User manual

Chapter 3 Software Overview
NI-DAQ User Manual for PC Compatibles 3-54 ni.com
In the cases where a digital I/O port has fewer than eight lines, the most
significant bits in the byte format are ignored.
You can configure most of the digital I/O ports as either input ports
or output ports. On the PC-TIO-10, 653X,DSA,671X, and E Series devices
(except for ports 2, 3, and 4 on the AT-MIO-16DE-10), you can program
lines on the same port independently as input or output lines. Some digital
I/O ports are permanently fixed as either input ports or output ports. If you
configure a port as an input port, reading that port returns the value of the
digital lines. In this case, external devices connected to and driving those
lines determine the state of the digital lines.
If no external device is driving the lines, the lines float to some
indeterminate state, and you can read them in either state 0 (digital logic
low) or state 1 (digital logic high). If you configure a port as an output port,
writing to the port sets each digital line in the port to a digital logic high or
low, depending on the data written. In this case, these digital lines can drive
an external device. Many of the digital I/O ports have read-back capability;
if you configure the port as an output port, reading the port returns the
output state of that port.
You can use digital I/O ports on the DIO-24, 6025E devices, and
AT-MIO-16DE-10 (ports 2 and 3 only), DIO-96, Lab and 1200 devices,
653X, DIO-32F, PCI-6115 and PCI-6120 devices for handshaking and
no-handshaking modes. These two modes have the following
characteristics:
• No-handshaking mode—This mode changes the digital value at an
output port when written to and returns a digital value from a digital
input port when read from. No handshaking signals are generated.
• Handshaking mode—This mode is for digital I/O handshaking; that is,
a digital input port latches the data present at the input when the port
receives a handshake signal and generates a handshake pulse when the
computer writes to a digital output port. In this mode, you can read the
status of a port or a group of ports to determine whether an external
device has accepted data written to an output port or has latched data
2 2
1 1
0 0 Least significant bit (LSB)
Table 3-6. Byte Mapping to Digital I/O Lines (Continued)
Bit Number Digital I/O Line Number