Understanding Switch Topologies

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 switch topologies from National Instruments, visit the Switch Product Selection Guide.

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 or 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.



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).



A National Instruments 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.

G.P. Topology	Max Voltage	Max Current (Switching/Carry)	Module
100 Channel SPST	100 V CAT I	1 A / 1 A	NI PXI-2569
100 Channel SPST	100 V CAT I	1 A / 1 A	NI SCXI-1169
32 Channel SPDT	150 VDC,125 VAC CAT I	2A / 5A	NI SCXI-1166
16 Channel SPDT	250 V CAT II	2A / 5A	NI SCXI 1160
16 Channel SPDT	150 VDC, 125 VAC CAT I	2A / 5A	NI PXI-2566
16 Channel SPST	125 VDC, 250 VAC CAT II	7A / 7A	NI PXI-2565
8 Channel SPDT	250 V CAT II	8A / 8A	NI SCXI-1161

Multiplexer Topology

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.

Multiplexer Topology	Max Voltage	Max Current (Switching/Carry)	Module
256x1 1-Wire, 128x1 2-Wire	60 VDC, 30 VAC	400 mA / 400 mA	NI SCXI-1130
128x1 1-Wire	60 VDC, 30 VAC	400 mA / 400 mA	NI PXI-2530
64 x1 4-Wire	60 VDC, 30 VAC	400 mA / 400 mA	NI SCXI-1130
64x1 2-Wire	60 VDC, 30 VAC	400 mA / 400 mA	NI PXI-2530
64x1 1-Wire	250 V CAT II	30 mA / 30 mA	NI SCXI-1128
64x1 1-Wire	250 V CAT II	1 A / 2 A	NI SCXI-1127
48x1 1-Wire	10 VDC, 7 VAC	3 mA / 3 mA	NI PXI-2501
48x1 1-Wire	60 VDC, 30 VAC	1 A / 1 A	NI PXI 2503
32x1 4-Wire	60 VDC, 30 VAC	400 mA / 400 mA	NI PXI-2530
32x1 2-Wire	250 V CAT II	30 mA / 30 mA	NI SCXI-1128
32x1 2-Wire	250 V CAT II	1 A / 2 A	NI SCXI-1127
24x1 2-Wire	10 VDC, 7 VAC	3mA / 3mA	NI PXI-2501
24x1 2-Wire	60 VDC, 30 VAC	1 A / 1 A	NI PXI-2503
Sixteen 16x1 1-Wire	60 VDC, 30 VAC	400 mA / 400 mA	NI SCXI-1130
Octal 16x1 1-Wire	60 VDC, 30 VAC	400 mA / 400 mA	NI PXI-2530
16x1 4-Wire	250 V CAT II	30 mA / 30 mA	NI SCXI-1128
16x1 4-Wire	250 V CAT II	1 A / 2 A	NI SCXI-1127
12x1 4-Wire	10 VDC, 7 VAC	3 mA / 3 mA	NI PXI-2501
12x1 4-Wire	60 VDC, 30 VAC	1 A / 1 A	NI PXI-2503
Octal 4x1 1-Wire	240 V CAT II	200 mA / 200 mA	NI SCXI-1163R

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.



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



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.


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.

Matrix Topology	Max Voltage	Max Current (Switching/Carry)	Module
16x16 2-Wire, Dual 8x16 2-Wire, 8x32 2-Wire	150 V CAT I	1 A / 2 A	NI SCXI-1129
8x32 1-Wire	60 VDC, 30 VAC	400 mA / 400 mA	NI SCXI-1130
8x16 2-Wire	150 V CAT I	1 A / 2 A	NI PXI-2529
8x16 1-Wire	60 VDC, 30 VAC	400 mA / 400 mA	NI PXI-2530
Quad 4x16 2-Wire, Dual 4x32 2-Wire, 4x64 2-Wire	150 V CAT I	1 A / 2 A	NI SCXI-1129
4x64 1-Wire, 4x32 2-Wire	60 VDC, 30 VAC	400 mA / 400 mA	NI SCXI-1130
4x32 2-Wire	150 V CAT I	1 A / 2 A	NI PXI-2529
4x32 1-Wire, 4x16 2-Wire	60 VDC, 30 VAC	400 mA / 400 mA	NI PXI-2530
4x8 2-Wire	250 V CAT II	1 A / 2 A	NI SCXI-1127
4x8 2-Wire	250 V CAT II	30 mA / 30 mA	NI SCXI-1128
4x6 2-Wire	10 VDC, 7 VAC	3mA / 3 mA	NI PXI-2501
4x6 2-Wire	60 VDC, 30 VAC	1 A / 1 A	NI PXI-2503

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. All NI multiplexers, other than the NI SCXI-1163R, can operate in 1-wire mode.

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.



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Relay Driver

National Instruments 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 proved 0.50 A of current.


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Module	Max Voltage	Max Current	Channels
PXI-2567	50 VDC	600 mA	64 Non-latching
SCXI-1167	50 VDC	600 mA	64 Non-latching

RF Modules

The National Instruments 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.

