Series 655T/656T Transmitter User's Manual mV/TC Input
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Refer to the Table 4 of the following page when using a
precision millivoltage source to configure the zero and full-
scale input range endpoints (with CJC turned Off).
Transmitter Zero/Full-Scale Configuration Procedure
1. Set DIP switch 12 of the channel to the up/ON or “Unlock”
position to allow the front panel toggle switches to make
adjustments to zero and full-scale.
2. Connect a precision voltage source or thermocouple
calibrator to the input, as required. Connect a current
milliampmeter (in series with the loop), or voltmeter (across
a precision load resistor), to accurately read the output
signal. Apply power to the transmitter.
3. Adjust the input source to the zero level (this level must be
within the input range selected). For our example: use 0
C.
4. If the measured output is not precisely at zero (4.000mA),
depress the
Zero
“Z” toggle switch to the
“UP”
or “
DN
”
position as required, to precisely adjust the output current to
4.000mA.
Note: The Zero Up/Down toggle functions as a trim
adjustment for the zero output loop current level.
Successive depressions of the “UP” or “DN” toggle positions
will increment or decrement the output current by a small
amount, while holding the toggle switch in the “UP’ or “DN”
position will increase the amount of increment or decrement.
Reverse Acting Outputs:
A reverse acting output can be
easily obtained by using the Up/Down toggle to adjust the
output level accordingly. For a reverse acting output, you
would adjust the zero output level to a higher level here
(20.000mA) in response to the zero input, and a lower level
(4.000mA) in response to a full-scale input signal (step 6).
5. Adjust the input source to the full-scale level (the input value
must be within the input range selected). For our example:
use 200
C.
Note: The full-scale input value must be greater than the
zero input value. If the zero and full-scale points are too
close together, performance will be degraded.
6. If the output is not exactly at the full-scale level (20.000mA),
press the
Full-Scale
“FS” Up/Down toggle to the
“UP”
or
“DN”
position, as required to precisely adjust the output
current to 20.000mA.
Note: The FS Up/Down toggle functions as a trim
adjustment for the full-scale output loop current level.
Successive depressions of the “UP” or “DN” toggle positions
will increment or decrement the output current by a small
amount, while holding the toggle switch in “UP’ or “DN”
position will increase the amount of increment or decrement.
Reverse Acting Outputs:
A reverse acting output can be
easily obtained by using the Up/Down toggle to adjust the
output level accordingly. For a reverse acting output, you
would adjust the output level to a lower level (4.000mA) in
response to the full-scale input here, and a higher level
(20.000mA) in response to a zero input signal (step 4).
7. After completing zero and full-scale adjustment, be sure to
return DIP switch 12 to the down or “Lock” position after
waiting at least 15 seconds. This will help prevent
inadvertent reconfiguration or tampering in the field by
locking out adjustment via the front panel zero and full-scale
toggle switches. Repeat this procedure for the second
channel of dual channel units.
IMPORTANT:
Note that the zero and full-scale adjustments
take effect immediately and are saved to non-volatile
memory after 15 seconds of toggle switch inactivity. Please
wait at least 15 seconds following adjustment before
powering down or setting DIP switch 12 to the “Lock”
position or your new configuration settings will be lost.
After setting the DIP switches as required by your application
and configuring zero and span, be sure to install the plastic
cover(s) over the DIP switch openings. To install these covers,
you will have to remove the left side cover by prying at each
corner with a screwdriver, then sliding the DIP switch covers into
the switch opening. Replace the left side cover by snapping it
into place and applying pressure at each corner to secure.
4.0 THEORY OF OPERATION
Refer to Simplified Schematic 4501-777 to gain a better
understanding of the circuit. A portion of the output loop current
is routed to a transformer while being driven in an alternating
push-pull fashion to generate an input supply. The remaining
output current is pulled through a current steering circuit that
regulates the loop current based on an optically coupled input
signal. The transmitter will accept a thermocouple or voltage
input, and condition it to a voltage signal for the A/D converter. A
reference temperature sensor signal voltage is also input to the
A/D. The A/D converter stage then applies appropriate gain to
these signals, performs analog-to-digital conversion, and digitally
filters the signals. The digitized signals are then transmitted
serially to a microcontroller. The microcontroller completes the
transfer function according to the input type and configuration,
and sends a corresponding output signal to an optocoupler and
filter circuit. A corresponding analog output voltage is generated
and used to drive the amplifier used to modulate the loop current.
The embedded configuration and calibration parameters are
stored in non-volatile memory integrated within the micro-
controller.