Apparatus interconnected for the communication of control messages

A system of at least first and second apparatuses interconnected for the communication of control messages by means of a serial data channel, the first apparatus having

• user input means defining a plurality of keys for activation by a user, the defined keys including "normal" keys for which each activation implies a single user command and "repeat" keys for which a prolonged activation implies a repetitive series of user commands for the duration of the activation;
• and
• first control means responsive to the user commands, including means for identifying a user command which requires to be processed by a second control means located in the second apparatus of the system, and means for in such a case generating a control message conveying the received user command to said second apparatus via the serial data channel,
• the second apparatus including:
• at least one functional element;
• and
• second control means for receiving via the serial data channel control messages conveying user commands received by the user input means of the first apparatus and for processing the conveyed user command to control the functional element(s) of the second apparatus in accordance with the user's wishes,
• wherein, to convey the prolonged activation of a repeat key the first control means operates to generate a first ("activation") control message conveying the user activationof the repeat key, followed by a second ("release") message in response to the user's eventual release of the repeat key, the second control means being responsive to the activation message for a repeat key to synthesise within the second apparatus the repetition of user commands in controlling the functional elements, until receipt of the release message.