Studio Technologies 47 - Manual (Page 20)

Issue 1, August 2010 Model 47 User Guide
Page 20 Studio Technologies, Inc.
Logic Refresh
While testing the Model 46, “cousin” to
the Model 47, it was found that in cases of
extreme ESD (electro-static-discharge or
“static”) an integrated circuit in the audio
signal path could “latch up.” This would
result in the audio signal no longer pass-
ing through this component and on to
the 4-wire audio output. Whenever this
situation arose the component was not
damaged, but restoring the flow of audio
required either a power-down/power-up
sequence, an auto null operation, or a
change to one of the level configuration
DIP switches to take place. While it is very
unlikely that this type of ESD event would
occur during normal field operation it is
of concern for critical applications. Since
the Model 47’s circuitry is very similar, to
minimize the chance that its audio path
could experience this problem a “refresh”
routine was included in the operating
software (“firmware”). To ensure that the
ESD-sensitive component always remains
functioning correctly the software re-sends
its operating instructions four times each
second. With this implementation the
worst case scenario would result in an
audio loss of 250 milliseconds or less in
the event of an extreme ESD “hit” being
experienced.
But nothing comes without a price and in
this case the logic refresh routine adds a
very slight audio click to the 4-wire output
and 2-wire party-line signals. During typi-
cal applications these refresh clicks will
not be noticeable. But there may be situa-
tions where any added audio artifact may
simply not be acceptable. To address this
possible, but highly unlikely, case back-
panel DIP switch 3 can be used to disable
the refresh routine. Referring to Figure 9,
when DIP switch 3 is in its off (down) posi-
tion the Model 47 operates in its standard
fashion. When DIP switch 3 is in its on (up)
position the logic refresh routine is dis-
abled. The Model 47 will continue to func-
tion normally with the exception that the
4-times-per-second logic refresh routine
will not occur. Goodbye click but hello to
the possibility that an ESD-induced audio
path problem could occur. A good “rule
of thumb” is that if a Model 47 is mounted
in an equipment rack with a known-good
ground connection, disabling logic re-
fresh shouldn’t pose any risk to reliable
operation. But if a unit is mounted in a
portable rack, or used loose as a single
device, logic refresh should always remain
enabled. The reasoning is quite simple:
the latter condition is much more likely
to allow an ESD “hit” to dissipate into the
Model 47’s enclosure, while a substantial
rack enclosure will offer a low-impedance
path for ESD energy to dissipate into.
Factory Test
Back-panel DIP switch 4 is used to select
between normal mode and factory test
mode. Referring to Figure 10, when DIP
switch 4 is in its off (down) position the
Model 47 operates in its standard fashion.
When DIP switch 4 is in its on (up) posi-
tion the factory test mode is selected. As
expected, when the Model 47 is deployed
Figure 9. DIP Switch 3—Logic Refresh