Conveyor Crossroad

As in my sample showed I have a crossroad in a conveyor system. I want to have the behaviour that a box passes the crossraod and when the crossroad is free the next box (by FIFO) will pass.
I tried to use closeinput (OnEntry) and openinput (OnExit). The OnExit event is started when the box enters the next conveyor. This is to early. I want to open the crossroad for next box when last box left the crossroad.

In short:
I've used a global variable, the triggers in conveyor2, conveyor4, conveyor7 and conveyor8. I've changed also the size of the center objects to prevent collisions.

The idea:
In the center should be only 1 item. I've used a global variable to remember this. If the center is used, no conveyor send an item to the center. If the center gets available again, it checks for blocked conveyors (with center-connection). The Conveyor4 will be preferred.

I hope I get your point