User manual

Chapter 3 Software Overview

section in the NI-DAQ Function Reference Online Help file to find out

which digital functions your device supports. The SCXI functions control

the SCXI digital and relay modules.

These devices contain a number of digital I/O ports of up to eight digital

lines in width. The name port refers to a set of digital lines. Digital lines

are also referred to as bits in this text. In many instances, you control the

set of digital lines as a group for both reading and writing purposes and for

configuration purposes. For example, you can configure the port as an input

port or as an output port, which means that the set of digital lines making

up the port consist of either all input lines or all output lines.

In NI-DAQ, you refer to ports by number. Many digital I/O devices label

ports by letter. For these devices, use port number 0 for port A, port

number 1 for port B, and so on. For example, the DIO-24 contains three

ports of eight digital lines each. Ports 0, 1, and 2 are labeled PA, PB, and

PC on the DIO-24 I/O connector. The eight digital lines making up port 0,

lines 0 through 7, are labeled PA0 through PA7.

In some cases, you can combine digital I/O ports into a larger entity

called a group. On the DIO-32F and 653X devices, for example, you can

assign any of the ports DIOA through DIOD to one of two groups. On the

PCI-6115 and PCI-6120, you can also create groups smaller than the port

size. For example, both the PCI-6115 and the PCI-6120 have one digital

port of eight lines, but you can configure five lines as an input group and

three lines as an output group. A group of ports are handshaked or clocked

as a unit.

The Digital I/O functions can write to and read from both an entire port

and single digital lines within the port. To write to an entire port, NI-DAQ

writes a byte of data to the port in a specified digital output pattern. To read

from a port, NI-DAQ returns a byte of data in a specified digital output

pattern. The byte mapping to the digital I/O lines is as follows.

Table 3-6.

Byte Mapping to Digital I/O Lines

Bit Number Digital I/O Line Number

7 7Mostsignificantbit(MSB)

6 6

5 5

4 4

3 3