TM 11-7025-234-23
In order to transmit data between widely separated terminals, two similar modems
are required. The transmitter section of each modem generates and modulates a data
carrier signal handled by the link with minimal distortion. The receiver section
of each modem demodulates the incoming carrier and prepares the data for use by the
other terminal. The following functional description is keyed to the V.29 modem
functional block diagram (fig. 2-4).
The power supply generates four dc voltages: +10 V, +7 V, -7 V, and -10 V. The
integrated circuits on the equalizer use +7 V and -10 V. The transmitter, and
receiver boards use +7 V, +10 V, and -10 V. Analog circuits (op amps) on the
interface board use +10 V and -10 V, and the logic circuits on the control board
use +7 V and -7 V. Two unregulated voltages are generated: one positive to feed
the +10 V and +7 V regulators, and one negative to feed the -10 V and -7 V
regulators. The unregulated input filter has a choke input, which provides
isolation of the regulators from the ac line.
a. Dc Power Distribution. The two unregulated voltages are routed through the
mother board before entering the regulator board. If the regulator board is
removed, power is cut in the unit.
b. Unregulated Assembly. The unregulated power supply assembly contains the ac
line filter, fuse, power switch, 115/230 V ac switch,' chassis ground stud, power
transformer, rectifier bridge, and two chokes. The two capacitors which form the
rest of the choke input filters are located on the regulator board. The
unregulated chassis forms a modular unit which has four electrical interconnects to
the motherboard; chassis ground, signal ground, and the two unregulated voltages.
These are connected to the motherboard via a box connector.
2-6. REGULATORS
The three regulators are located on the power regulator board (fig. 2-5). The +10
v, -7 V, and -10 V regulators are three terminal regulators which have their ground
terminal boosted to create higher output voltages then normally given by the
The booster circuits consist of a resistive voltage divider, an op amp,
regulator.
and either an NPN or PNP transistor dependent on the polarity of the +10 or +7 V
output voltage. The voltage divider determines the output voltage +10 or +7 V.
The three terminal regulators are short circuit proof, thus providing output
protection.
Data, commands, and clock information from a local terminal are sent through the
interface (A3) and control board (A4). This data is scrambled, and transmitted to
the input/output board (A9). From the 1/0 board the data is transmitted to remote
terminal modems.
a. Interface Board (A3). Input data from the computer, request-to-send command
and transmit timing signal enter the interface board. These are then sent to the
control board. Commands to send and transmit clock signals from the control board
(A4) are returned to the computer through the interface board.
2-6
