A new VME, VME64x, or CompactPCI system often reaches a validation stage before the final payload cards are available. The chassis, backplane, power supply, and cooling hardware may be ready for evaluation, while the application-specific boards remain in design, procurement, or qualification.
That gap creates a practical engineering problem. An empty chassis cannot demonstrate whether its power distribution can support the intended load or whether its cooling path can remove the resulting heat. Waiting for the payload cards postpones system-level verification and can allow power or thermal problems to appear late in the integration cycle.
A load board provides a controlled way to apply electrical demand and generate heat before the real payload exists. It uses the backplane power connections and occupies the intended card position, allowing engineers to evaluate the platform under defined conditions. The result is not a substitute for payload-card testing. It is an early validation of the chassis, backplane, power source, and airflow path.
What a Load Board Tests
A load board is designed to exercise selected power rails without requiring the application circuitry normally found on a payload card. In a VME or cPCI chassis, it can be used to evaluate four related conditions:
- Power delivery under controlled electrical load
- Supply margin as rail demand increases
- Heat generation within the card cage
- Airflow distribution and chassis hot spots
The thermal function is important because electrical load and cooling performance are coupled. A board that draws current also dissipates power as heat. Installing load boards in representative slot locations creates a controlled thermal source in the same general region where future payload cards will operate.
This allows the test team to identify hot spots, compare inlet and exhaust temperatures, and determine whether airflow is reaching the areas that require it. The test can also reveal whether the cooling solution is sensitive to slot position or to the presence of adjacent loaded cards.
The load board does not exercise signal lines, bus transactions, application logic, or payload-specific switching behavior. It is therefore a platform-validation tool, not a functional replacement for the final cards.
VME and VME64x Load Boards
Vector’s VME and VME64x load board is intended to test the power generation and cooling capability of VME systems. The 6U board applies a variable resistive load to the power supply for verification and generates heat to confirm chassis cooling. The resulting thermal profile can help locate hot spots so airflow can be directed appropriately.
The board conforms to the electrical and mechanical connections of ANSI/VITA 1 (1994) for VME and ANSI/VITA 1.1 (1997) for VME64 extensions. It uses the J1 and J2 power and ground connections.
Front-panel switches select the resistive load for each voltage. The available selection range is from 0 to 7 amps for thermal characterization. This makes it possible to test more than one operating condition rather than applying only a single fixed demand.
Primary test points include:
- +V1
- +V2
- -V1
- -V2
- ACFAIL
- SYSRESET
- SYSFAIL
- Ground
These points allow the test engineer to monitor the power rails and selected system signals while the board is under load. Measurements should be taken at the load board and correlated with power-supply output measurements where practical. This helps distinguish a source-level issue from voltage drop or distribution behavior within the chassis.
The board uses a 4-layer stripline design with 2 oz. copper power and ground layers. It is 0.062 inch thick and has a 6U x 160 mm form factor. The VME64x configuration uses a 160-pin J1/J2 connector and is backwards compatible to 96-pin J1/J2 connections. The standard front-panel width is 4HP, or 0.80 inch.
These mechanical details matter during a thermal campaign. The load board must occupy the same slot geometry and airflow region as the intended payload card. A test performed with a board that does not represent the actual card envelope or connector interface may not reproduce the system’s eventual thermal behavior.
cPCI Load Boards
Vector’s cPCI load boards are available in 3U or 6U configurations. They apply a fixed 25 W resistive load that generates heat to verify chassis cooling and locate hot spots so airflow can be adjusted.
The cPCI load board conforms to PICMG electrical and mechanical specifications for J1 power and ground only. Test points measure:
- +5 V
- +3.3 V
- +12 V
- -12 V
- Ground
The measurements are taken through P1 of the cPCI backplane. The J1 connection uses a 25-row, 110-pin 2 mm hard metric connector. The standard front-panel width is 4HP, or 0.80 inch.
The mechanical retention arrangement differs by board height. The 3U board has a lower Type IV injector/ejector. The 6U board has top and bottom Type IV injector/ejector pairs. These features support the physical installation and retention expected in the corresponding card cage.
Because the cPCI load is fixed, the test strategy differs from the switch-selected VME approach. The cPCI campaign should focus on slot placement, the number and arrangement of installed load boards, power-rail measurements, and thermal behavior under the defined fixed load.
