question

https://docs.flexsim.com/en/19.1/Reference/CodingInFlexSim/FlexScriptAPIReference/Conveyor/#Property-targetSpeed (emphasis added):

See also: Reversible Conveyors in FlexSim

Reversing Conveyor Direction

If you set a conveyor's targetSpeed to a non-zero negative value, the conveyor will reverse direction. When a conveyor's direction is reversed the following things happen.

• All stopped items on the conveyor will be fully resumed.
• Each item on the conveyor will be set to a moving state.
• Each item's target destination will be erased. For example, if the item previously had been sent to a point in the system using the Conveyor.sendItem() method, that destination will be forgotten.
• Each item will regenerate its list of traversal points, e.g. photo eyes, decision points, etc. as if the item is entering the conveyor at the point where it is when the conveyor is reversed.

The functionality for reversing conveyor direction is still in an early stage of maturity. It was designed with a very specific scenario in mind, which is to give modelers the ability to simulate accumulation buffers, where items can be pulled off a main conveyor line when the line gets backed up, and then moved back onto the line when it clears out. The functionality has been tested only for this scenario. Using reversible conveyors for other scenarios has not been tested and thus may not work perfectly.

Specific caveats that you should be aware of:

• If an item is straddling two inline transfers when direction reversal happens, then both conveyors that the item is straddling must be reversed at the same time. Doing otherwise will result in undefined behavior.
• An item cannot straddle more than two conveyors when direction reversal happens, or else an exception will be thrown.
• No updates are made to the internal routing table when a conveyor's direction is reversed. This means, for routing purposes, i.e. when sending an item to a destination point in the system, the conveyor is always assumed to be conveying in its original direction.