There are some devices that when you power off/on the device or in the middle of communication it can send some signal that the Jazz2 recognize it as a break signal and goes from MODBUS to PCOM protocol.
Please perform the following to solve this issue:
On power up bit we set SB134 to enable ignore break.
When one wish to connect with U90Ladder to the Jazz PLC, since the protocol by default is set to MODBUS, U90Ladder needs to send a break signal to change the protocol to PCOM.