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If two devices are interoperable, they can share information and commands without the need for a gateway or other special hardware or software to translate from one protocol to another.
But that doesn’t mean the devices are interchangeable. In other words, just because thermostats from two different manufacturers are interoperable, facility executives cannot simply replace one with another and expect the second to work exactly the way the first did.
For devices to be interchangeable, both the way protocols are implemented and the capabilities of devices would need to be standardized to a far greater extent than is currently the case.
If devices were interchangeable, facility executives would have far more freedom to mix and match devices from different manufacturers. But there would be fewer choices about features and the way those features are implemented.