National Instruments NI 781xR User Manual download pdf (Page 22)

Chapter 2 Hardware Overview of the NI 781xR

NI 781xR User Manual 2-4 ni.com

Figure 2-2 shows DIO<0..3> configured for digital input and DIO<4..7>

configured for digital output. Digital input applications include receiving

TTL, LVTTL, CMOS, or LVCMOS signals and sensing external device

states, such as the state of the switch shown in Figure 2-2. Digital output

applications include sending TTL or LVCMOS signals and driving external

devices, such as the LED shown in Figure 2-2.

The NI 781xR SH68-C68-S shielded cable contains 34 twisted pairs of

conductors. To maximize the digital I/O available on the NI 781xR, some

of the DIO lines are twisted with power or ground, and some DIO lines are

twisted with other DIO lines. To obtain maximum signal integrity, place

edge-sensitive or high-frequency digital signals on the DIO lines that are

paired with power or ground. Because the DIO lines that are twisted with

other DIO lines can couple noise onto each other, use these lines for static

signals or for non-edge-sensitive, low-frequency digital signals. Examples

of high-frequency or edge-sensitive signals include clock, trigger,

pulse-width modulation (PWM), encoder, and counter signals. Examples of

static signals or non-edge-sensitive, low-frequency signals include LEDs,

switches, and relays. Table 2-1 summarizes these guidelines.

RTSI Trigger Bus

The NI 781xR can send and receive triggers through the RTSI trigger bus.

The RTSI bus provides eight shared trigger lines that connect to all the

devices on the bus. In PXI, the trigger lines are shared between all the PXI

slots in a bus segment. In PCI, the RTSI bus is implemented through a

ribbon cable connected to the RTSI connector on each device that needs to

access the RTSI bus.

You can use the RTSI trigger lines to synchronize the NI 781xR to any other

device that supports RTSI triggers. On the NI PCI-781xR, the RTSI trigger

Table 2-1. DIO Signal Guidelines for the NI PXI-781xR

Digital Lines

SH68-C68-S Shielded

Cable Signal Pairing

Recommended Types

of Digital Signals

DIO<0..27> DIO line paired with power

or ground

All types—high-frequency or

low-frequency signals,

edge-sensitive or

non-edge-sensitive signals

DIO<28..39> DIO line paired with another

DIO line

Static signals or

non-edge-sensitive,

low-frequency signals

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National Instruments NI 781xR User Manual Page 22

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