USB Digital I/O Product Selection Guide
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Most test, measurement, and automation systems require some form of digital input and output, whether they involve generating test patterns to validate a custom circuit or controlling industrial actuators on a production line. Thus, selecting a digital device with the proper I/O characteristics and form factor is crucial for completing your system. USB is one of the fastest-growing bus technologies in the computer industry with an estimated 2 billion ports around the world (usb.org). National Instruments offers a range of USB digital I/O products to meet the digital needs of your portable applications.
Table of Contents
Selection Chart
There are three main categories of USB digital I/O products available from National Instruments:
- Static digital I/O: software-timed, 5V/TTL devices for simple digital applications
- Industrial digital I/O: high-voltage devices offering CAT I or CAT II isolation
- M Series: multifunction devices with some hardware-timed 5V/TTL digital lines
Use the following chart and guide to learn more about these digital I/O products and match your needs with the appropriate device. Click on the hyperlinks below for more specific information about each digital I/O module.
|
Device |
Isolation |
Voltage Range |
Logic Levels |
Sinking / Sourcing |
Max Current Drive |
# Channels |
Timing |
|
Ch-Ch |
60 V |
5 V1 |
Both |
500 mA |
8 DI, 8 DO |
Software-Timed |
|
|
— |
2 A |
4 DO |
|||||
|
Bank |
30 V |
24 V |
Sinking |
— |
8 DI |
||
|
Sourcing |
750 mA |
8 DO |
|||||
|
— |
5 V |
TTL |
Both |
8.5 mA |
24 DIO |
||
|
6509 NEW! |
24 mA |
96 DIO |
|||||
|
8 DIO2 |
1 MHz max |
||||||
|
32 DIO3 |
|||||||
1 The 6525 uses has a logic low threshold of 1 VDC and a logic high threshold of 3.2 VDC
2 The 6221 and 6251 each have 24 DIO lines, only 8 of which are hardware-timed
3 The 6229 and 6259 each have 48 DIO lines, only 32 of which are hardware-timed
Why is Isolation Important?
Isolation physically and electrically separates two parts of a circuit, while allowing for the two parts to interact. Isolation provides several key benefits:
- Isolation breaks ground loops
- Isolation improves common-mode voltage and noise rejection
- Isolation permits the two parts of the circuit to be at different voltage levels, which means one can be safe while the other side is at hazardous voltage levels
Many industrial applications require isolation to protect the electronics from transient voltage spikes and provide greater common mode noise rejection in electrically noisy environments containing machinery and inductive loads. There are three specifications key to isolated digital I/O devices: isolation topology, installation category, and voltage ratings.
Refer to the table below for details on the isolation capabilities of the industrial USB digital I/O devices.
|
Device |
Connector |
Isolation Topology |
Installation Category |
Input/Output Range |
Rated Voltage |
Withstand Voltage (5 s) |
|
Screw |
Ch-Ch |
Cat I |
60 VDC |
60 VDC |
60 VDC |
|
|
Screw |
Ch-Ch |
Cat II |
60 VDC |
250 Vrms |
2,300 Vrms1 |
|
|
Screw |
Bank |
Cat II |
30 VDC |
250 Vrms |
2,300 Vrms |
|
|
DSUB |
Bank |
Cat I |
30 VDC |
60 VDC |
1,000 Vrms |
|
|
Screw |
Bank |
Cat II |
30 VDC |
250 Vrms |
2,300 Vrms |
|
|
DSUB |
Bank |
Cat I |
30 VDC |
60 VDC |
1,000 Vrms |
1Channel-to-earth ground withstand voltage
Isolation Topologies
Generally speaking, there are three isolation topologies: channel-to-channel, bank-to-bank, and channel-to-earth:
-
Channel-to-channel: The most robust isolation topology; each channel is individually isolated from one another and from other non-isolated system components.
-
Bank-to-bank: Each bank of channels (for example, a port of 8 digital lines) shares the same ground but is isolated from other banks and other non-isolated system components.
-
Channel-to-earth: Each measurement circuit in its entirety has its own ground reference, separate from other non-isolated parts of the system referenced to earth ground.
Installation Categories
The IEC (International Electrotechnical Commission) defined the term Installation Category (sometimes referred to as Overvoltage Category) to address transient voltages. When working with transient voltages, there is a level of damping that applies to each category. This damping reduces the transient voltages (over-voltages) that are present in the system. As you move closer to power outlets and away from high voltage transmission lines, the amount of damping in the system increases. The IEC has created four categories to partition circuits with different levels of over-voltage transient conditions.
-
Installation Category IV: Distribution Level (transmission lines)
- Installation Category III: Fixed Installation (fuse panels)
- Installation Category II: Equipment consuming energy from a Category III fixed installation system. (wall outlets)
-
Installation Category I: Equipment for connection to circuits where transient overvoltages are limited to a sufficiently low level by design.
Voltage Ratings
Most USB digital I/O devices specify three voltage ranges, corresponding to the installation category and intended use of the device:
-
Input/output range: The intended voltage range for signals connected to each of the digital lines. Many times, this corresponds with the digital logic levels of the device. For example, TTL devices have an input/output range of 5 V, whereas 24 V logic devices typically have an input/output range of ±30 V.
-
Rated voltage: The maximum voltage a circuit can measure or reject continuously without causing damage to the device (also called “continuous” or “working” voltage).
-
Withstand voltage: The maximum transient voltage spike the device can withstand for five seconds (also called “transient” or “overvoltage” protection).
Sinking vs. Sourcing Current
“Sinking” and “sourcing” are terms used to define the control of direct current flow in a load. The concept of sourcing and sinking is independent of the component (transistor, mechanical relay) that implements the operation.
Sinking Device: A sinking device provides a path for the current to ground and is not responsible for powering the device. Terms used to describe sinking devices include NPN, Open Collector, Normally High, and IEC Negative Logic.
Figure 1. Sinking Current
Sourcing Device: A sourcing device provides the power or a positive potential. Sourcing devices 'push' the current through the load. Terms used to describe sourcing devices include PNP, Open Emitter, Normally Low, and IEC Positive Logic.
Figure 2. Sourcing Current
Where to Find More Information
You can find detailed information on each of the USB digital I/O devices by clicking on their respective hyperlinks in the tables above. Alternatively, refer to the following resources for additional material:
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