Topology	Max Voltage	Max Current (Switching/Carry)	Bandwidth	Module
36-Terminal Sparse Matrix 50/75 Ω Coaxial	150 V CAT I	500 mA / 1A	500 MHz	NI SCXI-1193
32x1 50/75 Ω Coaxial	150 V CAT I	500 mA / 1 A	500 MHz	NI SCXI-1193
18-Terminal Sparse Matrix 50/75 Ω Coaxial	150 V CAT I	500 mA / 1A	500 MHz	NI PXI-2593
Dual 16x1 50/75 Ω Coaxial	150 V CAT I	500 mA / 1 A	500 MHz	NI SCXI-1193
16x1 Terminated 50/75 Coaxial	150 V CAT I	500 mA / 1 A	500 MHz	NI SCXI-1193
16x1 50/75 Ω Coaxial	150 V CAT I	500 mA / 1A	500 MHz	NI PXI-2593
8 Channel SPDT 50 Ω Coaxial	30 V	NA** / 2 A	18 GHz	NI SCXI-1192
Quad 8x1 50/75 Ω Coaxial	150 V CAT I	500 mA / 1 A	500 MHz	NI SCXI-1193
Dual 8x1 Terminated 50/75 Ω Coaxial	150 V CAT I	500 mA / 1 A	500 MHz	NI SCXI-1193
Dual 8x1 50/75 Ω Coaxial	150 V CAT I	500 mA / 1 A	500 MHz	NI PXI-2593
8x1 Terminated 50/75 Ω Coaxial	150 V CAT I	500 mA / 1A	500 MHz	NI PXI-2593
8x1 50 Ω Coaxial	30 V CAT I	500 mA / 500 mA	2.7 GHz	NI PXI-2547
8x1 75 Ω Coaxial	30 V CAT I	500 mA / 500 mA	2.5 GHz	NI PXI-2557
Dual 6x1 50 Ω Coaxial	90 Vrms CAT I	1.73 Arms / 1.73 Arms	26.5 GHz	NI PXI-2596
6x1 50 Ω Terminated Coaxial	90 Vrms CAT I	1.73 Arms / 1.73 Arms	26.5 GHz	NI PXI-2597
Quad 4x1 Terminated 50/75 Ω Coaxial	150 V CAT I	500 mA / 1 A	500 MHz	NI SCXI-1193
Quad 4x1 50 Ω Coaxial	24 V	1 A / 1 A	1.3 GHz	NI SCXI-1190
Quad 4x1 50 Ω Coaxial	30 V CAT I	500 mA / 500 mA	2.5 GHz	NI PXI-2594
Quad 4x1 50 Ω Coaxial	30 V CAT I	500 mA / 500 mA	2.5 GHz	NI SCXI-1194
Dual 4x1 Terminated 50/75 Ω Coaxial	150 V CAT I	500 mA / 1 A	500 MHz	NI PXI-2593
Dual 4x1 75 Ω Coaxial	30 V CAT I	500 mA / 500 mA	2.5 GHz	NI PXI-2556
Dual 4x1 50 Ω Coaxial	30 V CAT I	500 mA / 500 mA	2.7 GHz	NI PXI-2546
4x1 50 Ω Coaxial	24 V	1 A / 1 A	1.3 GHz	NI PXI-2590
4x1 75 Ω Coaxial	30 V CAT I	500 mA / 500 mA	2.5 GHz	NI PXI-2554
4x1 50 Ω Coaxial	30 V CAT I	500 mA / 500 mA	2.7 GHz	NI PXI-2545
4x1 50 Ω Coaxial	30 V CAT I	500 mA / 500 mA	5 GHz	NI PXI-2595
4x1 50 Ω Coaxial	30 V CAT I	500 mA / 500 mA	5 GHz	NI SCXI-1195
4x1 75 Ω Terminated Coaxial	30 V CAT I	500 mA / 500 mA	2.5 GHz	NI PXI-2555
Nine 3x1 50/75 Ω Coaxial (Idependant Topology)	150 V CAT I	500 mA / 1 A	500 MHz	NI SCXI-1193
Quad 3x1 50/75 Ω Coaxial (Idependant Topology)	150 V CAT I	500 mA / 1 A	500 MHz	NI PXI-2593
Quad SPDT 50 Ω Coaxial	30 V CAT I	500 mA / 500 mA	2.7 GHz	NI PXI-2548
Quad SPDT 75 Ω Coaxial	30 V CAT I	500 mA / 500 mA	2.5 GHz	NI PXI-2558
Dual SPDT 50 Ω Terminated Coaxial	30 V CAT I	500 mA / 500 mA	2.7 GHz	NI PXI-2549
Dual SPDT 75 Ω Terminated Coaxial	30 V CAT I	500 mA / 500 mA	2.5 GHz	NI PXI-2559
Dual SPDT 50 Ω Coaxial	90 Vrms CAT I	1.73 Arms / 1.73 Arms	26.5 GHz	NI PXI-2599
Dual DPST 50 Ω Coaxial	65 Vrms CAT I	1.25 Arms / 1.25 Arms	26.5 GHz	NI PXI-2598

Related Links:
Switch Product Selection Guide
How to Choose the Right Relay
Selecting Switch Bandwidth

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