Open Systems - Controllers to HMIs


We covered open systems between controllers and I/O in this column late last year. This month’s column will examine open systems at the next level up, between controllers and HMIs.

As previously discussed, Ethernet has been a prime mover in standardizing connections between controllers and I/O. A similar situation is occurring at the next level up with the interface from controllers to HMIs. Vendors that make just HMI products love open systems as they need and want one industry-wide interface between all controllers and their HMIs. But some vendors that make a wide range of automation products would rather see proprietary interfaces.

For low end HMIs, proprietary links still exist. These HMIs typically have smaller screen sizes, 12” and below, and the price of their operator interface software is usually bundled into the total cost of the HMI. We often refer to these low-end HMIs as Operator Interface Terminals (OITs). OITs encompass everything from simple 2-line displays with 16 keys to 12” color LCD touch screens.

Vendors that make controllers and OITs usually tie them together via a proprietary digital interface. This locks machine and robot builder OEMs into using only that vendor’s family of controllers if they choose the same vendor’s OIT. This proprietary lockdown makes implementation very straightforward because the proprietary link between the controller and the OIT has been thoroughly tested and is optimized for the particular hardware.

The downside is that the OIT will almost always be overpriced when compared to offerings from vendors that specialize in open OIT hardware. These specialist vendors can’t lock purchasers into their products via proprietary links to controllers, so they have to compete fiercely on price and performance.

Increased openness between controllers and HMIs has drastically reduced prices and increased performance for both PC-based HMI software and for OITs. It is even reducing controller prices as OEMs are no longer forced to use a particular brand of controller just because they want or need to use a certain HMI platform.

See my March 2009 Mojo column for more info.