The Online Monitor is typically run from the "shift" account on the Online#3 computer using an automatic procedure combining many online monitor tasks.  For PCAL, it runs $PHAT2HOME/macros/online/dev/pcal_onlClient.C (not to be confused with the Event display macro $PHAT2HOME/macros/pcal/pro/pcal_online.C).  The macro pcal_onlmonitor_setup.C (might be in $PHAT2HOME/macros/online/dev/ but the official one sits in $PHAT2HOME/macros/setup) sets up various defaults for the process.

• Extracts data with TPhVMEUnpackerMod and TPhPCALCalibMod (the latter includes calibrations).
• Then most of the guts are in TPhPCALOnlClientMod (not be confused with the Event display code in TPhPCALOnlineMod).
• Control and communications are via a TPhMonitorGui, a small window with (usually) a big green box.

This code unpacks the event and checks for things like missing VME data and bad ADC values which might indicate latchups.  Error messages can also appear in the terminal window (usually hidden behind all the gui's) associated with this process.