Interfacing to the NI Digital Waveform Generator/Analyzer using the VHDCI Connector

Several single-ended NI digital waveform generator/analyzer devices (including the NI 6541/6542 (NI 654X) and NI 6551/6552 (NI 655X) devices) use the industry-standard VHDCI (Very High Density Connector Interface) connector to provide high channel count and high signal quality. The VHDCI connector is commonly used for SCSI 4 applications, but single ended NI Digital Waveform Generator/Analyzer devices also use this connector with a custom pinout and custom cable to maximize signal integrity.

This document describes the ways you can make connections to single-ended NI digital waveform generator/analyzers using the VHDCI connector and provides more detail on creating a custom printed circuit board (PCB) to interface to your NI device.

Connecting to Single-Ended NI Digital Waveform Generator/Analyzers

National Instruments offers multiple options for connecting the single-ended NI digital waveform generator/analyzer to your device under test (DUT). Each option is designed for controlled-impedance 50 Ohm traces and matched trace lengths.

SHC68-C68-D4 Cable

NI recommends that you use the NI SHC68-C68-D4 shielded cable for connections to your single-ended NI digital waveform generator/analyzer. This cable is designed specifically for the custom pinout of the VHDCI connector on these devices, and it provides 50 Ohm coaxial transmission lines for each signal.

Figure 1: SHC68-C68-D4 Cable

NI CB- 2162

The NI CB-2162 is a connector block and prototyping board for single-ended NI digital waveform generator/analyzer devices. The NI CB-2162 provides an easy way to do the following tasks:
• Terminate digital I/O (DIO) and control channels
• Connect with other devices for testing and debugging
• Develop and interface to prototype circuits
• Probe DIO and control channels

In Figure 2, the single-ended NI digital waveform generator/analyzer is connected to the NI CB-2162 using the NI SHC68-C68-D2 cable. In this figure, the DUT was placed directly on the prototyping area of the CB-2162 and wired to the header pins. Additionally, the header pins can be used to cable to an external DUT interface board.

NI SMB-2163

The NI SMB-2163 is a breakout box for single-ended NI digital waveform generator/analyzer devices. The NI SMB-2163 provides an easy way to connect to other devices for testing and debugging by routing every DIO and control channel on a separate SMB coaxial connector.

Figure 3: Connecting an NI SMB-2163 to a Digital Waveform Generator/Analyzer

In Figure 3, the single-ended NI digital waveform generator/analyzer is connected to the NI SMB-2163 through the SHC68-C68-D2 cable. The connection between the NI SMB-2163 and the DUT is made using coaxial SMB cables (as shown).

Custom PCB

Creating a custom printed circuit board (PCB) is the last option for connecting to the single-ended NI digital waveform generator/analyzer to the DUT. This option allows you to customize the interface to effectively meet the requirements of your DUT.

Figure 4: Connecting a Custom PCB to a Digital Waveform Generator/Analyzer

In Figure 4, the single-ended NI digital waveform generator/analyzer is connected to the VHDCI connector on the custom PCB using the NI SHC68-C68-D2 cable. The connection between the VHDCI connector and the DUT is routed on the custom PCB.

Using a custom PCB will be described in more detail in the following section, Creating a Custom PCB.

Creating a custom PCB

There are several factors to consider when you choose to build a custom PCB.

Connector Pinout

Figures 5 and 6 show the connector pinout for the NI 654X and NI 655X.

Figure 5: NI 654X Pinout

Figure 6: NI 655X Pinout

Choosing a Connector

There are four common styles of VHDCI connectors. Table 1 shows pictures of the VHDCI connectors that are available from National Instruments. Click on the connector to view the model page and pricing information.

Right Angle VHDCI Connector	Right Angle Dual Stack VHDCI Connector	Vertical Mount VHDCI COnnector

Layout Considerations

The following sections describe steps that should be followed when creating a custom PCB to interface with a single-ended NI digital waveform generator/analyzer.

Single-ended NI digital waveform generator/analyzer devices produce high-quality signals with fast edge rates and thus require special considerations when routing signals to maintain signal integrity. In addition to following standard design guidelines, be sure to use the following guidelines in your design:
· Route all traces with 50 Ohm impedance; this maintains the matched 50 Ohm system.
· Route ground pins directly into a solid ground plane.
· Reference all routed signals to at least one solid ground or power plane.
· Route all traces with sufficient trace spacing to maintain low crosstalk (Refer to Figure 7).
· Match all trace delays to maintain low skew (Refer to Figure 7).
· Trace delay depends on physical length and dielectric constant of the PCB. Therefore, surface traces will need different physical lengths than internal stripline traces.
· Maintain all ground return paths.

The ground pin connections to the ground plane are critical for keeping crosstalk low. Crosstalk is induced by the magnetic field that is produced when a signal is traveling along a trace. When two signals share a ground return path, the magnetic field from one signal is induced on the second signal, causing noise.

Figure 7: Trace Guidelines

Figure 8: Signal and Ground Routing

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Reader Comments | Submit a comment »

Ditto above comment. The PCB footprint is hard to find and not at all obvious.
- Jun 23, 2010


The connectors are made by AMP. There is a KB on this. Just search for 'mating connectors'
- May 26, 2010


Where's the footprint/layout for these connectors. that would be nice to have you your website. At least put a manufacturer part number on the page and please make it somewhere obvious
- Aug 12, 2009