Why can a device or SCADA send a message and change a coil I have forced?
Goals and Symptoms
An application has coils that have been forced but another device or application can send a Modbus write coil message and change the value.
Causes and Fixes
Forced coils, by default, can be written to and changed. It depends on the state of system bit 79 (%S79). If the value of %S79 is False (0), the coil can be written to and the value of the forced coil will change. If the value of %S79 is True (set by the user) the value of the coil will not be able to be changed by a message.
An application has coils that have been forced but another device or application can send a Modbus write coil message and change the value.
Causes and Fixes
Forced coils, by default, can be written to and changed. It depends on the state of system bit 79 (%S79). If the value of %S79 is False (0), the coil can be written to and the value of the forced coil will change. If the value of %S79 is True (set by the user) the value of the coil will not be able to be changed by a message.