MIDI Beat Clock is something that most keyboards and drum machines can generate and read. It is also known as beat clock. It is a system message sending quarter notes (read the deeper explanation here), similar to having everyone playing to a click in the studio. Depending on which one of the two you want to be the master, that will be the one you will have set the tempo. You will need to check the manual on both devices to show you how to set them up to send or receive beat clock.
Since most websites are dedicated to a specific product or brand, you won't be able to find too much more on it than just the textbook definitions. The biggest question about using beat clock that you need to ask is "what exactly do you want to accomplish and why?" I'd be glad to give you a few minutes if you want to call me to talk about it.
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