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 24 (973) 334-3100
Recommended Transfer Ratio
The Transfer Ratio (XFR) is a multiplier used by the internal digital math processor in the TMR. It is
applied to the signal in addition to the 5mA/V conversion factor resulting from the fact that the full-
scale input at the A/D converter is -10V to +10V and that the full-scale output at the D/A converter is
-50mA to +50mA.
The transfer ratio XFR is calculated as follows:
XFR = { output signal span at the D/A } / { (input signal span at the A/D) x 5mA/V }
In the above examples (4-20mA on input and output), the signals at the converters have these
spans:
input: 1.6 to 8V; span = 6.4V
output: 8 to 40mA; span = 32mA
Therefore, XFR = { 32mA } / { 6.4V x 5mA/V } = 1
For additional information, refer to RFL Telemetry Application Note 1.
Recommended Output Offset For TMR
If an Input Offset was set as recommended above, use an Output Offset equal to the desired value of
the left scale of a unipolar signal or of the center scale of a bipolar signal at the D/A converter. This is
required because the input offset has an opposite effect at the Transmitter and the digital data which
passes from the Transmitter to the Receiver represents either a unipolar signal starting at zero or a
bipolar signal centered around zero.
For example, for an output range of 4-20mA with output gain G
out
=1/2, the effective current required is
8-40mA at the D/A converter. If the internal digital data starts at zero as a result of input offset OS
in
,
then using an offset OS
ou t
= +8mA will add 8 mA to the internal digital data. This will cause the
internal range to start at 8 mA. For additional information refer to Application Note 1.
Parameter Values for Common Input-Output Combinations
Tables 11 and 12 contain recommended configuration parameter values for the TMX and TMR
modules. The parameters shown were calculated for the most commonly used input and output signal
ranges.
The tables contain values, not settings. For example, the table may indicate a required transfer ratio of
XFR = 1.5625. A configuration setting resulting in that value is: MSB = 0001100 binary, LSB =
10000000 binary.
For directions on how to determine parameter SCB or DIP-Switch settings resulting in desired values,
refer to the Remote Control Interface section of this publication and to Telemetry Application Note 1.
The symbols used in the Transfer Function and in Tables 11 and 12 are defined as follows:
R
in
- input current-to-voltage conversion resistor.
R
out
- output voltage-to-current conversion resistor.
G
in
- input gain value.
OS
in
- input offset value: polarity and magnitude.
XFR - transfer ratio value
OS
out
- output offset value: polarity and magnitude.
G
out
- output gain value.
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