NI EPM high-performance DAQ cable

NI SHC68-68-EPM Enhanced Performance Cable: The cornerstone of signal integrity for data acquisition systems

In modern data acquisition (DAQ) systems, cables are often overlooked but crucial links. A poorly designed cable may introduce crosstalk, electromagnetic interference, and grounding loops, thereby drowning valuable measurement signals. National Instruments has developed a series of specialized shielded cables for its high-performance DAQ devices, among which SHC68-68-EPM (Extended Performance Meter) has become the preferred solution for connecting M-series, DAQCard, and some S-series DAQ devices to 68 pin accessories due to its excellent noise suppression capability and signal integrity design. This article will provide a comprehensive and in-depth technical analysis of SHC68-68-EPM from the aspects of cable design concept, internal structure, technical specifications, compatibility of accessories, and selection suggestions.

Overview of NI Cable Design Advantages

National Instruments provides a variety of high-performance shielded cables for different series of DAQ devices, each designed specifically to enhance signal integrity. SHC68-68-EPM, as one of the enhanced performance models, integrates the following core technologies:

Single shielded twisted pair for analog input: Each pair of analog input signal lines is made of 30 AWG conductor and wrapped with an independent aluminum foil shielding layer. It is also equipped with drain wire for grounding connection, minimizing inter channel crosstalk and external electric field interference.

Analog output independent twisted pair: Each analog output channel also uses an independent twisted pair to avoid coupling the output signal to the input end.

The digital cables are individually wrapped and twisted with the ground wire: The digital signal cables are individually wrapped and shielded, and twisted with the ground wire to prevent high-speed digital switch noise from coupling to the analog part.

Partition bundling of analog and digital parts: The analog signal line and digital signal line are separated and bundled inside the cable to form two independent areas, further reducing the interference of digital noise on the analog circuit.

+The 5V power cord uses a 26 AWG large wire diameter conductor: providing sufficient power transmission capacity for DAQ devices and reducing voltage drop.

Integrated tin clad copper braided shielding: On top of the internal shielding layers, an additional layer of high coverage tin clad copper braided mesh is added to provide overall electromagnetic shielding and effectively suppress common mode interference.

XPbT outer sheath: made of lead-free thermoplastic elastomer material, wear-resistant, oil resistant, flame retardant, suitable for industrial environments.

These designs collectively contribute to the outstanding performance of SHC68-68-EPM in high-frequency and high-precision measurement applications.

Overview of SHC68-68-EPM Cable

SHC68-68-EPM is a high-performance shielded 68 core cable, with two end connectors:

One end: VHDCI (Very High Density Cable Interconnect) 68 pin male connector, used to connect the 68 pin I/O interface of M-series, DAQCard, or specific S-series DAQ devices.

On the other end: a 0.050-inch D-type 68 pin female connector, used to connect standard 68 pin accessories such as SCB-68 shielded junction box, SCC-68 signal conditioning carrier board, BNC-2110 BNC junction box, etc.

Available length (listed in the document):

0.5 meters (part number 192061-01R5)

1 meter (192061-01)

2 meters (192061-02)

5 meters (192061-05)

10 meters (192061-10)

Users can choose the appropriate length based on the system layout to balance signal attenuation and wiring flexibility.

Detailed explanation of internal structural technology

The internal structure of SHC68-68-EPM (see Figure 1 in the document) reflects a precise noise control strategy. Analyze layer by layer as follows:

1. Simulate input line pairs

Each analog input channel uses a pair of 30 AWG twisted wires.

Each pair of wires is covered with an independent aluminum foil shielding layer and equipped with a drain wire, making it easy for users to connect the shielding layer to the ground of the equipment or chassis.

This "individually shielded twisted pair" structure can effectively eliminate capacitive coupling and electric field crosstalk between channels, especially suitable for the transmission of low-level signals such as thermocouples and strain gauges.

2. Simulate triggering and external grounding reference line

Similarly, 30 AWG individually shielded twisted pair cables are used to ensure the purity of the triggering signal and avoid false triggering.

3. Simulate output line pairs

Each analog output channel uses independent twisted pair cables and also has separate shielding. This prevents high current transitions in the output driver stage from affecting adjacent input channels.

4. Digital signal line

Each digital signal line is twisted with a ground wire and wrapped as a whole for shielding. Digital signals usually have steep edges and radiate a large amount of high-frequency energy. By individually wrapping and twisting, the radiated noise can be limited to the interior of the line pair.

5. Analog and digital partitioning

The analog and digital lines are separated into two independent bundles by physical separation inside the cable, further preventing digital noise from entering the analog path through spatial coupling.

6.+5V power cord

Using 26 AWG thick conductors, much thicker than the 30 AWG of signal wires. This reduces the DC resistance during long-distance transmission, ensuring stable+5V voltage supply for DAQ devices, especially when supplying power to external sensors.

7. Overall shielding layer

In addition to all the internal structures mentioned above, cover with a layer of tin coated copper braided shielding mesh. This woven layer has high optical coverage (usually>85%) and provides good low-frequency magnetic field shielding effect.

8. Outer sheath

XPbT material (cross-linked polyolefin) has excellent mechanical strength and chemical resistance, and meets RoHS environmental requirements.

Comparison with other 68 pin NI cables

NI offers a variety of 68 pin cables to meet different cost and performance requirements. Understanding their differences can help in making the correct selection.

