Studio Technologies 42A 2013 - Manual (Page 19)

Model 42A User Guide Issue 2, December 2013
Studio Technologies, Inc. Page 19
continue to be displayed until the button is
released. Once the button is released the
unit will begin normal operation.
Note that while it’s easy to determine
which software version is loaded into
the Model 42A a trip back to the factory
is required to update it. The 8-bit micro-
controller that provides the unit’s logic
“horsepower” also includes internal
FLASH memory. This nonvolatile memory
is used to store the operating software
(“firmware”). Re-programming this mem-
ory requires using a specialized program-
ming unit. While not outrageous in price,
it still costs in the range of US$500. The
programmer uses a ribbon cable and
socket to interface with a 6-pin “header”
on the Model 42A’s printed circuit board.
And, as you would guess, once connected
reprogramming takes only a matter of
seconds. But unfortunately the program-
mer is not something that would be found
in a typical “field shop” or repair facility.
Cable Length
There are no hard and fast rules defining
the maximum cable length possible when
connecting user devices to the Model
42A’s IFB outputs. The maximum cable
length is directly related to the amount of
resistance in the connecting cable; the
lower the resistance per foot (or meter),
the longer the cable can be. (Although
cable capacitance affects high-frequency
performance, resistance is the limiting fac-
tor in this case.) For example, a traditional
20 AWG microphone cable is Belden
8412, which has 10.9 ohms resistance
per conductor per 1000 feet. Since we’re
using two conductors to carry the signal
(pins 1 and 2) you’d get 21.8 ohms per
1000 feet of cable. By knowing the cable
resistance value, along with the minimum
voltage and maximum load current re-
quired by an IFB user device, a simple
“ohms law” calculation will tell you the
maximum cable length.
Let’s use the example of a Studio Technol-
ogies Model 210 Announcer’s Console
being connected to a Model 42A IFB
output. We’ll select Belden 8412 as the
interconnecting cable. For correct opera-
tion, the Model 210 needs at least 24 volts
DC between pins 1 and 2 of its IFB input
connector. It has a current draw of 105
milliamperes. The Model 42A’s IFB out-
put presents an output voltage of 30 volts
across pins 1 and 2 and can supply a
maximum current of 220 milliamperes.
(As the Model 210’s current draw is well
within the Model 42A’s capability, this is
not a limiting factor.) The difference be-
tween the voltage supplied by the Model
42A (30 volts) and the voltage required by
the Model 210 (24 volts) allows a 6 volt
maximum drop over the interconnecting
cable. Using the current draw and maxi-
mum voltage drop figures, the maximum
cable resistance can easily be calcu-
lated: 6 volts divided by 0.105 amperes
equals 57 ohms. And finally, with 8412’s
21.8 ohms (total) per 1000 feet of cable,
a maximum of 2615 feet of cable can be
used and still be less than or equal to
57 ohms. Using this example as a guide,
entering the appropriate values will
allow you to determine the maximum
cable length for your application.
Cabling Issues – Crosstalk
The Model 42A’s IFB outputs conform to
a broadcast industry standard for sending
DC power and two channels of audio over
a single pair with shield audio cable. This
implementation allows standard portable
cables, such as are used for microphone