API Support for Switch Modules

Switch modules can support any of four different ways to control their relays. You may use the APIs interchangeably, but NI recommends using a single API for each application.

• Digital Output—Create your tasks, either programmatically with the Create Channel Digital Output function/VI or interactively through the DAQ Assistant, using the digital output physical channels. Use the digital versions of the Write function/VI to control the relays. Each digital port consists of 32 digital lines, and each line represents a relay on the switch. For example, if a module contains 64 relays, the first 32 are on port 0, and the rest will be on port 1. Writing a 0 to a digital line opens the relay and writing a 1 closes it.
• Immediate—The immediate API, supported by all switches, provides a switch channel-based interaction recommended for nonscanning operations. Functions/VIs such as DAQmx Switch Connect and DAQmx Switch Disconnect are considered part of the immediate API.
• Relay—The relay API provides a relay-based interaction. Functions/VIs like DAQmx Switch Open Relays and DAQmx Switch Close Relays are considered part of the relay API.
• Scanning—Scanning is a method of connecting channels and is often used when connecting instruments and devices under test (DUTs) in a specific order. In this operation mode, the switch cycles through each entry in a scan list downloaded to the switch. The triggers the switch receives initiate this cycling. Create scanning tasks using DAQmx Switch Create Scan List and control tasks using functions/VIs like DAQmx Start, DAQmx Stop, and so on.

Supported Topologies

Every switch module supports one or more topologies. Changing the topology alters the functionality of the switch and, in many cases, changes the list of supported channel names.

Special Considerations

Some switch modules have specific behaviors that you must consider when developing applications, described in the following table.

SCXI-1127 Considerations

To route signals to the analog bus backplane, you must enable the switch device property Auto Connect Analog Bus. As a result, if you connect a channel (ch1) to the common channel (com0), the signal is automatically routed from com0 to the analog bus (ab0).

The SCXI-1127 supports only continuous scanning.

If you have used immediate or relay operations to change relay states before starting a scan, all of those relays are opened when the scan starts. After the scan completes, the relays are returned to their previous state prior to the scan.

Early revisions of this hardware reserve the SCXI_TRIG1 line. If you place an older revision of this hardware (earlier than revision E) into an SCXI chassis that also contains an SCXI analog input module that performs track and hold (such as the SCXI-1140 or SCXI-1520), you may get reservation errors when trying to use the SCXI analog input module.

SCXI-1128 Considerations

To route signals to the analog bus backplane, you must enable the switch device attribute/property Auto Connect Analog Bus. As a result, if you connect a channel (ch1) to the common channel (com0), the signal is automatically routed from com0 to the analog bus (ab0).

The SCXI-1128 supports only continuous scanning.

If you have used immediate or relay operations to change relay states before starting a scan, all of those relays are opened when the scan starts. After the scan completes, the relays are returned to their previous state prior to the scan.