Codan NGT 2010 RF Technical & Service Manual page 773

8571—Brief description
Transmit audio path
In Transmit mode, microphone audio from the master junction box is applied to the 8571
via an interface cable. The audio passes through a 300 Hz notch filter IC3 and IC4 to
remove any 300 Hz that may be present in the transmit audio that could possibly corrupt
the performance of the system. The signal output from the filter is connected to a gain-
controlled amplifier/compressor IC7 to maintain a constant audio output level.
At the output of IC7, the audio is fed via the signalling modem IC6 to the line driver
IC102 and is then connected to either a 2-wire private line via the hybrid and first
isolating transformer, or to a second isolating transformer for the 4-wire system.
At the other end of the 2-wire or 4-wire link, the transmit audio is applied to the 8571 via
the appropriate isolation transformer, to a hybrid and then to the first amplifier IC101.
From the output of this amplifier, the 300 Hz control tone is fed via a 300 Hz BPF IC106
(to remove the transmit audio) to a threshold detector IC104a/IC107. The transmit audio
(still containing the control tone) is applied to a DC gain-controlled amplifier IC103a,
then to a DC-controlled equaliser IC103b/IC104b. From here it passes via a 3 kHz LPF
IC105a through a switch, selected to a 300 Hz notch filter (used also in the Receive
mode) to remove the 300 Hz control tone, and finally via an interface cable to the
transceiver transmit audio path.
Microprocessor control and signalling
All 8571s and transceivers in the system communicate via RS232 commands to the
microprocessor IC103 on the General Purpose Input/Output PCB in the 8571s. The
microprocessor then communicates via the I
transmit/receive audio path switching.
All 8571s in the system communicate with each other by means of the signalling modem
IC6 using the same audio paths as the transmit and receive signals described earlier.
The signalling employed is FSK using tones of 1070 Hz and 1270 Hz. Signalling
integrity is ensured by parity checks and echoing each command. Signalling time varies
depending on the complexity of the information exchanged, but is typically 150 to
200 ms including acknowledgment.
Automatic line equalisation
When automatic line equalisation is initiated, the receive path is checked first. The slave
8571 sends a 300 Hz tone followed by a 2 kHz tone. These tones are received by the
master 8571 and the levels measured by the microprocessor. The 300 Hz tone is adjusted
for the correct level by the DC gain-controlled amplifier, and the 2 kHz signal level is
adjusted by the DC-controlled equalisation circuit. Because both controls interact, the
cycle is repeated until balance is achieved.
When equalisation of the receive path has been completed, the transmit path is then
checked in the same way as described above.
334
2
C bus to IC12, which controls all of the
NGT Transceiver System Technical Service Manual