Abstract: This invention is a security system for computer terminals which system provides means for preventing the unauthorized access to digital computers from illicit terminals. The invention provides a local code unit which includes means for manually selecting a sequence of digits by means of a keyboard. When the selected sequence and the pre-set code in the local code unit are the same, the terminal equipment is connected to the transmission path. This prevents unauthorized personnel at an authorized terminal from gaining access to a transmission path. A second code, pre-set into the code transmitter by any suitable means such as switches, is transmitted over the transmission path to the code receiver. A code receiver at the other end of the transmission path connects the computer to the transmission path only when the second code is the same as the code pre-set into the code receiver.