Data Acquisition Case Study

Introduction

This white-paper compares solutions from data-acquisition vendors (Real Time Devices and National Instruments) with a similar solution made using the SIMM-Sys paradigm and components. All three solutions have similar features and performance. The purpose of this document is to show that SIMM-Sys is a viable alternative for PC-based data-acquisition solutions with lower price and many benefits over traditional add-on card-based solutions.

The cards selected for this comparition are the Real Time Devices' DM5401 PC104 card and the National Instruments' PCI-6024E PCI add-on card.

Disclaimer

All information used in this comparation is based on documentation available on the World-Wide-Web. RCS Ltd. put a lot of effort in ensuring the correctness of the information presented here however cannot guarantie that.

Feature and price comparation

SIMM-Sys price estimation

Benefits of SIMM-Sys solution

• 2 ch. simultaneous sampling
• 8 ch. digital output, double sample rate
• 24 bit I/O with industry-standard programming
• Analog signal travels short, digital signal travels long
• UTP ethernet can cover >100m distances
• No slots needed in PC
• Standard communication stack can be used
• USB gives predictable performance
• Low-power (3.3V) operation
• Significantly lower price
• Intelligent device with around 300MIPS processing power!

Sacrifices of SIMM-Sys solution

• No programmable gain
• If programming the CPU card, no familiar environment available
• 10 bit DAC resolution
• External casing required
• External power required
• Real-time, µs delay to PC impossible
• No sustained full-speed operation available

Features and benefits

Feature	Benefit
Close-coupled architecture	No need for expansive HW buffering/DMA/bus-mastering
Modular solution	Pay for what you need
	Increased volume in common parts
SW as a value-added solution	Major portion of the SW development cost is not included in the price
Single-chip solutions wherever possible	Lowers design, manufacturing, material costs
	Lowers power consumption
	Shrinks module size
Use of standards (SNMP, HTTP, STDC++ etc.)	Well-documented, compatible, familiar environment
Local processing power	Intelligent triggers
	High-level communication protocols
	Filtering
	Noise-sharping (dynamic resolution enhancement at lower acquisition speed)
	FFT output
	Virtualy independent acquisition

Pitfalls

No programmable gain

In real-world applications programmable gain (i.e. different gains on the same channel for different samples) is not required. what is required is different gains on differnet channels. That can be acheaved with external resistor-deviders or amplifiers as application requires.

If programming the CPU card, no familiar environment available

For this DSP processor a C compiler is available from Analog Devices. Various libraries are available from both Analog Devices and RCS Ltd. With these tools C programming and use of most of the standard C run-time library can be used. However for efficient use of the DSP-specific features of the processor assembly programming is recommended. RCS is planning to provide a data-acquisition libraries programmable through the RCS propery-interface from the host computer. With that interface the user can create highly-efficient customized data-acquisition applications without directly programming the SIMM-Sys processor board.

External casing required

External casing lowers noise transmitted through EMI. PC is not a friendly environment for sensitive analog signals.

External power required

Power supply independent from the host PC can increase signal-to-noise preformance by de-coupling noise transmitted on power lines.

Real-time, µs delay to PC impossible

Applications' real-time processing procedures should be placed on the SIMM-Sys CPU board.

No sustained full-speed operation available

Full-speed operation means 200ksps acqusution with both ADCs and 200ksps conversion on all eight DACs. That requires 2 x 200000 x 12 + 8 x 200000 x 10 = 21.8 Mbps transfer rate. That's well over the 12Mbps available over USB and higher even the 20Mbps of the full-douplex operation of the 10Mbps ethernet too. Thus no sustained operation at that speed is available. However this rate is much higher than the concurrent cards. If we count the maximum data-rate required by the (faster) RTD card we get 1 x 200000 x 12 + 2 x 200000 x 10 = 6.4 Mbps which is well within the limitation of the network layer. A second argument is that such a burst-performance can be achieved up-to the limitations of the available memory of the controller. The RTD card has the same limitation bound to the slow 8-bit ISA-bus. The designers of that card provide high burst-performance with the use of a 1k FIFO buffer. The same applies to the SIMM-Sys solution. Also it's not very common that such high-speed sustained performance is required. In most cases the analog signals must be monitored at a given rate and when a pre-defined event occures (trigger-condition) a relatively small array of samples is acquired and processed by the host PC. When sustained acquisition of high-speed data is required it's often because of the lack of proper trigger on the acquisition HW. This is not the case in the SIMM-Sys solution as the local intelligent controller can be programmed for sophisticated trigger-conditions. Conclusion: this functionality is either not available on competitive solutions or not likely to be required by the application.

Conclusions

The SIMM-Sys approach for data-acquisiton problems can lead to an easy-to-use, high-preformance, competitive and reasonably priced solution for low- to middle-volume products.