We’ve been using Arduinos for all our projects, and it was time I got around to benchmarking the serial communication performance. It’s actually not very fast; even at the maximum baud rate of 115200 bits per second, delivered performance is only a little over 10KB/second each way, and it only hits this bandwidth when sending over 100 bytes at a time.
The problem for small messages seems to be a 4 millisecond minimum roundtrip latency. Messages over about 40 bytes seem to take several such latencies, so there’s a stair step pattern to the latency. Paul Stoffregen says this is due to the Uno firmware’s 4.1 millisecond transmit timeout.
Evidently, the Teensy (with direct USB to the chip, not a USB-to-serial onboard) gets about 1ms serial latency. The same page reported the Duemillanove at 16ms minimum (62Hz!).
Overall, this means you’re only going to get a 250Hz control rate if you’re shipping sensor data from an Arduino Uno up to a PC, and then sending actuator signals back down to the Arduino. But 250Hz is enough for most hardware projects we’ve been thinking about.
The other annoying problem? After opening the serial port, the Arduino Uno takes 1.7 seconds to boot before it responds to serial commands. Anything you send from the PC before that time seems to be lost, not buffered. The fix is probably to have the Uno send the PC one byte at startup, so the PC knows the Uno is ready.