PLC as Modbus Master with 6 HMIs as Modbus slave on Rs485

Hello friends, I am having a PLC as a Modbus master on serial 485 connected to 6 HMIs as slaves. I read from and write into the HMIs registers by the PLC. The application is for a passenger elevator with Ground + 4 floors. All these floors have an HMI as a LOP device (Landing station operator panel) and another HMI in the Car Cabin as the 6th HMI. The problem I face is Destination Calls are registered without WITHOUT a request made from either the LOPs or the COP. The programming is done in Basic Language. Can someone advise me how to overcome this problem. Thanks in advance. Regards. Vilas Desai.