validation
no matter what you do, if you read data into the plc and are going to use it for part 11 compliance, the system will need to be validated for change controls, etc. You still will be required to write the data to non-rewritable medium, if this is a "critical" record. the fda (?) (depending on your individual inspector) will probably want the documentation showing that the record could not be tampered with. Document control procedures will be necessary, explaing who is allowed access to plc program, or, if the program interacts with HMI, then all of those links will have to be validated as well. If the data goes to a histroian, then usually the "newer" histroians have "part 11" controls, such as time/date stamps, login, electronic signatures. So, I would look at it from the approach that the regualtors will require validation no matter what, so what is easier to validate? Complex hooks between external clocks, BCD, etc. or document and user control of program and data? This probably didnt help, so I will stop rambling.
tj