Scada web hmi system

A SCADA web HMI system comprising:

• a plurality of HMI clients that run a web browser displaying a human machine interface (HMI) screen, wherein parts indicative of a state of a plant are arranged on the HMI screen;
• and
• a plurality of web HMI servers that connect to a programmable logic controller and transmit a display signal for updating of states of the parts to the web browser in accordance with a PLC signal received from the programmable logic controller, wherein at least two of the web HMI servers pertain to the same server group,
• each of the web HMI servers including
• at least one processor and
• a memory that stores a program, wherein the program, when being run by the at least one processor, causes the at least one processor to perform processing including:
• a table sharing process of sharing a server connection priority list table by all of the web HMI servers pertaining to the server group;
• and
• in response to a list request signal being received from the HMI client, a connection priority list transmission process of transmitting a server connection priority list, the server connection priority list defining connection priorities of all of the web HMI servers pertaining to the server group, wherein the server connection priority list is transmitted in accordance with order of assignment defined in the server connection priority list table,
• the server connection priority list table defining the order of assignment such that, for each of a plurality of the server connection priority lists, numbers of the HMI clients connecting to each of the web HMI servers pertaining to the server group are made close to an equal number,
• each of the HMI clients including:
• at least one processor;
• and
• a memory that stores a program, wherein the program, when being run by the at least one processor, causes the at least one processor to perform processing including:
• an alive monitoring process of monitoring operating states of all of the web HMI servers pertaining to the server group;
• a connection priority list request process of transmitting, to any one of the web HMI servers pertaining to the server group, the list request signal for requesting the server connection priority list;
• in response to the server connection priority list being received, a server connection process of establishing connection to the web HMI server having a highest connection priority defined in the server connection priority list from at least one or more of the operating web HMI servers pertaining to the server group;
• and
• in response to the currently connected web HMI server entering a non-operating state, a fail-over process of switching connection from connection to the currently connected web HMI server to connection to the web HMI server having a highest connection priority defined in the server connection priority list from at least one or more of the operating web HMI servers pertaining to the server group.