NMIS will try to make sure it's own fpingd.pl is running every time a collect or update is run if configured to use it (detailed below). If fpingd is not enabled or the loss value for the node to ping cannot be found in the fping status file nmis-fping.nmis or nmis-fping.json (on newer systems) then NMIS will run a ping of it's own. This code can be found in nmis.pl in sub runPing.
The fping status file nmis-fping.nmis, or nmis-fping.json (on newer systems) is automatically created and updated by fpingd. This file should NEVER be hand edited. At the very least hand editing will be overwritten by fpingd and at the worst a bad edit could result in disabling fpingd completely.
fpingd is enabled by default, the config setting (in Config.nmis) to control this is:
'daemon_fping_active' => 'true',
If you are NOT using DNS make sure to set daemon_fping_dns_cache in /usr/local/nmis8/conf/Config.nmis to false, the default value is true.
fpingd runs a constant cycle of pinging all nodes updating the NMIS event table, sleeping and then doing it all over again. Every 10 poll cycles the Nodes.nmis file is re-read to look for changes.