A flash programmable microprocessor-based control module is operated in a manner to protect the integrity of event data stored in the programmable memory of the module while permitting authorized manufacturing and field alteration of the programmable memory with a Download and Execute routine. The Download and Execute routine is resident in a designated sector of the module's read-only memory, and download access to the module's random access memory after module manufacture has been completed is denied. During manufacture of the module, and during field programming of the controller prior to the writing of event data, the programmable memory may be externally altered by an authorized service tool by transferring the Download and Execute routine from read-only memory to random access memory for execution by the module's microprocessor, and downloading the new data or code over a data link coupling the service tool to the module. After event data has been written to the programmable memory, external requests to alter the programmable or read-only memories are denied, and the transfer of the Download and Execute routine to random access memory is not permitted.