UART Serial Read Error (Device reports readiness but returned no data)

Project help
1

I’m trying to hook up an RFID reader to pin 15 on my raspberry pi. When I test this on stock Raspberry OS, it works, but when trying it on balena, I get errors on scanning an RFID. I have enabled UART in the fleet variables, and I’m running FROM balenalib/%%BALENA_MACHINE_NAME%%-python:3-stretch-run as my image. The code is pretty basic (and as I mentioned works fine on stock) but here it is:

import serial, time

# Setup Serial Port
ser = serial.Serial(
    port='/dev/ttyS0',
    baudrate = 9600
)

while 1:
    rfid = ser.readline()
    data_left = ser.inWaiting()
    rfid += ser.read(data_left)
    print (rfid)

When I scan an RFID I get the message
Traceback (most recent call last):
File “serialtest.py”, line 12, in
rfid = ser.read()
File “/usr/local/lib/python3.6/site-packages/serial/serialposix.py”, line 596, in read
'device reports readiness to read but returned no data ’
serial.serialutil.SerialException: device reports readiness to read but returned no data (device disconnected or multiple access on port?)

Anyone have any ideas on this? Previously I used USB readers and they worked great, but this year I’m working with serial over the GPIO and having no end of trouble.

2

If you’re using a balenaOS dev image, the serial port is already used by balenaOS for debug infos.
So this could be a problem. Only the “production” image has a clean serial port :slight_smile:

3

login to your device, balena ssh <device ip> and type the following commands,

mount -o remount,rw /
systemctl mask serial-getty@ttyAMA0.service
systemctl mask serial-getty@serial0.service
systemctl mask serial-getty@serial1.service
reboot

These commands will prevent from being occupied by balenaOS.

1 Like
4

Ok, so this worked. My question now is, how do I do this programmatically instead of having to manually ssh in to change each device in the fleet? or do I just do like @nmaas87 mentions above and use a production image?

5

Maybe you should use production image or build your own image with yocoto.

1 Like
6

I really would go with a production image. It is also more secure (you do not have the root, empty password ssh login but need to integrate an ssh key into the config.txt to access it) - and I don’t know what happens if you update that modified dev version down the road. There is always the chance your changes will be lost and your services will then fail again.

7

Hello,

Did you have any luck using the production image? Please let us know if you’re still having trouble.