Test Report: USB to Serial Adapter Designs

August 15, 2007

“We were stunned at the performance difference between brands of multi-port USB to serial adapters,” states Earle Foster, VP Sales & Marketing for Sealevel Systems. So, the company arranged a lab test to determine why performance varied so dramatically.

The testing revealed that design architecture is the critical element. The two basic design architectures were termed “shared throughput” and “dedicated USB UART.”

The “shared throughput” design is multi-port adapters using a USB microcontroller and a single FPGA wired to multiple serial ports. This design creates a data throughput bottleneck because the serial devices are sharing the throughput of the microcontroller. Another disadvantage of “shared throughput” is that the data is transmitted in bursts rather than in a continuous stream.

With one port on the “shared throughput” adapter open at the standard baud rate of 115.2K bps, the actual data rate was only approximately 65K bps. When multiple ports were opened, actual data rates were severely depressed on all ports because of the “shared throughput” design.

The “dedicated USB UART” design couples each port with a dedicated USB UART and eliminates performance problems inherent with the other design. The “dedicated USB UART” design is equivalent to connecting multiple single-port USB serial adapters to the host. In this design, each port on the adapter runs at its specified maximum speed.

According to Foster, “These findings have been enlightening and helpful for customers needing USB to serial adapters. Certainly customers with mission-critical applications, but the performance difference is so substantial that all customers express a preference for the dedicated USB UART design.” He then adds with a smile, “All Sealevel USB adapters have this design advantage.”

Sealevel Systems, founded in 1986, provides industrial computing solutions in addition to a variety of serial communications and digital I/O products including PCI Bus cards, Ethernet serial servers, USB serial adapters, PCMCIA cards, and PC/104 modules. Their product line includes multi-port RS-232, RS-422/485, RS-232/422/485 multi-interface high-speed sync/async, and digital/relay I/O.