Security system comprising three apparatuses sharing a time-varying code

In an electronic security system comprising an access-granting apparatus, a linking apparatus and a central computer issuing a link code segment, the access-granting apparatus comprising an access clock having an output, and an access computer having an access memory, the linking apparatus comprising a link memory, the linking apparatus receiving the link code segment from the central computer and communicating the link code segment to the access-granting apparatus which thereupon grants access to a protected apparatus if the link code segment matches an access code segment stored in the access memory, the access-granting apparatus otherwise thereupon denying access thereto, an improvement wherein

• the central computer comprises a master clock having an output communicated by a first synchronization means to the access clock whereby the access clock is synchronized to the master clock, and
• the linking apparatus comprises a linking clock and a linking computer operatively associated with the linking memory, the output of the master clock communicated by a second synchronization means to the linking apparatus whereby the linking clock is synchronized to the master clock.