The BIDC/4000 is a communications device which allows the transfer of data from a computer system operating at a lower security level to another operating at a higher level, whilst providing a high degree of confidence that data flow in the reverse direction is either prevented entirely (In Overflow Mode) or limited to a very low rate (whilst in Disable Mode).
It functions as a transparent device inserted in the communications link between the low security system and the high security system. It must be located within the protected area of the high system with safeguards to prevent tampering.
The BIDC/ 4000 is not a COMSEC device and does not have any cryptographic function. The letter C in the BID name means the device is commercially available. It is believed to have been made by ICL Defence .
1) Crypto Bloke