Keep Previous Value applies to Communications Failure and modifies how communications errors are displayed. Keep Previous holds the last value if Communications to the PLC or Device Fails. If Keep Previous Value = Yes, and there has been at least one successful read, the SCADA node will Display the last value read from the PLC or Field Device. Otherwise, if Keep Previous Value = NO, an asterisk (*) is displayed as the value of the Tag when communications to the field device fails.
If "KEEP PREVIOUS VALUE" is YES, then the last good value appears as the value of the tag indicating during a communications failure.
If "KEEP PREVIOUS VALUE" is NO, then an Asterisk (*) appears as the value of the tag indicating BAD DATA.
The QCODE is displayed next to the Tags value in the Point Info Dialog Box if there is a communications failure and KEEP PREVIOUS VALUE = NO. AN asterisk * is typical for a loss of communications error.
If there is a communications error on start-up, and KEEP PREVIOUS VALUE is YES, this Initial Value will remain until communications is restored or an operator changes the value.