Multiplexer Expansion with NI Switches

I need a multiplexer with more channels than any individual NI switch module has.  Is there a way to expand multiplexer channel count using multiple modules?

If you need to multiplex through more channels than the number present on your switching module, you can expand the size of the multiplexer by connecting the COM lines of multiple modules. There are a couple of methods for doing so:
 

Method 1: Directly tie the COM lines of each module together

The COM lines of two 4x1 multiplexers are tied together to create an 8x1 multiplexer
 

With this approach, there is some risk of shorting input channels and damaging hardware. Therefore, you need to ensure that only one of the channels from any module has a path to COM. In other words, it is very important that when one relay is closed on a given module, all relays on any other module tied to the same COM need to be open.

In order to guarantee that only a single channel on any of the modules tied together is connected to the shared COM, you can define software exclusions in NI Switch Executive. This utility facilitates the implementation of this method for matrix expansion, and it allows you to save the custom configuration for use in your program or sequence.
 

Method 2: Connect the COM lines of multiple modules to the input channels of a multiplexer

The COM lines of two 4x1 multiplexers are connected to input channels of a third multiplexer to create an 8x1 multiplexer
 

This technique implements the required exclusions in hardware, inherently allowing only one channel a path to COM. This is the recommended approach for RF matrix expansion, because when working with high-frequency signals, the configuration prevents reflections between multiplexers. 

For a similar expansion with fewer channels, you can connect the COM channels of fewer modules to the input channels of a multiplexer and use the remaining inputs as normal channels:

The COM line of a 4x1 multiplexer is connected to an input channel of another to create a 7x1 multiplexer
 

Again, NI Switch Executive can be very helpful in defining and maintaining these combined topologies.

Additional Information

If using Switch Executive, you can define software exclusions as follows:

1. Physically connect the COM terminals of all the modules that will be combined.
2. Create a new Switch Executive Virtual Device that includes all modules.
3. On the Hardwires/Buses tab, add a hardwire that associates the COM lines of each module.
4. On the Channels/Exclusions tab, add a mutual exclusion between all the input channels across all the modules.

After establishing these exclusions, calls to this virtual device will only execute in such a way that one single channel connects to the shared COM at any given time. This is a true multi-module multiplexer topology.