Replacing Wire-wrap Boards with Software, FPGAs, and Custom Signal Conditioning

Challenge

Amentum (www.amentum.com) supports a decades-old system deployed in the early 1980s. While the mechanical subsystems were still functioning, the wire-wrapped discrete logic and analog circuitry was having intermittent problems.

Systems designed and built decades ago can sometimes have wonderful documentation packets. Nevertheless, we’ve been burned too often when the docs don’t incorporate the latest redlines, last-minute changes, or other updates.

The replacement system needed to be a form-fit-function replacement to land in the same mounting locations as the original equipment with the same behavior and connections. Below is an image of the existing wire-wrap boards and their enclosure. We had to fit the new equipment in this same spot.

Figure 1 – Original wire-wrap boards

Finally, Amentum wanted to work with Viewpoint in a joint development approach. While our joint capabilities looked complementary, we didn’t know at the start how well we would mesh with our technical expertise and work culture – it turns out we worked extremely well together as a team and neither one alone could have easily delivered the solution.

Solution

Since the team treated the existing documentation package with suspicion, we adopted a “trust but verify” approach. We would use the documents to give overall direction, but we would need details from the signals to verify operation.

Leveraging Amentum’s experience with the fielded systems, the team decided early on to record actual signals to understand the real I/O behavior. We used the system’s “test verification” unit to run the system through some check out procedures normally run prior to system usage. This verification unit enabled us to use a logic analyzer for the I/O to and from the discrete logic digital signals and an oscilloscope and DMM for the analog signals. The available schematics were reviewed to assure that the signals made sense.

With a trustable understanding of system operation, Amentum created a requirements document. We jointly worked on the design of the new system. There were both an “inside” system (in a control shelter) and an “outside” system (in the unit’s pedestal).

Some overall tasks were:

• Viewpoint recommended an architecture for the inside application running on PXIe LabVIEW RT and FPGA layers.
• Amentum created the system control software on a Linux PC.
• Viewpoint developed the more intricate parts of the inside application and mentored Amentum on other parts they developed. This work recreated the existing discrete logic and analog I/O using PXIe NI FPGA boards.
• Viewpoint designed custom interposer boards to connect harnesses to the NI PXIe equipment, including a test point and backplane boards.
• Amentum designed and developed the cRIO-based outside system application and Viewpoint created a set of custom interposer boards to connect harnesses to the cSeries modules.

The PXIe FPGA boards handled the required 60 MHz clock-derived signals with correct phases, polarity, and so on. Furthermore, the wire-wrap boards were register-based so the PXIe had to decode “bus signals” sent over a Thunderbolt bus to emulate the programming and readouts from the various wire-wrap boards.

Figure 2 – PXIe replacement to wire-wrap boards

Amentum wanted to be able to support the LabVIEW FPGA VIs used to replace the functionality of the discrete logic. So, Viewpoint acted as mentor and code reviewer with Amentum to ramp them up on using LabVIEW FPGA effectively. Neither one of us alone would have been successful coding the applications in the allotted time. Joint knowledge and experience from both Viewpoint and Amentum were required.

Signal conditioning and harnesses needed to be reworked or replaced as well, of course, since the landing points for the wires were different in the new system. Viewpoint suggested a technique, which we’ve used frequently in past obsolescence upgrade projects, to create PCB boards that accepted existing connectors.

For the cRIO, these interposer “connection” PCBs plugged directly into the cRIO cSeries module. For the PXIe, these interposer PCBs accepted the field wiring connectors and converted them to COTS cables that connected to the PXIe modules. These interposer PCBs could have signal conditioning incorporated as needed. This approach significantly reduced the need for custom harnesses. All told, about 200 signals were passed between the PXIe and various other subsystems, and about 100 for the cRIO. This approach saved significant wiring labor and cost.

Figure 3 – cRIO with interposer boards between cSeries and field harnesses

The work to design and build the signal conditioning custom electronics was split between Viewpoint and Amentum. Viewpoint did more design than build and handed over the schematics and Gerber files to Amentum so they could manage the builds while also being able to make modifications to the boards as needed.

Benefits

Amentum wanted an engineering firm that was willing to work along side them as a partner. Joint discussions about architecture and design led to a collaborative development effort where Amentum benefited from Viewpoint’s extensive expertise and guidance on LabVIEW architectural implementation and FPGA coding style.

The main outcomes were:

• As a partner of the team, Viewpoint acted as staff augmentation by providing experienced engineers with technical capabilities that Amentum initially lacked.
• This team approach delivered a stronger product to the end-customer more quickly than either of us could do alone.
• The combination of Viewpoint’s and Amentum’s experience reduced the amount of reverse engineering needed due to the lack of firm requirements.
• Reduction of electronics obsolescence by using software-centric FPGA-based functionality. Recompiled LabVIEW FPGA could target future boards models.
• Increased software-based functionality simplifies future updates and modifications.
• Decrease in number of parts leading to simpler maintenance.
• Lower wattage consumed eliminated need for an anticipated HVAC upgrade.
• Cybersecurity concerns were reduced by using Linux-based systems and FPGA coding.

System Overview

Using software to emulate the old hardware was a critical success factor. Since the requirements were not 100% solid at the start of the project, some field-testing was required for final verification and validation. The flexibility of the software approach eased modifications and tweaks as development progressed. A hardware-only solution would have necessitated difficult and costly changes. For example, some of the changes occurred very near the final deployment after the system was finally connected to an actual unit in the field.