National Instruments NI 781xR User Manual

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National Instruments NI 781xR User Manual Manual pdf 22 page image

Chapter 2 Hardware Overview of the NI 781xRNI 781xR User Manual 2-4 ni.comFigure 2-2 shows DIO<0..3> configured for digital input and DIO<4..7>configured for digital output. Digital input applications include receivingTTL, LVTTL, CMOS, or LVCMOS signals and sensing external devicestates, such as the state of the switch shown in Figure 2-2. Digital outputapplications include sending TTL or LVCMOS signals and driving externaldevices, such as the LED shown in Figure 2-2.The NI 781xR SH68-C68-S shielded cable contains 34 twisted pairs ofconductors. To maximize the digital I/O available on the NI 781xR, someof the DIO lines are twisted with power or ground, and some DIO lines aretwisted with other DIO lines. To obtain maximum signal integrity, placeedge-sensitive or high-frequency digital signals on the DIO lines that arepaired with power or ground. Because the DIO lines that are twisted withother DIO lines can couple noise onto each other, use these lines for staticsignals or for non-edge-sensitive, low-frequency digital signals. Examplesof high-frequency or edge-sensitive signals include clock, trigger,pulse-width modulation (PWM), encoder, and counter signals. Examples ofstatic signals or non-edge-sensitive, low-frequency signals include LEDs,switches, and relays. Table 2-1 summarizes these guidelines.RTSI Trigger BusThe NI 781xR can send and receive triggers through the RTSI trigger bus.The RTSI bus provides eight shared trigger lines that connect to all thedevices on the bus. In PXI, the trigger lines are shared between all the PXIslots in a bus segment. In PCI, the RTSI bus is implemented through aribbon cable connected to the RTSI connector on each device that needs toaccess the RTSI bus.You can use the RTSI trigger lines to synchronize the NI 781xR to any otherdevice that supports RTSI triggers. On the NI PCI-781xR, the RTSI triggerTable 2-1. DIO Signal Guidelines for the NI PXI-781xRDigital LinesSH68-C68-S ShieldedCable Signal PairingRecommended Typesof Digital SignalsDIO<0..27> DIO line paired with poweror groundAll types—high-frequency orlow-frequency signals,edge-sensitive ornon-edge-sensitive signalsDIO<28..39> DIO line paired with anotherDIO lineStatic signals ornon-edge-sensitive,low-frequency signals