Studio Technologies ISS - Manual (Page 24)

ISS User Guide Issue 3, June 1990
Studio Technologies, Inc. Page 25
ISS
positions of the ribbon cable are described
in Figure 2, located at the end of this
manual. As the 20 conductors on the ribbon
cable bus are .050" apart, care was taken
during the design phase to limit the chance
of cross talk occurring between adjacent
signal paths. This was achieved by two
means: signal path selection and limiting
signal level. Physical isolation was imple-
mented by keeping the audio signals away
from control (logic) signals. The audio
signal levels are limited by the nominal
internal audio operating level of –6dBu.
Logic signal transitions are limited in num-
ber due to the non-synchronous design of
the system. No clock signals or reoccurring
logic transitions are produced within the
system.
I/O Card
There are four main sections of circuitry on
the I/O Card: ±18V power supplies,
undervoltage and I/O Bypass sensing and
control, line input, and line output.
+18V Power Supplies: With nominal input
and output levels of up to +8dBu, the input
and output audio circuitry on the I/O Card
requires ±18Vdc for excellent audio perfor-
mance. Operation at lower power supply
rails, such as +15Vdc, will not provide
adequate peak signal levels; i.e., will give
inadequate headroom. The mainframe
provides ±15Vdc and ±24Vdc. The I/O
Card utilizes two integrated circuit type
voltage regulators (and supporting circuitry)
to reduce the ±24Vdc to ±18Vdc. As a
note, other sections of the I/O Card use the
±15Vdc and ±24Vdc.
Undervoltage Sensing and I/O Bypass: As a
product intended for continuous on-air duty,
major ISS failures must not take a station’s
audio off the air. Relays on the Transfer
Relay Assembly act as a “hard” (mechani-
cal) bypass switch. We call this the I/O
Bypass function. A control signal is gener-
ated by the I/O Card which controls the
Transfer Relay Assembly. When the ISS is
operating normally, the relays are held in
the energized state. The left and right line
input signals connect to the line input cir-
cuitry; the left and right line output circuitry
connects to the line output connectors.
When the I/O Card goes into the I/O Bypass
state, the transfer relays de-energize, con-
necting the line input signals to the line
output connectors. In this mode, the ISS’s
line input and output circuitry is discon-
nected from the outside world.
The I/O Bypass function can occur because
of three reasons: an undervoltage condition
on one or more of the four power supply
voltages; a manually initiated command
from the front panel switch; or a command
via the remote control input.
A 5.1V zener diode provides a reference
voltage for the undervoltage sensing cir-
cuitry. This reference voltage is scaled and
connected to four sections of integrated
circuit voltage comparator. Two sections of
comparator monitor the mainframe
±15Vdc, and two sections monitor the
±18Vdc from the I/O Card voltage regula-
tors. If the +15Vdc or +18Vdc drop below
+10Vdc, an error condition is detected
and the transfer relays de-energize. If the
–15Vdc or –18Vdc go less negative than
–12Vdc, an error condition is detected and
the transfer relays de-energize.
A switch located on the front edge of the I/O
Card allows manual I/O bypassing.