Understanding Switch Topologies

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A switch topology is an organizational representation of the channels and relays in a switch module. The topology establishes the default states for all relays in a module and defines the channel names. Some switches can use multiple topologies or variations of each topology type. Some terminal blocks or accessories may force the switch to use a given topology or set of topologies.

For the most up-to-date list of all switches from NI, visit ni.com/switches.


General-Purpose Topology

A general-purpose switch card consists of a group of independent relays. These relays are typically capable of switching high (relative to the multiplexer or matrix) current loads. Examples of use include power switching (connecting power to a unit under test (UUT)) and load switching.

The Form A and Form B switches are single pole single throw (SPST) switch types. The differentiation is in the rest state. In this state, Form A switches are open. Form B switches are closed when at rest.

Figure 1. SPST and SPDT Relay Forms

Form C and Form D switch operation is based on a single pole double throw (SPDT) switch. The operational difference is the Form C switch opens the connection before it closes the other connection (this is also known as break before make or BBM). The Form D switch closes both connections before opening the original connection (this is also known as make before break or MBB).

Figure 2. SPDT Form C (BBM) and Form D (MBB) Variants


An NI general-purpose card is usually composed by a series of Form A or Form C switches. The user decides which circuits to close and which input to connect at any given moment with the corresponding output. This architecture can be used to allow a current to flow through a circuit or to route a voltage from the input to the output.

For available NI switch modules that utilize the General-Purpose Topology, please visit NI PXI Switch Product Selection Guide.

Multiplexer Topology

Figure 3. Multiplexers are built with either one level of Form A relays, or multiple levels of Form C relays. 


A multiplexer, or mux is a topology in which you can connect one input to multiple outputs or one output to multiple inputs. This topology is often used for scanning when you need to automatically connect a sequence of channels to a common line. This topology can also be used to synchronize source and measure connections by using a pair of multiplexers. One multiplexer would be configured as 1:N, the other as N:1. An example of this would be a scope measuring four different signals one at a time and a function generator sourcing a signal to different point on the UUT.

For available NI switch modules that utilize the Multiplexer Topology, please visit NI PXI Switch Product Selection Guide.

Matrix Topology

A matrix is one of the most flexible switching configurations. Unlike a multiplexer, a matrix can connect multiple inputs to multiple outputs organized as columns and rows. You can connect any column to any number of rows and any row to any number of columns. At each intersection of a row and column, there is a switch. When the switch is closed, the row is connected to the column.

Matrix size is often described as M rows by N columns (M x N). The figure below depicts a 1-wire, 2 x 4 matrix.

Figure 4. Matrix rows are connected to columns through individual Form A relays.


Two common matrix configurations are shown in the two figures below.

Figure 5. Matrices can be configured with instruments on the rows and UUT test points on the columns.


Putting instruments on the rows and the units under test (UUTs) on the columns allows for easy UUT expansion. In this example to example, we could add more UUTs with another module and would need to connect just the four rows. If we needed to add more rows for the instruments we would need to connect all of the columns.

Figure 6. Matrices can be configured with instruments and UUT test points on the columns, reserving the rows for signal routing. 


Putting both the instruments and the UUTs on the columns of a matrix allows for further expansion of both by adding only one more module and connecting the rows. It is limited in that expansion occurs only by adding columns.

For available NI switch modules that utilize the Matrix Topology, please visit NI PXI Switch Product Selection Guide.

Other Switch Considerations

x-Wire Switching

NI switch modules are capable of switching 1, 2, and/or 4-wires. In 1-wire mode, you connect the positive leads to the relays and the negative leads to a common connection. All signals are referenced to this common connection. 

Figure 7.  Single-Ended Multiplexer

Sometimes more than one signal needs to be switched at the same time. In this situation a switch that employs 2-wire or 4-wire mode can be used. In 2-wire mode, you connect both positive and negative leads to the terminals of a channel. An advantage of 2-wire switching is great common-mode noise rejection. Some applications in which 2-wire is typically used are differential measurements, low voltage, high current, and resistance measurements in the 100 – 10 M Ohm range. 4-wire mode is usually used for 4-wire resistance measurements. Two leads are used for the current excitation and another two leads are used for measuring the voltage drop across the resistor.

Figure 8. 2-wire (differential) and 4-wire Multiplexers

Relay Driver
NI relay drivers are the ideal choice when the current and voltage requirements for relays exceed those found in existing relay modules (or for relays embedded in a test system). Like NI switches, relay drive modules are controlled with NI-SWITCH driver software, so engineers can program external relays connected to the relay driver and standard PXI and SCXI switch modules identically. For added safety against flyback voltages, a flyback diode has been added across the relay. The SCXI-1167 has a 5 V source and the PXI-2567 has a 5 V and a 12 V source available to drive relays. The 5 V source on the SCXI-1167 can provide up to 0.75 A of current. The 5 V source on the PXI-2567 can provide up to 1.25 A of current and the 12 V source can provide 0.50 A of current.

Figure 9. PXI-2567 External Relay Driver 


RF Modules
The NI RF (radio frequency) switch modules are ideal for expanding the channel count or increasing the flexibility of systems with signal bandwidths greater than 10 MHz. RF is not a topology but RF switches can be any topology. High-density multiplexers, dimensionally flexible sparse-matrices, and general-purpose relays are among the available configurations in PXI and SCXI switch modules. Each of these modules has been optimized for minimal insertion loss, reflection, cross-talk, and maximum isolation between channels. For more information on these parameters see the links below for the Complex RF Switching Architectures – Part I and Part II.


For available NI switch modules designed for RF switching, please visit NI PXI Switch Product Selection Guide. For the design and operational concepts of RF switches, please visit Complex RF Switching Architecture -- Part I and Complex RF Switching Architecture -- Part II.

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