pySerial buffer won't flush

Question

I'm having a problem with serial IO under both Windows and Linux using pySerial. With this code the device never receives the command and the read times out:

import serial
ser = serial.Serial('/dev/ttyUSB0',9600,timeout=5)
ser.write("get")
ser.flush()
print ser.read()

This code times out the first time through, but subsequent iterations succeed:

import serial
ser = serial.Serial('/dev/ttyUSB0',9600,timeout=5)
while True:
    ser.write("get")
    ser.flush()
    print ser.read()

Can anyone tell what's going on? I tried to add a call to sync() but it wouldn't take a serial object as it's argument.

Thanks, Robert

Answer 1

Put some delay in between write and read e.g.

import serial
ser = serial.Serial('/dev/ttyUSB0',9600,timeout=5)
ser.flushInput()
ser.flushOutput()
ser.write("get") 

# sleep(1) for 100 millisecond delay
# 100ms dely
sleep(.1)
print ser.read()

Answer 2

Question is really old, but I feel this might be relevant addition.

Some devices (such as Agilent E3631, for example) rely on DTR. Some ultra-cheap adapters do not have DTR line (or do not have it broken out), and using those, such devices may never behave in expected manner (delays between reads and writes get ridiculously long).

If you find yourself wrestling with such a device, my recommendation is to get an adapter with DTR.