TTL/UART communication issue

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I have (an awesome) Onion Omega2+ SBC and an A9G GPS/GSM module. The module has two UART ports.
I can easily read data from the UART_GPS but for some reason I cannot communicate with the UART port for the GSM chip.
When I connect it to my Mac however, I can easily send and read commands.
I can also connect to the UART of the Omega2+ and write to and from it. I probably miss some small thing but I just can’t find what that is.

When using on my Mac:

Radeks-MBP:~ neo$ stty -f /dev/tty.usbserial-0001 
speed 9600 baud;
lflags: -icanon -isig -iexten -echo
iflags: -icrnl -ixon -ixany -imaxbel -brkint
oflags: -opost -onlcr -oxtabs
cflags: cs8 -parenb

When trying to use it from Omega2+:

root@Omega-9A9F:~# stty -F /dev/ttyS2
speed 9600 baud; line = 0;
min = 100; time = 2;
-icrnl -imaxbel
-opost -onlcr
-isig -icanon -echo

Can anyone see what the issue might be here?

Regards,
Radek

Answer

My assumption:

  1. HW Flow control for GPS UART enabled by default, try to connect line RTS & CTS to host

  2. Crossing RXD & TXD line. Try to swap both lines between themselves.

Attribution
Source : Link , Question Author : Radek Suski , Answer Author : Doka

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