pySerial can write, but not read

Question

I am trying to use pySerial on a Windows 10 machine (Python 3.6.4, 32 bit) to read serial data from a piece of lab equipment that would normally log its data to a serial ASCII printer. Connecting using a USB-to-serial adaptor.

If I connect the computer to the printer, I can print using serial.write(), so I know my adaptor is working. However, when I connect the computer to the lab equipment and try to read data using the following code, I get nothing all:

import serial
ser = serial.Serial('COM5')
while True:
    if ser.in_waiting != 0:
        datastring = ser.read(size=ser.in_waiting)
        print(str(datastring))

I know the lab equipment is transmitting when I run the code. Have also tried connecting two USB-to-serial adaptors to the computer with a serial cable in-between the adaptors and sending data from one serial port to the other. Again, I can write without problem, but the other port receives nothing.

EDIT: I turned out to have a hardware problem. I had connected the lab equipment to my USB-to-serial adaptor (and, for testing purposes, the two USB-to serial adaptors to each other) using a standard serial cable. Connecting using a null modem solved the problem.

Do you need to assert DTR or RTS? – quamrana Mar 21 '18 at 14:27 — quamrana, Mar 21 '18 at 14:27

Dadep · Answer 1 · 2018-03-21T15:44:41.627

0

you may have a problem of baud rate you should declare something that look like :

import serial
ser = serial.Serial('COM5', 9600, timeout=None) #<- 9600 is the baud rate

while True:
    data = ser.readline()
    print(data)

EDIT

While True:
    bytesToRead = ser.inWaiting()
    data=ser.read(bytesToRead)
    print(data)

edited Mar 21 '18 at 15:44

answered Mar 21 '18 at 14:27

Dadep

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Thanks, but I still don't get any output when running this. – Martin Rune Hansen Mar 21 '18 at 14:34
did you check what is the baud rate from your equipment ? – Dadep Mar 21 '18 at 14:35
do you have anything else you can use to test your python script (arduino, other computer, etc...) ? check also edit in my post... – Dadep Mar 21 '18 at 15:45

score 0 · Answer 2 · answered Mar 21 '18 at 14:30

0

There are many causes to this kind of problem, the first things that comes to my mind is that you have a low timeout for your serial, and you close the connection before anything can be received:

ser = serial.Serial('COM5', timeout=None)

other values that you may check are the following:

ser = serial.Serial('COM5', baudrate=9600, bytesize=8, parity='N', stopbits=1, timeout=None, xonxoff=False, rtscts=False)

answered Mar 21 '18 at 14:30

Gsk

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Tried connecting both USB-to-serial adaptors to the computer again with a serial cable in-between. Specified all the values as you suggested for both ports (except for the port names and the timeout that was set to 100). Wrote to one serial port, the other one still did not receive anything. The problem can't be values that are incompatible with my lab equipment. – Martin Rune Hansen Mar 21 '18 at 14:50

pySerial can write, but not read

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