The distinction between the two platforms is significant. A VME/VME64x campaign can vary resistive loading by voltage through front-panel switches. A cPCI campaign using the fixed-load configuration establishes the chassis response to a defined electrical and thermal condition. In both cases, the load board verifies the platform before the application electronics are available.
Planning a Load-Board Test Campaign
A useful test campaign begins with a defined load matrix rather than an installation of boards followed by informal temperature checks.
First, define the target load points for each rail. For VME/VME64x, document the intended switch settings for +V1, +V2, -V1, and -V2. For cPCI, document the fixed-load condition and the rails being monitored through P1.
Next, define slot population. An empty chassis is not a representative thermal condition, but loading every slot is not always necessary for every test. The campaign should identify the worst-case slot positions, adjacent loaded slots, and any locations known to be sensitive to airflow. Test plans should distinguish between:
- Single-board electrical characterization
- Key-slot thermal evaluation
- Adjacent-slot loading
- Full intended slot population
Instrument the load-board test points and record the supply condition at each load step. Measurements should include rail voltage, current where available, and any relevant power-supply status signals. Startup and shutdown behavior should also be observed, particularly when more than one load board is installed.
Temperature measurements should include chassis inlet air, exhaust air, and representative locations within the card cage. The test team should identify the hottest regions rather than relying only on a single chassis outlet measurement. If the system includes adjustable cooling, record the cooling configuration used for every test condition.
The load should be increased in controlled steps, with sufficient time at each step for the thermal response to stabilize. The report should identify the load setting, slot map, rail measurements, inlet temperature, exhaust temperature, observed hot spots, and any alarm or abnormal behavior.
Once representative payload cards become available, their results should be compared with the earlier load-board data. The comparison should account for differences in power distribution, component placement, heat spreading, and operating duty cycle. Load boards establish a controlled platform baseline; they do not predict every payload-specific thermal condition.
Common Mistakes
Testing with an empty chassis
An empty chassis may show acceptable fan operation and low temperature, but it does not demonstrate performance under system load. The absence of heat sources also prevents the team from observing temperature gradients and slot-to-slot differences.
Loading only one slot
A single loaded slot may validate local power delivery while missing airflow interactions between adjacent cards. A thermal campaign should include the slot arrangements that are most likely to create a restrictive or uneven airflow pattern.
Treating thermal load as functional validation
A load board can draw power and generate heat, but it does not exercise signal integrity, bus arbitration, data transfers, application processors, or payload-specific transients. Those functions require the actual electronics or an appropriate functional test fixture.
Measuring only at the power supply
Supply output voltage alone does not show the voltage available at the card interface under load. Measurements should include the load-board test points and the relevant backplane paths so distribution effects can be identified.
Failing to document configuration
Cooling results depend on slot population, load settings, fan configuration, ambient conditions, and measurement locations. Without configuration control, a later comparison with payload-card data becomes difficult and may produce misleading conclusions.
What Engineers and Program Teams Should Consider
The principal value of a load board is timing. It allows chassis and backplane validation to proceed before the payload design is complete. That can expose inadequate rail capacity, unexpected voltage behavior, airflow imbalance, or thermal hot spots while corrective action is still practical.
The test should be planned around the questions the platform must answer:
- Can the power distribution support the defined rail demand?
- Does the supply remain stable as load increases?
- Do the hottest slot locations receive adequate cooling?
- Does the chassis maintain acceptable inlet-to-exhaust temperature behavior?
- Are alarm, reset, and failure signals behaving as expected under the test condition?
- Which results must be repeated with the final payload cards?
For VME/VME64x, the variable front-panel loading supports a stepped characterization of the specified rails. For cPCI, the fixed 25 W resistive load provides a repeatable thermal condition through the J1 power and ground interface. The appropriate test method depends on the platform, backplane configuration, cooling architecture, and intended payload distribution.
Load-board testing is most effective when treated as a controlled engineering baseline. It verifies the infrastructure that supports the payload rather than attempting to imitate the payload itself. The governing principle is straightforward: validate power delivery and heat removal at the chassis level before application hardware makes those problems more difficult to isolate.