Rockwell Automation 1762-IR4 RTD/Resistance Input Module - Manual (Page 50)

Publication 1762-UM003A-EN-P - February 2003
3-16 Module Data, Status, and Channel Configuration
Selecting Cyclic Lead Compensation (Bit 4)
For each channel, the module measures lead resistance in one of two
ways. Set bit 4 to 0 to enable measurement and compensation of lead
resistance every five minutes. One channel is measured per module
update to limit the impact to channel throughput. You can also
implement a lead wire calibration cycle any time, at your command,
by enabling and then disabling this bit in your control program.
Regardless of the state of bit 4, lead wire compensation occurs
automatically on a system mode change from Program-to-Run or if
any online configuration change is made to a channel.
Selecting Excitation Current (Bit 3)
The module is capable of exciting each individual RTD/resistance
device with either 0.5 mA or 1.0 mA of current. Setting bit 3 to 0
provides 1.0 mA, while a setting of 1 provides 0.5 mA.
The 0.5 mA excitation current is recommended for use with 1000
Ω
RTDs and 3000
Ω
direct resistance inputs. An excitation current of 1.0
mA is recommended for all other RTDs except the 1000
Ω
devices, and
all other direct resistance devices except the 3000
Ω
devices. Refer to
the input device literature for the manufacturer’s recommendations.
Setting Filter Frequency (Bits 0 to 2)
The module supports filter selections corresponding to filter
frequencies of 10 Hz, 50 Hz, 60 Hz, 250 Hz, 500 Hz, and 1 kHz. Your
filter frequency selection is determined by the desired range for the
input type, and the required effective resolution, which indicates the
number of bits in the channel configuration word that do not vary due
to noise. Also consider the required module update time when
choosing a filter frequency. For example, the 10 Hz filter provides the
greatest attenuation of 50 and 60 Hz noise and the greatest resolution,
but also provides the slowest response speed.
TIP
A lower excitation current reduces error due to RTD
self-heating, but provides a lower signal-to-noise
ratio. See the manufacturer’s recommendations for
your particular RTD.