Most of remote devices, connected to WebHMI always have some settings or setup data which do not have to be read in every WebHMI scan. They are actually accessed by operator only during commissioning or installation.

For example temperature regulator may have only 2-3 process-related registers (current temperature, set point, control output value), but tens or even hundreds of setting registers, like sensor types, calibration data, control profile (ON-OFF, PWM setup, analog control setup, PID coefficients, hysteresis) etc.

So when you add many such controllers with all registers to the WebHMI project, it will communicate 90% of time in vain, and only 5-10% to read important data.

To avoid this situation, there is a special attribute for the registers which marks them as “Read on-demand” so that WebHMI skips them when polling external devices thus minimizing scan time. This also minimize memory consumption and makes possible to have thousands registers in the project.

Tip: Group all read-on demand registers in one or two 'settings' category then they can be easily accessed and 
viewed via Analytics -> Raw values menu. 

To set any register into this mode select “On demand” in Priority dropdown at Edit Register page. To activate reading such register either click on (R) icon in register list or go to Analytics → Raw values page, select needed connection and registers category and click “Read on-demand registers”.

Once activated this register will be read for 60 seconds in every scan. And then it will be deactivated again.