Ethernet Device Servers and Small HMI System DesignThe theory of operation is quite simple. Since most serial protocols only allow a single master, HMI-A is set up as the "I/O Server". It moves data between the HMI application "object database" and the slave devices by the serial protocols. The second HMI-B must indirectly access this data by network. If HMI-A goes down, HMI-B must be reconfigured to act as "I/O Server" and the serial cables moved from HMI-A to HMI-B. For the EWS to access the slaves, the serial cable is likely moved from HMI-A to the EWS, which puts the device "off-line" and allows the device vendor's special configuration tools to be run.
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