Abstract: A receiver having an interface arrangement for connection between a television circuit and a conditional access sub-system conveys data via a bus having a four frame bus cycle period with a bit rate corresponding to the line rate. During a first part of the bus cycle period data is conveyed from the receiver to the sub-system sequentially as follows:i. an address block of 1 byte with the unique address of the sub-system with which the receiver is in communication,ii. a status block of 1 byte providing status information for the following blocks in the first part,iii,iv,v. three data blocks each of 93 bytes and each containing an entitlement message,vi. a keying block of 1 byte conveying keyboard or remote control commands, andvii,viii. a further block of either 1/2 byte (vii) or 21/2 bytes (vii+viii) conveying frame count information.