Rockwell Automation 1746-NR8 SLC 500 RTD/Resistance Input Module User Manual - Manual (Page 66)

Publication 1746-UM003A-EN-P
4-18 Channel Configuration, Data, and Status
While the channel enable bit is cleared (0), the channel data word and status
word values are cleared. After the channel enable bit is set, the channel data
word and status word remain cleared until the RTD module sets the channel
status bit (bit 11) in the channel status word.
Table 4.23 Bit Descriptions for Channel Enable Selection
Excitation Current Selection (Bit 12)
The following table shows the description for bit 12. Use this bit to select the
magnitude of the excitation current for each enabled channel. Choose from
either 1.0 mA or 0.25 mA. This bit field is active for all inputs. A lower current
reduces the error due to RTD self heating, but provides a lower signal-to-noise
ratio. Refer to RTD vendor for recommendations. See Appendix A for general
information.
Table 4.24 Bit Description for Excitation Current Selection
Calibration Disable (Bit 13)
The module can disable or enable periodic calibration by setting the
calibration disable bit for channel 0. Setting this bit to 0 enables the periodic
calibration, which occurs once every 5 minutes. Setting this bit to 1 disables
the periodic calibration
Lead Resistance Measurement Enable (Bits 14 and 15)
The module can disable lead resistance measurement, periodically measure the
lead resistance, or measure the lead resistance on each acquisition for each one
of the 8 channels. Setting a channel’s lead resistance enable bits to 00 disables
the lead resistance measurement. Setting a channel’s lead resistance enable bits
to 01 enables the periodic measurement of the lead resistance, which occurs
once every five minutes. Setting a channel’s lead resistance enable bits to 10
enables measurement of the lead resistance on each acquisition cycle.
Binary Value Select If you want to
0 channel disable disable a channel. Disabling a channel causes the
channel data word and the channel status word to be
cleared.
1 channel enable enable a channel.
Binary
Value Select Description
0 1.0 mA Set the excitation current to 1.0 mA.
1 0.25 mA Set the excitation current to 0.25 mA.