. “But wait, what about middle-sized systems?” Hull says Pilz has a modular and configurable safety “system,” aimed at handling six to 14 safety devices. The logic is configurable off-line, diagnostics are handled through LEDs, and components are modular so they have flexibility. PNOZplus series, Pilz’ largest modular safety system, reportedly is “as simple as a relay and as flexible as a controller.” Does it matter what it really is anymore?

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