8.15.12
T2000-A70 Data Modem Kit
M2000-00
31/12/97 Copyright TEL
8.15.6 Circuit Description
The T2000-A70 data modem PCB is based on a CML FX469LS 1200/2400 baud FFSK
modem IC (IC1) and the baud rate is selectable by PCB links.
Receive audio is recovered and fed into IC1 via a buffer amplifier, with filtering (IC5), to
attenuate all but the FFSK tones. The signal is demodulated in IC1 and the data fed into
the microprocessor, IC2. The data, complete with its length & checksum information, is
fed to the DTE via IC4, which converts the 0V/5V TTL levels to RS232.
On transmit, data is sent from the DTE to IC2, via IC4. It is processed and sent on to IC1,
where it is modulated and the output goes to the TX-SIG-IN input of the radio, via a
unity gain buffer, IC6. IC2 also monitors inputs from the radio’s logic PCB and controls
various output lines.
The microprocessor checks that the radio is in lock, and is not busy. It then mutes the
microphone, activates the radio PTT and 30ms later, the PWR-CTRL signal goes low,
releasing the transmit inhibit.
Both buffer amplifiers are biased at half rail by IC1, pin11.
If a channel change is required, the data is sent to IC7, which toggles the appropriate
BCD lines to change the radio channel.
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