TM 11-7440-232-12/NAVELEX 0967-LP-275-5010
Section II. MODEM SELF-TEST
4-3. GENERAL
Digital Data Modem MD-701(*)/UY provides a test pattern generator circuit in the transmitter and loopback circuit
in the receiver to allow troubles in the modem to be quickly isolated to the modem or local data terminal or to the
transmission path.
4-4. TEST PATTERN GENERATOR
By rotating the TRANS MODE switch on the transmitter from the NORM position to LOW, HIGH, REV, or PATT
position arranges the transmitter to produce four types of test pattern output signals derived from the test pattern
generator. The test patterns are directed into the receiver by use of the loop-back mode, refer to paragraph 4-5. This
allows modem troubleshooting without the need for external pattern generating equipment.
LOW with TRANS MODE switch in this position a 1200 Hz signal is applied to the transmit line.
HIGH with TRANS MODE switch in this position a 2400 Hz signal is applied to the transmit line.
REV with TRANS MODE switch in this position, the test pattern generator in the transmitter produces a
reversal pattern of alternate ones and zeros. The signal appearing on the transmit line resembles the alternate
one and zero data input pattern for the particular data rate applied.
PATT with TRANS MODE switch in this position, the test pattern generator in the transmitter produces a
random data input, which originates from a noise source containing a wide band of signal components; the
binary pattern is applied in a random manner and results in a random one and zero output on the transmission
line.
4-5. LOOP-BACK MODE
By rotating the REC MODE switch on the limiter discriminator to BACK-TO-BACK, the receive signal path will open
and the transmitter output is directed into the receiver. This provides means for monitoring the complete send and
receive signal path and enables the technician to pinpoint the trouble to the data terminal or to the transmission path.
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