I should have read down further.... The zoom direction is combined with the speed, unlike the pan and tilt. The first 4 bits represent the direction, the lat 4 the speed. If it's \x24 the camera should be zooming in at a speed of 4. If it's \x37 the camera should be zooming out at a speed of 7.
I created my own Visca module a while back and haven't looked back. You can do everything in Simpl really easily (though I do store presets on the camera, not the control system). I also like to have the speeds change the longer you hold the direction, which none of the stock modules do. It's just analog to serials and vice versa.
