I needed a small PLC or Smart-Relay with a real time clock to augment an embedded controller application that I had running in Canada. The embedded controller was a 20 year old design and had no real time clock. The customer wanted this embedded controller to control certain functions only between 8:00PM and 8:00AM when demand on his system was the lowest. There were two ways to approach this problem: One, incorporate a real time clock into the existing embedded controller firmware and hardware or, two, modify the embedded controller firmware (software) to take a permissive bit during the specified times of operation. Modifying the embedded controller hardware and firmware so that it could have a RTC would have been a very expensive proposition. The hardware would have to be redesigned and the firmware would have to be re-written and thoroughly tested. I decided to to use a small discrete controller with a RTC and simply activate an output on this device to feed my embedded controller a permissive bit.

I looked around a bit on the Internet and found the Factorymation.com site and took special interest in the SG2 series of Programmable Logic Relays (PLR's). As I read the product specifications for the small SG2-12HR-D, 12-30VDC PLC I was pleasantly surprised to find that not only did this small unit have a built in programmable RTC, but it also came standard with six (6) DC inputs, four (4) relay outputs, and even two (2) 0-10VDC analog inputs. All for just $79 per unit. Wow! I was even more surprised to find that the software for these small PLR's was FREE via download at the Factormation.com site.

I downloaded the software and found it very intuitive. The software even has a simulation mode which is very thorough. You can even configure your simulation to include DC input, analog, and output simulations via an interactive user interface. I was disappointed to learn that I had to purchase a programming cable, but in light of the huge problems that this little unit was solving for me, it wasn't that big a deal. I programmed the first output of my SG2 to activate during the 12 hours between 8:00PM and 8:00AM. I modified the firmware of the existing application to accept a digital input from SG2 and have it act as a conditional input to activate the critical process. Project complete.

I've since had the opportunity to use the SG2's in a few other applications. To date, all the one's that I've installed are still working and I've not yet had to replace or reprogram a single unit. I'll keep using these powerful little PLR's when applications arise that lend themselves to the small, but powerful SG2.