Studio Technologies IFB Plus 2001 - Manual (Page 36)

Issue 7, November 2001 Model 2 User Guide
Page 36 Studio Technologies, Inc.
Four sections of analog switch set the
audio flow through the interface. Control
signals for the analog switches are derived
from the front-panel audio routing switch,
along with some simple logic gates. One
section of analog switch controls the
output of the receive operational amplifier.
It is active only when the interface is in the
receive mode. This analog switch prevents
channel 1 or 2 audio from “feeding back”
to other Model 2 circuits when the inter-
face is in the send mode. The other three
sections of analog switch control the flow
of send audio, as well as adjusting the
interface termination impedance. The
secondary of the transformer, via a 1000
ohm resistor, connects to one of the three
remaining sections of analog switch. In the
standard audio mode this switch is always
in the on state, keeping the 1000 ohm
resistor effectively connected across the
transformer’s secondary. This resistor,
along with the 2200 ohm resistor on the
input of the receive op-amp, provides a
700 ohm termination impedance. In the
standard audio mode this analog switch
is active when the routing switch is set to
send channel 1 or channel 2 audio. The
two remaining analog switches control if
channel 1 or channel 2 audio is connected
to the send driver operational amplifier.
The interface mode switch, located on the
back panel, controls two functions: relay
status and termination impedance. In the
telephone line mode the interface control
switch is active, allowing the operator to
control the relay. In the standard audio
mode the switch is disabled, the relay
is held energized, and the LED labeled
STD is lit.
A simple logic circuit implements the
telephone interface control function. Two
NAND gates form a flip-flop, the two states
being on-hook and off-hook. In the tele-
phone line mode, pressing the interface
control switch to the manual off-hook posi-
tion sets the flip-flop to the off-hook state,
causing the relay to energize via one gate
of a power driver integrated circuit. The
output of the ring voltage detector can
also set the flip-flop. If ringing voltage is
detected for a sufficient time period the
resistor/capacitor circuit on the input of
the flip-flop is brought to the logic low state,
and the flip-flop is set. If loop current is
detected the circuit will stay in the off-hook
state. If current is not detected the circuit
will time out and return to the on-hook
state. Pressing the interface switch to the
manual hang-up position forces the circuit
to the on-hook state. Notice that there is
what appears to be a “bug” in this circuit
when in the telephone line mode. If the
interface control switch is moved to the
manual off-hook position, and then moved
immediately to the hang-up position, the
phone line will not “hang up.” This is be-
cause approximately three seconds are
required for the ringing detection capacitor
to recharge after being discharged by the
manual off-hook switch action. If a pause
of three seconds occurs, or the interface
control switch is held in the hang-up posi-
tion for about three seconds, the telephone
line will hang up. Normal operation will not
be affected by this condition.
Access Station Interface
The Model 2 contains interface circuitry to
allow up to four Model 22 Access Stations
to be connected. Four types of signals are
sent to and received from the access sta-
tion: interrupt audio, lamp voltage, control
signals, and +18Vdc power.
Two identical circuits connect access
station interrupt audio into the Model 2.