Tandy 102 Service Manual page 33

RS-232C /MODEM Selection Circuit
The RS232C signal (PB3 terminal of the PIO) determines whether the serial port is to be used for
RS-232C or for MODEM. When the RS232C signal is H, the serial port is used for MODEM.
The reception signals, including the control signals, are demultiplexed at M33.
During the RS-232C mode, the CD (Carrier Detect) signal is not used. To make this condition, pin
14 of M33 is connected to the ground.
During the MODEM mode, the CL/AS signal is used as the sensing signal for the ORG/ANS switch,
and CP/TL signal is used as the sensing signal for the ACP/DIR switch. In order to detect the
carrier signal from the telephone line, the CD signal is connected to the RXCAR terminal of the
MODEM IC.
1A 2 RXR
1A 2
1 0
/2A 11 CTSR
Ao
2A 5
r-------- S = 3 DSRR
S =3
3A
14
I //// //4A I
to 4A
RRI 4 1Y
Z;
w 1B 3 RRI 4 1Y'
'+
1B RXM
CTS
T
2Y 2B
6
CTS 7 Y 2B
WAS
DSR
9 3y
3B 10 DSR 9 ' 1 CP/TL
CD 12 4Y 4B 13 CD 12
3Y
4Y 1 RXCAR
9 9
RTS
10 M13
r-
1
9
TXR 10 M13
8
1011 13
.8 38
12
M RTSR
TRO
3
2
M38
9
10 8
M38
2
38
1
3 3
A
RS-232C
W
N
n
MODEM
TXM(M31)
RTSM
Figure 4-15. RS-232C/ MODEM Selection Circuit