Because RFL™ and Hubbell® have a policy of continuous product improvement, we reserve the right to change designs and specifications without notice.
RFL TMX/TMR RFL Electronics Inc.
January 25, 2012 23 (973)
334-3100
RECOMMENDED GUIDELINES FOR THE SELECTION
OF PARAMETER VALUES
The telemetry system operates as a voltage-to-current converter. The input signals must be presented
in the form of voltage, while the output signals are generated in the form of current. The maximum
input range is -10V to +10V and the maximum output range is -50mA to +50mA. This equates to an
internal conversion factor of 5mA/V as the signal passes from the TMX to the TMR.
If you want to use current as an input instead of voltage, the input current must be passed through a
conversion resistor R
in
in order to obtain input voltage for the TMX Transmitter. If you want to use
voltage as an output instead of current, the output current must be passed through a conversion resistor
R
out
in order to obtain output current for the TMX Transmitter.
Transfer Function
The telemetry system processes signals according to the following transfer function:
[Output Signal, mA] = {([Input Signal, Volts] ∗ G
in
– OS
in
) ∗ XFR ∗ 5mA/V + OS
out
} ∗ G
out
See page 24 of this document for symbol definitions.
Recommended Input Gain For TMX
Use the highest possible gain which results in the largest possible voltage at the A/D which fits within
the -10V to +10V maximum input range.
For example, consider the 4-20mA input current range with R
in
=200Ω, resulting in an input voltage
range of 0.8-4V at the terminals. The highest possible gain is G
in
=2, resulting in an input signal of 1.6-
8V at the A/D converter. The next highest gain, G
in
=4, would cause the voltage at the A/D to exceed
the maximum input range.
Recommended Input Offset For TMX
Use an offset which causes the effective unipolar voltage after the A/D converter to have a “left scale”
of 0, or to center a bipolar signal around zero.
For example, for the input range of 4-20mA with R
in
=200Ω, and with input gain G
in
=2, the effective
voltage is 1.6-8V at the A/D converter. Using offset OS
in
= +1.6V will subtract this value from the
signal, causing the range to begin at zero. Note that magnitude and polarity of offsets are set
separately.
Recommended Output Gain For TMR
Use the lowest possible gain (highest attenuation) which causes the D/A to generate as large a current
as possible which still fits within the -50mA to +50mA maximum output range.
For example, consider the 4-20mA output range. The lowest possible gain is G
out
=1/2 , resulting in an
output signal of 8-40mA required at the D/A converter. The next lowest gain, G
out
=1/4, would cause
the current at the D/A to exceed the maximum output range.
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