Serial Port read() gets only less bytes at a time

Question

Operating system : Ubuntu

Serial Ports Using : USB->Serial adapter (prolific PL2303)

I have a sample serial port read and write programs. While running the applications,I am trying to send 4100 bytes ,write program able to do it in single shot.

n = write (s_port,msg,4100);

Here I am checking 'n' value and it is 4100

I*strong text*n receiving end

n = read(s_port,msg,5000);

Above line is in a loop, I am getting not more than 32 bytes in a single read attempt.So more than 128 read attempts(32+32+32 etc...) to get full bytes(4100)

In sending side If it is able to push the whole bytes in a single shot, why not able to receive in single shot?

"why not able to receive in single shot?"* -- Simply because you have not (properly) configured the serial terminal to do so. You have neglected to post/mention any termios settings and I/O modes. — sawdust

Richard Chambers Richard Chambers · Accepted Answer · 2012-12-11T12:48:58

Serial communications take time for individual bytes to be transferred depending on the baud rate. CPU is much faster in processing the bytes once received.

With the write the serial communication device interface is able to buffer up the bytes to be sent. The read however has to take the bytes as they are received from the other device so there is a time lag.

Here is an wikipedia article on the RS-232 Serial Communications standard.

Compare the serial port to this wikipedia article on the Parallel port.

The parallel port had one wire per bit so that all of the bits are transferred at the same time where as the serial port transferred the bits serially, one at a time. Parallel ports were used for higher speed transfers such as for printers at a time when serial port speeds were pretty low. Serial port speeds have dramatically improved over the years though still no where near USB speeds.

Serial Port read() gets only less bytes at a time

2 Answers