SWITCHING CIRCUITS

TM 11-5805-387-34-2

Send Sensing Circuit. The send sensing circuit develops a positive dc output pulse whenever dc loop current flows

(mark signal) and no output pulse when no current flows (space signal). Switching circuitry (para 1-42) connects the send

steering quad into the send dc loop. When the tty machine in the dc loop closes the loop, current flows through steering

quad diode A5A2CR14, send oscillator voltage supply zener diode A5VR6, and steering quad diode A5A2CR11. Thus,

when loop current flows, +6.8 volts dc appears across zener diode A5VR6. This dc level is filtered through choke

A5A2L3 and is simultaneously applied to the base and collector of send oscillator A5A2Q4, gating the oscillator on. The

frequency of the oscillator is determined by the resonant circuit composed of capacitor A5A2C11 and the inductance of

the primary of transformer A5A2T2. Transformer A5A2T2 isolates the following circuitry from the dc loop and associated

tty machines, preventing problems associated with grounding. The send oscillator signal is rectified by an envelope

detector-doubler consisting of A5A2C12, A5A2CR15, A5A2CR16, and A5A2C13. The resulting dc pulse (send pulse) is

applied to transmitter module A3 through pin 9 of connector A5J1. When the send dc loop is open (space signal), no

voltage drop occurs across zener diode A5A2VR6. Consequently, send oscillator A5A2Q4 is cut off and no output pulse

appears. Steering quad diodes A5A2CR11 through A5A2CR14 allow an external power supply to be used without regard

1-42. SWITCHING CIRCUITS

(fig. FO-9)

Internal-External Switch. Combinations of the dc-to-ac inverter and current regulator, receive sensing and send

sensing circuits (para 1-41) will be used for each mode of operation of the modem. Internal-external switch A5S1 selects

an internal or external power source for each dc loop, regardless of mode of operation. The internal current supply

actually used does depend on the mode of operation. Power sources corresponding to each switch setting of A5S1 for

duplex mode are listed in the following chart.

INT-EXT SWITCH

POWER SOURCE

SETTING

DC LOOP 1

DC LOOP 2

Loop 1-Ext Loop 2-Int

External source

Dc-to-ac inv 2, current reg 2

Loop 1-Int Loop 2-Ext

Dc-to-ac inv 1, current reg 1

External source

Loop 1 and 2-Ext

External source

Loop 1 and 2-Int

Dc-to-ac inv 1, current reg 1

Dc-to-ac inv 2, current reg 2

Loop Current Paths. Relays A5A2K1 and A5A2K2 are controlled by grounds supplied by front panel switches

RCV/SEND S5 and ONE WAY/DUPLEX S1, respectively. These relays switch the loop currents to the send and receive

sensing circuits, depending on the settings of the front panel switches. The following chart shows the loop current paths

for each setting of the ONE WAY/ DUPLEX and RCV/SEND switches (module A5 circuit interconnection for various

modem modes).

1-30