8 - 6
8.5 MODEM (CNM-162BA)
The MODEM is used to transmit or receive control or monitor
signals of the VHF Transceiver to or from VHF Controller at a distant
place. For the MODEM, a transmission rate of 200 or 1200 baud can
be selected. When 200 baud is selected, voice signals can be
superimposed. When 200 baud is selected, control or monitor signals
with the CPU are converted to FSK signals by the FS MODEM
comprising IC12, IC9, IC1, and IC13 or to digital signals from FSK signals.
When 1200 baud is selected, IC2 is used in the FSK MODEM.
The band-pass filter and the low pass filter are used to eliminate an
unnecessary frequency component of a voice signal or FSK signal. They
are switched capacitor filters (SCF) comprising many resistors such as
IC3 to IC6, four circuits in total. IC3 and IC4 are transmitting filter circuits,
and IC5 and IC6 are receiving filter circuits.
The RS-422 is used for data transfer with the CPU at both 1200 baud
and 200 baud. IC11 is the RS-422 interface IC. For transfer at 200
baud, the FSK frequency is 3235±85 Hz and data sent from the CPU
using the RS-422 is transmitted by IC1 and IC9. For transfer at 1200
baud, the FSK frequency is 1700±400 Hz because the transmission rate
is high, and data is transmitted by IC2. To transmit or receive a FSK
signal from a communication line, IC15 and IC16 are used as buffer
amplifier circuits. The transmission level is also adjusted by these ICs.
When a communication line connected to the MODEM is extremely
reduced in level by some cause, it is detected and an alarm is outputted.
It is referred to as a carrier loss alarm and detected by IC8. (At 200
baud)
