1-33. RECEIVE DC LOOP
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.