By Dan Hebert, PE, Senior Technical Editor
We talked about open systems between controllers and I/O in this column late last year. This month, let’s examine open systems at the next level up, between controllers and HMIs.
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 in. and smaller—and the price of the operator-interface software usually is bundled into the total cost. We often refer to these low-end HMIs as operator interface terminals (OITs), which encompass everything from simple two-line displays with 16 keys to 12-in. color LCD touchscreens.
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 almost always is 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.
In the past, these open OIT vendors had to create and maintain hundreds of software drivers to link their OITs to each different type of controller. These links were proprietary at both the hardware and software level, making support a nightmare. Not only did the open OIT vendors have to write and maintain software drivers for every different type of controller, they had to design and implement multiple hardware communication ports on their OITs. This was very costly and naturally drove up the price.
Nowadays, most controllers have Ethernet ports. Instead of supporting scores of communication protocols, many open OIT vendors now focus on Ethernet. Different controller vendors favor various industrial Ethernet protocols, but different protocols can run simultaneously over the same Ethernet network. An open OIT vendor now can provide just one Ethernet port on its OIT, along with software support for the leading industrial Ethernet protocols.
Ethernet thus makes the OIT-to-controller link more open. This gives OEMs more choices to mix and match, and it even drives down the prices of proprietary OITs because they must compete with open alternatives.
For higher-level HMIs, systems are even more open. These HMIs typically run on Windows-based platforms, with other operating systems, such as Linux, a minor factor. The platform of choice most often is a PC, but it also could be a Windows CE box. Regardless, the link between high-level HMIs and controllers is almost exclusively via Ethernet.
This means virtually any controller can be connected to just about any Windows-based HMI via Ethernet. If the controller has an Ethernet protocol that requires special hardware, it is relatively cheap and easy to buy an adapter card that plugs into the HMI platform.
The OPC communication standard also eases communication between controllers and HMIs. “Instead of writing a custom software driver to connect a controller with an HMI, one can simply download an off-the-shelf OPC HMI server and be communicating the same day,” says Scott Kortier, marketing communications manager at InduSoft. “Many OPC servers have installations for Windows CE as well as XP and Vista, allowing for a scalable solution,” adds Kortier.
Increased openness between controllers and HMIs has reduced prices drastically and increased performance for both PC-based HMI software and for OITs. It even is reducing controller prices as OEMs no longer are forced to use a particular brand of controller just because they want or need to use a certain HMI platform.