Operator`s manual

Model VR240 Audio Logging Recorder
A-2
January 2000
CHU Canada 7.335 MHz
c. Call your local telephone company’s time number.
d. Log on to Internet at www.bldrdoc.gov.
Another way is to adjust the internal clock for your location’s temperature. You do not
necessarily need an extremely accurate counter, although it should have very good resolution.
Observe the clock for several days or weeks and note how much time it gains or loses. Let’s
say it is losing one second per day. This means that the frequency of the internal oscillator is
32768 multiplied by the number of seconds per day it is counting, (24*3600)-1, divided by the
number of seconds in a day, (24*3600). Because the clock test point gives the frequency of the
crystal divided by four, the frequency you should read will be 8191.905 Hz. If your counter
reads this frequency as well, simply readjust the clock to give 8192. If your counter reads
something else, just note the difference between the actual frequency (8191.905) and 8192, or
.095 Hz. Readjust the oscillator to read .095Hz higher on your counter, regardless of what the
actual frequency is.
Many modern frequency counters actually can read small fractions of a Hz quickly, by using a
“multiple period average” time count and converting this count to frequency with their
microprocessors. If your counter is an older model, you may have to explicitly use the multiple
period average function is available, or a long “gate time” if it is not.
A-2. CLOCK FREQUENCY ADJUSTMENT PROCEDURE.
Refer to the VR240 maintenance manual for the CPU-3 time clock adjustment procedure.
A-3. LITHIUM BATTERY VOLTAGE CHECK.
A lithium battery that allows it to keep time even if no power is applied to the VR240 powers the
clock. This battery also powers the non-volatile RAM that maintains unit configuration
information such as channel descriptors. Since this battery has a nominal 5-year lifetime, and
only provides power when the unit is turned OFF, it should last indefinitely. Nonetheless, as
long as you have the unit open, you can, if you wish, check the battery voltage. Confirm that it
is greater than 2.8V by measuring the voltage between the chassis ground and the top surface
of the battery, using a high input impedance digital voltmeter. (Note: If the battery voltage is
low, the VR240 will normally sense this during self-test and advise you to replace it.)
A-4. TIME CODE INPUT.
The VR240 provides an input for a time code generator. By sending time data, the clock of the
VR240 can be automatically updated and corrected so that it remains within two seconds of the
generator at all times. The time code input overrides the internal clock; so that setting the clock
(or clock frequency) as described in the VR240 maintenance manual is unnecessary. The time
code generator should be connected to PORT B on the VR240 rear panel using an RS-232
serial connection. The input is on PIN 6; the return is on PIN 3. Note that PORT B is also used
for the optional label printer and interconnection of two VR240s in the alternate recording mode.
The PORT B pins used are different for each and it is possible to use any combination of