Tandy 102 Service Manual page 38

MODEM Connector Interface Circuit
When the acoustic coupler is used, the transmission and reception signals are directly connected
to the connector (TXM, RXM). When the MODEM cable is used, they are connected to the
secondary side of the driver transformer. The primary side of this transformer is connected to the
telephone line via the connector (TXMD, RXMD).
The ACP/DIR switch is used in the MODEM mode, relay RY3 separates the telephone receiver
audio signal (TL) to prevent interference. RY2, another relay, separates the MODEM circuit and the
telephone at the conclusion of use in the MODEM mode and is also used as an automatic dialer for
the pulse-type telephone line.
D16 VDD
ADO
Y2
WR*
VDD
MODEM
RTSM R49
T7 RY2
C1 03
6
CN4
3.3K
R 48
I
220j
2
GND
6
F rom
Transmission
Filter Circuit
To Reception
Filter Circuit
ACP
DIR 620n
R63
ACP
N
DIR Iv
""
D
M36
p
C R134
3.3K
OT1
R135
68K
R32
TXM
22n 4
RXM
T
1 TL
TX MD
D24 D18
R171 I
RY3
VDD
T24
D21
C102
Figure 4-21. MODEM Connector Interface Circuit
D I D2 R21
RXMD
RP