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CIRCUIT DESCRIPTION
6-43
March 1999
Part No. 001-2009-600
This modem operates half-duplex, therefore two
are used to provide full duplex operation which is the
simultaneous transmission and reception of data. The
data rate is 1200 baud (bits per second). Full duplex
1200 baud operation is possible because a 4-wire line
is used which provides two separate audio paths. The
path is selected by programming switches S100 and
S101 as shown in Figure 6-17.
Figure 6-17 FSK MODEM SWITCH SETTINGS
If an RS-232 link is used for data communication
with the Switch (on P100, pins 22-23), the modem is
not used. This type of data path is selected by pro-
gramming switches S100 and S101 as shown in Figure
6-18.
Figure 6-18 RS-232 SWITCH SETTINGS
The level of the transmit FSK signal from U109
to the Switch is set by amplifier U165A. The output
of the amplifier is routed to main audio amplifier
U166A by FSK To Audio Gate U116A. This gate is
controlled by A/D processor U111/latch U108. Buffer
amplifier U166B converts the single-ended input to a
balanced 600 ohm output.
When the FSK data signal uses a different 4-wire
link as the voice signal, the FSK output of U165A is
applied to secondary audio amplifier U131A. Buffer
amplifier U131B converts the single-ended input to a
balanced 600 ohm output.
The FSK signal is also applied to FSK amplifier
U165B and fed to gate U115D. Gate U115D is con-
trolled by A/D processor U111/latch U155. When
enabled the FSK signal is passed to voice amplifier
U120B and Receive Voice To Backplane gate U115C
to the Controller Backplane.
The FSK input from the Switch is applied to dif-
ferential amplifier U131C. This stage converts the
balanced input to a single-ended output. The FSK sig-
nal is then applied to U131D which sets the input level
on the RC input of the modem. When enabled by A/D
processor U111/latch U107 the transmit Secondary To
FSK gate passes the FSK signal to U110 RC input.
When the FSK data signal uses the same 4-wire link as
the voice signal, the signal from U167A is applied to
transmit audio amplifier U167B and passed by gate
U116D to U131C.
If an RS-232 serial data path (1200 baud) is used
instead of 600 ohm 4-wire lines, the data signal com-
pletely bypasses modem U110. In this case, RS-232
driver U162B converts data to the Switch to RS-232
logic levels of -5V and -7.5V. Conversely, when data
is received from the Switch, RS-232 receiver U162C
converts the RS-232 logic levels to TTL levels of 0V
and +5V.
The 1200 Hz carrier is constantly transmitted by
the Switch modem and received by the MAC modem.
In turn, the MAC modem sends a constant carrier to
the Switch modem. This is used as a constant check
that a completed link is established between the two
modems. If the carrier does not return from the MAC,
it notifies the Switch system operator that the link has
been broken.
ON
2
1
8
7
6
5
4
3
ON
2
1
4
3
R
XS
-
(
GND)
R
XS
-
R
XS
+
(
AUDI
O)
R
XS
+
(
F
S
K)
R
XS
-
(
GN
D)
T
X
S
-
T
XS
+
(
AUD
I
O)
T
XS
+
(
F
S
K)
S
1
0
0
S
1
01
F
S
K
T
XS
+
O
N
2
1
8
7
6
5
4
3
ON
2
1
4
3
R
XS
-
(
GND)
R
XS
-
R
XS
+
(
AUD
I
O)
R
XS
+
(
F
S
K
)
R
XS
-
(
GND)
T
X
S
-
T
XS
+
(
AUD
I
O)
T
XS
+
(
F
S
K)
S
1
0
0
S
1
0
1
F
S
K
T
XS
+