Serial port performance

[marcvanriet]

Hi,

I've been evaluating several Qt serial port libraries :
- Inbiza labs qserialport
- Kuzulis QSerialDevice 0.4
- Kuzulis serialport 2.0
(QExtSerialport will be next when I have the time)

One of the tests involves putting a serial port in loopback (using a small wire), sending a character on the port, waiting until the character is received back, and then send another one, and see how many can be sent per second. This is (I think) a measure for how fast the library handles read and write calls, and receive notifications (using the signal-slot mechanism).

Since I have no native serial port on my laptop, I'm using USB-to serial cables, from different manufacturers.

It seems that - for each USB port - no more than about 60 send-and-receive cycles can be achieved per second.

I wonder if it is related to the implementation of the libraries, or if it is a limitation in USB itself (which is unlikely, since the frame rate on USB is much higher than that : 1ms for a USB interrupt packet). I don't think it is a limitation of Qt (the signal-slot mechanism is much faster than that).

Does anyone have experience with this ?

I intend to control embedded devices through a serial port, and if I can only send 60 'commands' per second, that's rather slow.

Best regards,
Marc

[marcvanriet]

Some progress here...

On a 'native' serial port inside a desktop computer, the Kuzulis 0.4 library reaches 2500 send-and-receive cycles, and the Inbiza labs library 250.
On the same computer using a USB port, only about 150 cycles are achieved.

So USB is certainly a limiting factor, but also the use of the library.

More insight would be much appreciated...

Best regards,
Marc