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Unformatted text preview: 3-7. National Instruments Corporation 3-11 SCB-68 Shielded Connector Block User Manual Chapter 3 Connecting Signals +5 V LED DIO<4..7> TTL Signal +5 V Switch DIO<0..3> DGND I/O Connector SCB-68 Figure 3-7. Digital I/O Connections Connecting Timing Signals
If you are using a 68-pin or 100-pin DAQ device, all external control over device timing is routed through the programmable function input (PFI) lines <0..9>. These PFI lines are bidirectional; as outputs they are not programmable and reflect the state of many DAQ, waveform generation, and general-purpose timing signals. The remaining timing signals use five different dedicated outputs.
Note For more information, refer to the device user manual at ni.com/manuals for detailed signal description and connection information. SCB-68 Shielded Connector Block User Manual 3-12 ni.com Chapter 3 Connecting Signals All digital timing connections are referenced to DGND. Figure 3-8 demonstrates how to connect two external timing signals to the PFI pins of a DAQ device. PFI0 PFI2 PFI0 Source PFI2 Source DGND I/O Connector SCB-68 Figure 3-8. Timing I/O Connections Noise Considerations
Environmental noise can seriously affect the measurement accuracy of your application if you do not take proper care when running signal wires between signal sources and the device. The following recommendations apply mainly to AI signal routing to the device, although they also apply to signal routing in general. Minimize noise pickup and maximize measurement accuracy by taking the following precautions: Use differential AI connections to reject common-mode noise, if the DAQ device that you are using supports DIFF input mode. Use individually shielded, twisted-pair wires to connect AI signals to the device. With this type of wire, the signals attached to the National Instruments Corporation 3-13 SCB-68 Shielded Connector Block User Manual Chapter 3 Connecting Signals ACH+ and ACH inputs are twisted together and then covered with a shield. You then connect this shield at only one point to the signal source ground. This kind of connection is required for signals traveling through areas with large magnetic fields or high electromagnetic interference. Route signals to the device carefully. Keep cabling away from noise sources. A common noise source in DAQ applications is the computer monitor. Separate the monitor from the analog signals as far as possible. The following recommendations apply for all signal connections to the DAQ device: Separate DAQ device signal lines from high-current or high-voltage lines. These lines can induce currents in or voltages on the DAQ device signal lines if they run in parallel paths at a close distance. To reduce the magnetic coupling between lines, separate them by a reasonable distance if they run in parallel, or run the lines at right angles to each other. Do not run signal lines through conduits that also contain power lines. Protect signal lines from magnetic fields caused by electric motors, welding equipment, breakers, or transformers by running them through special metal conduits. For information about minimizing noise in your application, refer to the NI Developer Zone tutorial, Field Wiring and Noise Considerations for Analog Signals, located at ni.com/zone. SCB-68 Shielded Connector Block User Manual 3-14 ni.com Using Thermocouples 4 This chapter describes how to take thermocouple measurements using the SCB-68. A thermocouple is created when two dissimilar metals touch, and the contact produces a small voltage that changes as a function of temperature. By measuring the voltage of a thermocouple, you can determine temperature using a nonlinear equation that is unique to each thermocouple type. Thermocouple types are designated by capital letters that indicate their composition according to the American National Standards Institute (ANSI) conventions. To determine the type of thermocouple that you are using, refer to Table 4-1. For more information on the theory of operation of thermocouples, refer to the NI Developer Zone tutorial, Measuring Temperature with Thermocouples, at ni.com/zone.
Table 4-1. Thermocouple Coloring Thermocouple Type B C E J K N R S U T Positive Color Gray White/Red Trace Purple White Yellow Orange Black Black Black Blue Negative Color Red Red Red Red Red Red Red Red Red Red Thermocouple Cover Color -- -- Brown Brown Brown Brown -- -- -- Brown Extended Grade Cover Color Gray White/Red Trace Purple Black Yellow Orange Green Green Green Blue National Instruments Corporation 4-1 SCB-68 Shielded Connector Block User Manual Chapter 4 Using Thermocouples The maximum voltage level thermocouples generate is typically only a few millivolts. Therefore, you should use a DAQ device with high gain for best resolution. You can measure thermocouples in either differential or single-ended configuration. The differential configuration has better noise immunity, but the single-ended configurations have twice as many inputs. The DAQ device must have a ground reference, because thermocouples are floating signal sources. Therefore...
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