The bus, having 4 separate 19.2kbps channels, has a total available bandwidth 76.8 kbps. And perhaps more importantly, the device-control circuitry is moved out to being immediately adjacent to the device being controlled. Simpler wiring, simpler diagnostics. In the example above, the PC wants to send the value 0xFF to a device that's connected to tap 0x3F, EMB slot 2, PED port 5. Some devices consider the received byte as junk, only using it as a trigger for processing.
Some devices have nothing to return, but do so anyhow in order to honor the protocol.
One additional, very important feature of these devices is that, on power cycling, they remember their last setting and initialize themselves to it. Separate PCs could each control each of the 4 bus channels, or a single PC, with 4 COM ports, could control all 4 channels simultaneously. The most basic function of the control app is to constantly monitor all its input devices, crunch the data to determine the layout's state, then drive all the output devices. I also intend the layout's dispatcher to control the layout via a Windows app, so the dispatcher app works in concert with the various control apps to configure the layout. Throwing turnouts is as easy as clicking the mouse.