Interoperability of AdvancedTCA modules depends on their thermal compatibility. This article describes the methods used to measure AdvancedTCA shelf and board characteristics to ascertain their thermal interoperability. Automated test tools that measure the flow volumes through each slot in an AdvancedTCA shelf and a special wind tunnel created to measure the impedance of an AdvancedTCA board are also presented. These tools and test processes are derived from the Communications Platforms Trade Association’s (CP-TA) Interoperability Compliance Document (ICD) and the Test Procedure Manual (TPM).
CP-TA interoperability documents
In order to address interoperability issues, CP-TA has released its first ICD and TPM. The current release addresses the top interoperability issues of thermal, manageability, and data transport for PICMG’s AdvancedTCA specification.
The ICD defines a set of requirements to build interoperable communications platforms. The TPM defines test procedures to test compliance for those requirements. Together, these two documents will allow vendors to design and deliver interoperable open specifications-based building blocks, thus reducing integration costs and time-to-market.
Interoperability Compliance Document (ICD 1.0)
Foundation document for the CP-TA certification program
Contains the interoperability requirements for building blocks and is based on PICMG’s AdvancedTCA specification
Thermal interoperability
In an open specification-based architecture, such as AdvancedTCA, proper interoperability between modules determines the commercial success of the platform. Interoperability is required for all interfaces between modules from mechanical fit, electrical power, signal integrity, and inter-module communications protocol compliance, to thermal compatibility.
Thermal Interoperability is defined as the ability for a compliant Shelf to adequately cool a compliant Blade. To achieve this, it is necessary to match the thermal and airflow characteristics of Shelf Slots and Blades.
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An AdvancedTCA Shelf has the responsibility of providing a minimum airflow and pressure performance in each Slot. A compatible Blade is required to utilize available Shelf airflow effectively for cooling devices mounted on the blade.
To facilitate the thermal interoperability of both the Shelf and Blade it is necessary to characterize them independently using a standard test process and tool. Here the test method and test tools that would automate the process of characterizing the flow through the chassis slots as well as the impedance of the boards are discussed.
Characterization of a shelf
The airflow characteristics of a Shelf are primarily defined by two quantities: (1) the volumetric airflow it supplies to each slot under certain standard flow impedance, and (2) the airflow distribution within a slot (front to back). Using this information, it can be determined if a given blade with known impedance and power dissipation can be cooled adequately. Degree Controls, Inc. developed the Chassis Scan test tool to evaluate the airflow characteristics of a chassis.
Comparing shelves
Currently, no industry accepted standard measurement technique can compare shelves in terms of airflow volume through each slot in the shelf. This section describes an automated methodology of measuring the volumetric airflow delivered to each slot in an AdvancedTCA shelf. Adopting this technique allows the data to be used to benchmark each shelf.
The Chassis Scan system primarily consists of four types of modules that fit into AdvancedTCA front and rear slots. Two of these modules have airflow sensors in them, namely, the Front Flow Measurement Board (FFMB) and the Rear Flow Measurement Board (RFMB). The other two modules, Flow Impedance Boards for front and RTM slots (FFIB and RFIB, respectively), are matched impedance modules. All modules contain a mechanism to straighten the airflow at the intake and exhaust. The impedance of the FFMB and RFMB is equivalent to the FFIB and RFIB impedance.
The FFMB and RFMB have low profile airflow sensors, specifically hot wire anemometers, positioned at the midline of the board. The FFMB is built with twelve sensors, and the RFMB has three sensors as shown in Figure 1. The twelve sensors in the FFMB are distributed in four quadrants. These sensors are calibrated to measure and transmit linear velocity to a PC, where a software application converts the data to volumetric airflow. The application provides the total volumetric airflow for the slot and the volumetric airflow through each quadrant. The FFMB and RFMB are stepped through each slot to obtain the overall airflow profile across slots in the shelf.
