Patent ID: 7313704

Claim:
A control system in communication with a host control system and having a download function, comprising: a first storage element storing execution program data to execute a control function in a rewritable status; a second storage element to store a download module including fresh pieces of update target execution program data and module identifying information; a first control unit receive the download module encrypted in the host control system by an encryption key generated from the same program data as the execution program data stored in said first storage element and from the module identifying information, and to store the received download module in said second storage element; and a second control unit to decrypt the download module by an encryption key generated in the control system itself from the execution program data in said first storage element and from the module identifying information of the download module in said second storage element and to replace, when a storage start address, a data length and a check digit that are encrypted in the download module are decrypted into valid values in a plain text, the execution program data in said first storage element with the fresh execution program data decrypted.