Feature request: Unbuffered write

[autowp]

In my application i need unbuffered synchronous write to provide equal spacing between the end of each and start of subsequent transmits. (using ScheduledExecutorService.scheduleWithFixedDelay)

Transmit time can be greater than delay between transmits. Thats means write buffer overflows if used buffered write.

ScheduledExecutorService service = Executors.newScheduledThreadPool(N);
service.scheduleWithFixedDelay(new Runnable() {
  @Override
        public void run() {
            Serial.write(buffer);
        }
}, 0, delay, TimeUnit.MILLISECONDS);

Quick patch to add unbuffered synchrowrite: https://github.com/autowp/UsbSerial/commit/a6ab307909f09f71d30be570902c94f1ed7f8a72

Maybe that needs not only for me and unbuffered interface appears in the library

[felHR85]

Hi @autowp there are more people with that need. I will try to add a way to choose between sync and async api as soon as possible

[autowp]

I think it is also necessary to read for symmetry.

It's useful for dialog transfers where next submition wait for previous response

[felHR85]

I've added a synchronous way to write and read in UsbSerial (version 4.1).

public boolean syncOpen();
public int syncWrite(byte[] buffer, int timeout)
public int syncRead(byte[] buffer, int timeout)
public void syncClose();

It would be nice if you could give it a try! @autowp

[autowp]

In my apps it works, but ...

syncRead works in poll mode. Non blocking. I think that is becase FTDI 2 bytes response. Hmm

[felHR85]

In the FTDI devices I am just ignoring the 2 byte header and in case I've only got those two bytes I return 0 bytes read. It is a blocking call

[autowp]

Nope, i think it is not blocking.

while (true) {
    mSerial.syncRead(buffer, 60000);
    System.out.println("timeout");
}

That prints "timeout" continously without 60s delay on empty input.

[felHR85]

Ok! you set the timer to 60 seconds but in the FTDI devices just return every 40 ms because the modem status is sent continuously (returning 0 bytes read). I have to ignore those bytes till the timeout is over

[autowp]

Yes. That means the read is not blocking. I think its no way to fix that with FTDI, but interface can be little better if not only ignore 2 bytes, but wait up to timeout for payload bytes.

Quick fix: https://github.com/felHR85/UsbSerial/pull/30

[felHR85]

I was trying to fix it right now! Basically the same as you did so I will check it out and I will merge it

[felHR85]

Thank you very much for your help @autowp Pull request accepted