Sealevel, represented in Australia by Allied Data Systems , has released a series of test reports outlining the differences between the popular USB to serial interfaces.
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 are 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.”