Often legacy electronic equipment needs to be used with modern computers that do not support the interface or protocol for the device. Disparate electrical signaling levels, contrasting data rates, and missing or mismatched handshaking signals are among the problems the system engineer may face. Overcoming these challenges is simplified through the use of Field Programmable Gate Array (FPGA) technology as demonstrated by a custom USB to synchronous data cable designed for the U.S. military that, largely through the use of an FPGA, meets the core application requirements for fast throughput, low power, and small, rugged mechanical design.

Communication on the battlefield is vital to the success of any military operation and the safety of today’s warfighters. Unfortunately, the military uses several different manufacturers for tactical radios that provide data communications and situational awareness on the battlefield, making it difficult to achieve interoperability from one radio to another. To solve this problem, Sealevel Systems, Inc. worked with the Defense Information Systems Agency (DISA) to develop the ACC-188, a non-proprietary, interoperable USB synchronous interface that works seamlessly with the PDA-184 tactical data communications software application developed by DISA. The ACC-188 adapter and PDA-184 software enable PC compatible computers (mainly mobile devices) to connect to tactical radios to transmit and receive IP data such as email, text messages, GPS maps, images, coordinates, and other communications.

Fast Data Throughput
Maximizing the benefits the ACC-188 brings to warfighters requires fast data throughput necessary for transmitting and receiving documents, images, and other relatively large data files. USB is an asynchronous interface that relies on the USB host controller to poll the USB I/O device for data. Since the radio transmits continuous high-speed synchronous data, often for extended time periods, the conversion to asynchronous USB required careful attention to a variety of critical timing elements. Accomplishing this design without the use of an FPGA would have been very difficult given the timing constraints involved.

The heart of the design is an 8-bit microcontroller with integrated USB port and an FPGA. The FPGA includes a 256-byte Receive FIFO (First In/ First Out) buffer for receiving high-speed serial data from the radio. Using the clock signal supplied by the radio, the incoming data is clocked into the FIFO and stored until the microprocessor organizes the data into byte format and transfers the formatted data to the PC via USB packets. On message transmission, the USB packets are sent to the Transmit FIFO and subsequently clocked out one bit at a time to the radio. The configurable routing of the FPGA made possible the optimal design for the circuit and maximizes the throughput of the ACC-188.

Figure 1. ACC-188 hardware architecture.

Low Power Consumption
To eliminate the need for an external power supply that would need to be transported with the device, the ACC-188 is designed to operate from the 5 V supplied by the USB port.  Since the ACC-188 is used most frequently with mobile computers operating from battery power, low power draw is a huge consideration in order to lengthen the life of the computer’s battery. 

Implementing the ACC-188 using an FPGA helped to lower the power required by the ACC-188 by reducing the number of overall gates to the minimum needed to accomplish the required functionality. Sealevel engineers were careful to select an FPGA that uses a small geometry process, and future upgrades to newer process geometries as they become available will continue to provide improvement in power usage. As a result, the ACC-188 is optimized to consume less than 0.3 W of power when operating with maximum throughput. 

Small Size Requirements
For optimal usability, the cable adapter needed to be small and lightweight while rugged enough for battlefield conditions. Using an FPGA allowed much of the circuitry necessary for the USB to synchronous conversion to be done in a single IC, eliminating the space that would have been needed for a traditional design using a synchronous communication controller with external RAM for the FIFOs and the associated glue logic. At 1.5” x 3.5” in size, the ACC-188 PCB is smaller than a credit card.

Figure 2. ACC-188 adapter connects tactical radio.To protect the PCB and cable interconnects from the environment, the board is encapsulated using an overmold process, commonly called a “bump” in the cable. One end of the cable includes a standard type-A USB connector, suitable for use with any USB enabled computer. The other end of the cable includes a connector specific to a brand or model of tactical radio.

FPGAs Offer Flexible Design Alternatives
The decision to implement the ACC-188 USB to synchronous serial adapter using an FPGA was critical in meeting the core requirements for fast data rates and a suitable power and size footprint for the application. Other advantages of using FPGAs include the ability to reprogram the device in the field to add functionality without changing hardware, and the easy ability to switch the FPGA technology in the event of part obsolescence. As FPGA technology progresses, the benefits they present will only become greater and lead to more widespread adoption in a variety of applications.

About the Author

Earle Foster is Vice President of Sales and Marketing at Sealevel Systems, Inc. Earle has over 20 years experience in industrial computing and I/O connectivity solutions. He holds a B.S. degree in computer engineering from Clemson University. For more information, visit or call 864-843-4343.