What does the 'Modbus Exception Code 03 Illegal Data Value' error mean when writing to a External Modbus Control Register on a 7650?

Product Line
ION7650/7550

Environment
Register Writing

Resolution
The meter is locked into a Read-Only mode when the password security is anything other than the default of 0 (zero).  To write to an External Modbus control registers requires password security to either be disabled, or set to 0.