Cable model, shielding structure, typical use, applicable equipment

SHC68-68-EPM enhanced performance: analog/digital partition, individually shielded twisted pair for analog input, overall braided shielding, high-precision, low-noise measurement for M-series DAQCard、 Partial S series

SHC68-68 basic shielding: twisted pair cable, but without analog/digital partition, no separate shielding, cost sensitive, and low noise requirements M-series (connector 1 optional)

RC68-68 flat ribbon cable, unshielded, lowest cost prototype development, OEM integrated M series, E series

SH68-68-EP shielded, but without VHDCI connector, both ends are D-type 68 pin female E series, S series, B series devices with 68 pin quality end connected to USB M series, etc

SHC68-68-RMIO/- RDIO is similar to EPM and is designed specifically for the MIO/DIO connector of the R series intelligent DAQ. The R series FPGA devices NI 783xR, 781xR

SHC68U-68-EP has a reverse VHDCI male connector on one end, used for adjacent slots of DAQCard compact PCMCIA dual card configuration E-series DAQCard

Key difference: SHC68-68-EPM is currently one of the models with the strongest noise suppression capability among NI 68 pin cables, making it particularly suitable for use in multi-channel, high-resolution measurement systems. If there are high-power digital signals or switching power supplies in the system, EPM cables can significantly improve measurement accuracy.

Compatibility of accessories and application scenarios

The other end (68 pin female head) of SHC68-68-EPM can be connected to various NI and third-party accessories to achieve signal conversion, conditioning, or expansion.

1. SCB-68 shielded junction box

Provide 68 screw terminals that can be directly connected to on-site signals.

Built in cold junction compensation temperature sensor, suitable for thermocouple measurement.

Large area grounding plane and shielding shell, combined with EPM cable to form a complete shielding link.

2. SCC-68 portable signal conditioning carrier board

Up to 4 SCC signal conditioning modules (such as isolation, amplification, filtering, etc.) can be inserted.

Small footprint, suitable for desktop or portable systems.

3. BNC-2110 / BNC-2111 / BNC-2120 / BNC-2090

Transfer the DAQ signal to a BNC coaxial connector for easy connection to standard test lines.

Suitable for direct connection of high-frequency or high-voltage signals.

4. TBX-68 DIN rail junction box

Industrial grade installation method, suitable for wiring inside cabinets.

5. CB-68LP/CB-68LPR low-cost junction box

Economical solution, suitable for OEM or teaching experiments.

Special configuration:

When a 50 pin accessory needs to be connected, the 68M-50F-MIO adapter (mentioned in the document) can be used to convert the SHC68-68-EPM to a 50 pin interface.

For USB M-series quality termination modules (such as USB-6251), SH68-68-EP cable should be used instead of SHC68-68-EPM, as the latter has VHDCI connectors that do not match.

Cable selection rules for multi connector devices

M-series DAQ devices (such as NI 6225, 6255) typically have two 68 pin connectors (Connector 0 and Connector 1). The document provides clear recommendations:

Connector 0 (primary analog I/O): Use SHC68-68-EPM for optimal analog performance.

Connector 1 (auxiliary digital/timed I/O): Use SHC68-68 (low-cost shielded cable) because the signal on this connector is less sensitive to noise.

For DAQCard (PCMCIA card), when two cards are inserted adjacent to each other into the slot of a laptop, due to space limitations, the standard VHDCI connector may not be able to be inserted side by side. At this point, the SHC68U-68-EP cable should be used, with its connector direction reversed to enable and arrange the wires.

Overview of other related cables and accessories

Although this article focuses on SHC68-68-EPM, NI provides a complete cable accessory ecosystem covering various pin numbers, briefly introduced as follows:

100 needle device

SH100100: 100 needles to 100 needles, used for E-series 100 needle equipment.

SH1006868:100 pin to two 68 pin accessories can be connected simultaneously.

SCB-100: High density shielded junction box.

50 needle device

SH6850:68 pin to 50 pin conversion cable.

CB-50LP: Low cost 50 pin junction box.

37 needle device

SH37F-37M: 37 pin D-Sub shielded cable.

CB-37F-HVD: High voltage isolation junction box (150 V CAT II).

Signal conditioning platform

SCC: Portable modular signal conditioning with up to 4 modules.

SCXI: High performance modular signal conditioning, expandable to thousands of channels.

CRIO-9151: R series expansion chassis, capable of connecting C series I/O modules.

Selection suggestions in practical applications

1. High precision analog measurement (such as thermocouples, strain gauges, pressure sensors)

Preferred SHC68-68-EPM+SCB-68 or SCC-68.

Ensure that the cable length is as short as possible (1 meter or 2 meters) to reduce noise pickup.

Use the grounding jumper inside SCB-68 to correctly configure the signal reference (differential/RSE/NRSE).

2. Mixed signal system (including both analog and high-speed digital I/O)

The analog part uses EPM cables, while the digital part can use SHC68-68 or RC68-68 to save costs.

Keep analog and digital cables routed separately to avoid parallel routing.

3. Portable or battery powered system

Use DAQCard device+SHC68-68-EPM+SCC-68 to form a compact low-power measurement front-end.

4. Industrial site and cabinet installation

Choose TBX-68 or CB-68LPR (right angle connector) and install it with DIN rail.

Pay attention to the bending radius of the cable to avoid damaging the shielding layer.

Maintenance and troubleshooting tips

Although the SHC68-68-EPM cable is sturdy and durable, improper use may still lead to a decrease in signal quality. It is recommended to follow the following principles:

Regular inspection of connectors: VHDCI connectors have high-density pins and should be aligned during insertion and removal to avoid bending or breaking the pins.

Maintain shielding integrity: Damage to the cable sheath can reduce shielding effectiveness, and repair should be done with heat shrink tubing or electrical tape.

Avoid excessive bending: Especially at the root of the connector, excessive bending can cause internal wire breakage.

Use ferrite magnetic rings: In environments with strong radio frequency interference, ferrite magnetic rings can be clamped at both ends of the cable (some NI cables have built-in ferrite magnetic rings).