This airflow profiling process is repeated for several operating conditions of the shelf such as:
Fans operating at 7.2 Bels acoustic noise level (NEBS limit)
Fans operating at maximum speed (failure condition)
Fans operating at minimum speed (normal operating conditions)
Each of the fans under failed condition (single fan failure, locked rotor failure)
Each fan module removed (service condition)
Figure 2 shows FFIB (top) and FFMB (bottom). Figure 3 shows RFIB (top) and RFMB (bottom). The airflow profile within the shelf is reported for each test condition mentioned before, as a graph showing flow through each quadrant in each slot (Figure 4).
This information helps the user in selecting the best slot in the shelf to locate a blade with high power and impedance characteristics. The Interoperability Compliance Document (ICD 1.0) from CP-TA categorizes shelves into classes per its airflow characteristics as tested using this method. Figure 5 shows a shelf with Chassis Scan flow measurement boards. The Chassis Scan tool used in an AdvancedTCA Shelf can be seen in Figure 6.
Characterization of blade
The airflow over a blade in a shelf primarily depends on two factors: the flow characteristics of the shelf slot and the flow impedance of the blade. The impedance is defined as the Z = P/Q2 where, P is the pressure drop across the blade and Q is the volumetric airflow over it.
Matching the airflow impedance with airflow performance of a slot is necessary to ensure its reliable operation and adequate cooling.
The impedance measurement consists of measuring airflow over the blade and pressure drop across it when placed in a shelf slot. Often, the impedance distribution over the blade is not uniform. There may be a channel for air to bypass high density (high wattage) devices altogether. The measured impedance may look low, but most of the critical devices may not experience any airflow. To address this it is important to measure airflow at multiple locations across the exhaust end of the blade.
The blade can be profiled for both gross impedance and airflow distribution, in a wind tunnel with pressure and flow sensors.
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Mimicking an AdvancedTCA shelf
The Blade Profiler is a wind tunnel specifically designed to mimic an AdvancedTCA slot of a shelf and measure the impedance presented by the blade. It has several airflow sensors that measure linear flow distributed across the channel. A sensor measures the pressure drop across the blade. Varying the fan speed generates different airflow levels by the PC (Figure 7). Figures 7 and 8 show the Blade Profiler system. The PC application determines the flow impedance by varying fan speed and measuring the airflow and pressure drop. It is then plotted as in Figure 8.
The flow distribution across the blade is measured by the array of sensors. Airflow seen by the quadrants is plotted as in Figure 9. This helps the blade designer evaluate component placement distributing to address flow blockage and heat dissipation.
A look ahead: AdvancedMC and MicroTCA
CP-TA is now addressing interoperability issues for PICMG’s AdvancedMC specification, including thermal, manageability and data transport interoperability. CP-TA members are also working closely with Degree Controls, the developer of the Chassis Scan and Blade Profiler (Figure 10), on developing the thermal test tools for AdvancedMC.
CP-TA will target interoperability issues for MicroTCA in the second half of 2007. This effort is charged with identifying the interoperability requirements within PICMG’s MicroTCA specification and creating test procedures to verify that MicroTCA building blocks are in compliance with those interoperability requirements.
CP-TA focuses on enabling interoperability between building blocks for the communications platforms. Currently CP-TA has released compliance documents for thermal, data transport, and manageability interoperability for AdvancedTCA (www.cp-ta.org).
CP-TA encourages companies to join CP-TA and to participate in defining the interoperability requirements and test procedures as well as future test tools. For more information on CP-TA test tools, including the Chassis Scan and Blade Profiler, visit www.cp-ta.org.
Rajesh Nair is the founder and CTO of Degree Controls, Inc, which specializes in sensing, engineering, and managing airflow and heat in electronics packaging with special focus on high-power, high-availability telecommunications products. He also chairs the Thermal Task Force in CP-TA, a group that develops test standards for thermal interoperability among AdvancedTCA shelves and blades from different vendors.
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