DC Loop. The dc loop performs as
an interface, as previously stated, but
filter is located between the DC LOOP
is also a constant current source for
and TTY KEY terminals and the TTY-FSK
the teletypewriter. This current source
converter. The rfi filter blocks rfi
is switchable on the amplifier-converter
that could damage internal components
front panel for either 60 or 20 milli-
or cause erroneous outputs.
amperes, depending on the type of tele-
typewriter unit used. Also mounted on
the front panel is a dc loop current
2-34. Relay type teletypewriter units
adjust potentiometer. This potentiom-
having an external loop voltage source
eter is used to adjust the dc loop cur-
can produce relay voltage spikes. These
rent to exactly 20 or 60 milliamperes,
spikes are suppressed by CR1 in the rfi
if necessary. VR1 is a 4.3v Zener diode
filter. The Zener diode (VR1) is used
which is temperature-compensated by CR6.
to shunt to ground any incoming voltages
VR1 and CR6 in parallel with the dc loop
if the amplifier-converter dc loop
current adjust potentiometer, and R19
develops a major malfunction.
and R20, comprise the major components
of the dc loop.