Mobile data terminal channel busy arrangement

In a data transmission system wherein data terminals transmit digital data messages including a synchronizing signal and digital data modulated on a carrier signal over a radio channel and wherein a receiver, associated with a data terminal, receives a digital data message, demodulates to produce a demodulated audio signal and generates a carrier activated switch (CAS) signal indicative of a carrier or noise on the radio channel indicating that the channel is busy, said CAS signal being utilized to inhibit data transmission by the data terminal, the improvement comprising:

• providing a pulse generator for generating, in response to said CAS signal, a CAS pulse having a predetermined duration;
• a SYNC detector for generating a SYNC signal whenever said receiver detects a synchronizing signal;
• and
• logic OR function means responsive to said CAS pulse and to said SYNC signal for inhibiting data transmission for an interval of time corresponding to the duration of said CAS pulse while determining whether a SYNC signal will appear or not and providing a channel busy signal extending beyond the duration of SYNC signal.

A method for determining the busy/free status of a radio channel used for transmission by a data terminal of a data message including synchronizing signals and data signals modulated on a carrier signal, comprising:

• first detecting, with a radio receiver, a signal from said radio channel to provide (a) a demodulated audio signal and (b) a carrier activated switch (CAS) signal indicating the presence of a carrier signal or noise on the radio channel;
• second generating, in response to said audio signal, a SYNC signal whenever said audio signal includes a data synchronizing signal as part of a digital data message;
• third generating, in response to said CAS signal, a CAS pulse having a predetermined duration;
• and
• OR-gating said SYNC signal and said CAS pulse to generate a channel busy signal for inhibiting the transmission of data by said data terminal.