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Scan Lists

Last Modified: October 17, 2018

A scan list is a string composed of device names, channel names, and characters that define connections, disconnections, triggering, and timing of the scan.

Switch Modules in NI-DAQmx

MAX supplies the default device name (or resource name) you use in a scan list. To find the default name, open MAX, and go to Devices and Interfaces»NI-DAQmx Devices. An example name is SCXI-1130:"SC1Mod1". The part of the name in quotes—for instance, SC1Mod1 or Dev1—is what you use in the scan list.

Scan List Characters

The following characters can be used in a scan list.

Character Definition
-> Used in a connect action (/Dev1/channel1->channel2). For example, the string /SC1Mod1/ch0->com0 connects CH0 to COM0.
~ Used with '->' in a disconnect action (~/Dev1/channel1->channel2). Valid only in No Action mode. For example, ~/Dev1/ch0->com0 means disconnect CH0 and COM0.
; Wait for debounce, send scan advance output signal, then wait for trigger input.
& Separates connect and/or disconnect actions. For example, the string /Dev1/ch0->com0 & /Dev1/ch9->com1 means connect CH0 to COM0 and CH9 to COM1 (in no particular order).
&& Wait for debounce. For example, /Dev1/ch0->com0 && /Dev1/ch9->com1 means connect CH0 to COM0, wait for the relays to settle, then connect CH9 to COM1.
: Used in a channel range (channelX:Y, where X and Y are integers). Text containing a channel range represents multiple scan list entries. For example, the string /SC1Mod4/ch0:7->com0; represents 8 scan list entries. A semicolon must appear after the connect action using a channel range.
spd-note-tip
Tip  

NI-DAQmx ignores white space and line returns. Use these to format the appearance of lengthy scan lists.

Scan List Entries

A scan list entry is the text delimited by semicolons (;). Scan lists are composed of one or more scan list entries. For example, the following scan list contains two scan list entries:

/Dev1/ch0->com0; /Dev1/ch1->com0;

Scan Modes

The scan mode affects how NI-DAQmx interprets the scan list string. Typical scanning applications use the Break Before Make scan mode.

Mode Description
Break Before Make (default) Connections from the previous scan list entry are automatically disconnected before executing the current scan list entry. Disconnect actions (~/Dev1/channel1->channel2) are not valid in this mode.
No Action Connections remain connected until they are explicitly disconnected by a disconnect action.
Break After Make Currently unsupported.

Scan List Examples

Example 1

Scan Mode: No Action

Scan List: /Dev1/ch0->com0; ~/Dev1/ch0->com0 && /Dev1/ch1->com0; ~/Dev1/ch1->com0 &&

Meaning:

  1. Connect ch0 to com0.
  2. Wait for debounce, send scan advance signal, then wait for trigger input.
  3. Disconnect ch0 from com0, and wait for debounce.
  4. Connect ch1 to com0.
  5. Wait for debounce, send scan advance signal, then wait for trigger input.
  6. Disconnect ch1 from com0, and wait for debounce.
  7. If the scan is set to continuous, return to step 1; else, stop.

Example 2

Scan Mode: Break Before Make

Scan List: /Dev1/ch0->com0; /Dev1/ch1->com0;

Meaning:

This scan list is equivalent to Example 1. Notice that the disconnect actions described in Example 1 are no longer required.

Example 3

Scan Mode: Break Before Make

Scan List: /Dev1/ch0:1->/Dev1/com0;

Meaning:

This scan list is equivalent to Example 1 and Example 2. This scan list uses a channel range to reduce typing.


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