For control system applications where the Ethernet I/P PLC needs to take specific action to recover from a loss of communications due to a module that has had its power cycled off and on; there is an additional set of instances implemented that provides 2 new registers that allows the PLC to manipulate the function of the module for recovery.
When a ConveyLinx Module in ZPA Mode has lost power and then is powered back up, due to perhaps a system E-stop that disconnects control power, some of the module’s working register values are reset to 0. Among these are the arrival counters, departure counters, and the accumulate commands for each configured zone(s) on the module. Upon power cycle to the ConveyLinx Module, if the PLC can establish its full Ethernet I/P connection prior to the ZPA task becoming fully functional, the preceding PLC programming examples would still function as expected. However, because PLC Ethernet I/P connection time is variable and not fixed; a robust control system design cannot count on the PLC establishing Ethernet I/P connection prior to the ConveyLinx Module’s ZPA task commanding the module as if no PLC was connected. A consequence of this in the previous programming example is that if a load happens to be accumulated in a PLC controlled zone at the time of power loss, upon powering back up, the load can release without the PLC commanding it to do so. This release could be caused by the PLC logic detecting a change in arrival count and thus incrementing the release or it could be caused by the fact that the accumulate command is cleared in the ConveyLinx Module due to power cycle and because the PLC has not established communications to set the accumulate command bit, the module releases the zone because there is no command present to accumulate.
For ConveyLinx Module’s in PLC I/O mode, Reset Protection may not be as much a concern, however reset protection assemblies are available for PLC I/O mode. Some items such as current servo position, etc. are reset upon restoration of power and thus the PLC programmer may want to detect this condition and act accordingly.