TM 11-7440-232-12/NAVELEX 0967-LP-275-5010
Section III. TECHNICAL PRINCIPLES OF OPERATION
1-14. GENERAL
a. The Digital Data Modem MD-701(*)/UY is a 4-wire duplex terminal consisting of a transmit branch, receive
branch, and a common power supply and common RF oscillator.
(1) The digital data modem is used for transmitting and receiving serial-by-bit binary digital data at rates of
600, 1200, or 2400 bits per second (bps) over a standard 3-KHz voice frequency (vf) path which interconnects two data
processing terminals.
(2) The vf path can be any facility such as open wire, cable, or wire or microwave carrier that accommodates
a 3-kHz signal band and meets the minimum requirements for signal/noise and amplitude and delay distortion.
(3) Typical data processing terminals can include computers, card readers, computer and card reader
combinations, high speed telegraph, facsimile, or digitalized voice equipment.
(4) The digital data modem operates with any type of equipment that produces and accepts an NRZ serial-by-
bit binary signal having voltage and impedance characteristics within the ranges listed in Chapter 1, Equipment Data.
b. Indicators, visible at the front cover when the front cover is closed, allow monitoring the operation of the digital
data modem. The POWER indicator when lit, shows that power is applied to the modem modules. The ERRORS
indicator when lit, shows that there is trouble with the modem.
c. The data carrier alarm module provides alarm signals to a peripheral, visual and audible alarm located on the
local data center console if a malfunction occurs in the digital data modem transmit branch, receive branch, or power
supply.
1-15. TRANSMIT
a. Transmit branch contains two modules: a transmitter and a transmitter clock. The transmit branch converts
serial-by bit NRZ binary data to a form which can be transmitted over the transmission path.
b. TRANS MODE switch on transmitter module is used for selecting the mode of transmission and during the
mission is set to NORM. Four other positions provide test patterns for use during modem maintenance.
c. BIT RATE switch on transmitter module is used to select the operating bit rate of the transmit branch. Although
located on digital receiver clock module in the receive branch, the BIT RATE switch controls the operating rate of
transmitter clock module and is set to the same bit rate position as the BIT RATE switch on the transmitter during
transmission.
1-25
