Tandy 102 Service Manual page 35

MODEM IC
The Tandy 102 employs the IC MC14412 as a MODEM control device. This IC modulates/
demodulates data to be transmitted/received in accordance with frequencies suitable for originate
or answer mode respectively.
The RXRATE and TYPE terminals of the MC14412 (M31) are pulled up to VDD. The baud rate is set
to 300 bps and the U.S.Standard is selected. Since the ECHO and SELFTEST terminals are not
needed, they are connected to ground.
The Q output (EN signal) of the M36 selected by bit 1 of the Y2 port is input to the ENABLE terminal
when the MODEM mode is selected.
In addition, the signal detected by the ORG/ANS switch is input to the MODE terminal, and it
switches between the ORIGINATE mode or ANSWER mode.
R47 15M
0
MHz
VDD
XTAL1
R46
VDD
16
33K
RXRATE
6
15
TO TRANSMISSION
TTLD
14
FILTER CIRCUIT
TYPE
TXCAR
13
ECHO
2
9
Y2
FROM RECEPTION
FILTER CIRCUIT
SLFTEST
Vss
RXCAR
B
12
C
M13
TT C)02
7H
IOOP
10
WR*
VDO
ENABLE
0 D
AD)
10
MODE
7
ORIG
ANS
RXM
1
RS-232C/MODEM
J
R12
TXM
SELECTION CIRCUIT
1K
RESET
5
RESET
MB
(MC14412)
Figure 4- 17. MODEM IC and Peripheral Circuit
Transmission Filter Circuit
The DC component of the carrier output from the TXCAR terminal is removed by C61. The signal
level is adjusted by the potentiometer VR2. The signal then passes through the transmission
band-pass filter and is sent to the telephone line or the acoustic coupler.
The transmission filter circuit is composed of an active filter (consisting of an operation amplifier)
and the intermediate frequency of the active filter is 1170 Hz for the originate mode, and 2125 Hz
for the answer mode. They are changed by switching T4 ON or OFF.
C60
3500P
R44 242K
CN4
Pin-5
TXMC
OTI
R63620n
DIR 1
ACP
C59
2
3300P R43 10K
M30
+3
R45
R42
7.97K 2.3K
M31
MC14412
C61 4700P
9 TXCAR
VR2
50K
VDD
R32 22a
C103 2 20,u
CL AS
ANS
R33
ORIG
T4 10K
R11
1K
Figure 4- 18. Transmission Filter Circuit
4-17