RECEIVE DC LOOP

TM 11-5805-387-34-2

1-33. RECEIVE DC LOOP

(fig. FO-6)

The receive dc loop is a dc series circuit containing loop current switch A5A2Q1 in series with receive steering quad

A5A2CR1, CR2, CR3, CR4, a current supply, and tty receive equipment (attached to the modem). The receive dc loop

also contains relay A5A2K1 for switching between receive dc loops. As explained in the preceding paragraph, a mark

tone input to the MD-522A/GRC results in loop current switch A5A2Q1 being biased into conduction. Current supplied be

either the modem or an external supply flows through the receive steering quad, conducting loop current switch A5A2Q1,

and to attached tty receive equipment. The source of dc and the supplied loop depends upon the setting of SEND/REC

switch S5, DUPLEX/ONE WAY switch S1, and loop battery module internal-external switch A5S1.

Receive Dc Loop in Duplex Operation. When ONE WAY/DUPLEX switch S1 is set at DUPLEX, receive-send relay

A5A2K1 is energized, automatically switching dc loop No. 1 to current supply 1, which is used for the transmit (send) half

of duplex operation. In duplex, duplex/one-way relay A5A2K2 is unenergized, connecting the positive output of current

supply 2 to receive steering quad A5A2CR1, CR2, CR3, CR4. The positive side of current supply 2 is connected to loop

current switch A5A2Q1 through the receive steering quad. Because the rest of the dc loop is a closed series circuit when

loop current switch A5A2Q1 is biased on, loop current supplied by current supply 2 flows through switch A5A2Q1, the

other duplex contacts of relay A5A2K2, and out pin A of jack J7 to dc loop No. 2, the loop used for duplex receive, or

pony operation in one way. The current pulses key the tty equipment in series with dc loop No. 2. Return is applied

through pin C of J7 to the negative side of current supply 2.

Receive Dc Loop in One-Way Operation. With ONE WAY/DUPLEX switch S1 set at ONE WAY, relay A5A2K2 is

energized, switching current supply 2 out of the receive dc loop. With RCV/SEND switch S5 at RCV, relay A5A2K1 is

deenergized. The positive side of current supply 1 is applied to loop current switch A5A2Q1 through the RCV contacts of

A5A2K1 and receive steering quad A5A2CR1, CR2, CR3, CR4. When the loop current switch is biased on, loop current

from current supply 1 flows through the switch, the other pair of A5A2K1 RCV contacts, and out pin A of jack J6 to dc

loop no. 1, the loop used for one-way receive and duplex send. The current pulses controlled by loop current switch

A5A2Q1 key the tty equipment in series with dc loop No. 1. Return is applied through pin C of J6 to the negative side of

current supply 1.

External Operation. When internal-external switch A5S1 is set at external for a given loop, the associated diode

bridge rectifier and current regulator are switched off and out of the dc loop. The resulting loop current path is exactly the

same as for internal operation, except that the supply is external and unregulated. The steering quad permits connection

of the external supply without regard for the polarity of the external supply. If the external supply is connected

backwards, the loop current will be routed by the steering quad so that the direction of the current flow through loop

current switch A5A2Q1 is the same, regardless of the polarity of the external supply.

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