TTL/USB converter usage question

[Ramiro Aceves <ea1abz%gmail.com@localhost>]

Hello,

I have a Jupiter-T GPS timing module board that is connected to a home
made board with a Microchip PIC 12F629 microcontroller. They form a
simple amateur radio GPSDO unit(GPS disciplined Oscillator for my
radio lab) They communicate eachother using serial TX and RX 5 volt
signals. Recently it has problems to get the fix and want to debug it
(antenna problem or module problem, who knows).

I have a NetBSD RaspberryPi4 board with a TTL/USB converter (TX, RX
and GND pins, ftdio driver). I have connected only the RX and GND pins
of the TTL/USB converter  to the TX pin of the GPS module to see what
information the GPS board is sending to the PIC board.

I added rw permissions to device the wheel group so I can talk to the
converter from the wheel group without root privileges

netbsd-raspa4$ ls -lh /dev/ttyU0
crw-rw----  1 uucp  wheel  74, 0 Oct 14 11:25 /dev/ttyU0

"cu" command works just fine:

cu -l /dev/ttyU0 -s 9600|hexdump -C|less

netbsd-raspa4$ cu -l /dev/ttyU0 -s 9600|hexdump -C|less
00000000  43 6f 6e 6e 65 63 74 65  64 0d 0a 0a 40 40 45 61  |Connected...@@Ea|
00000010  0a 0e 07 e8 09 1b 0e 3b  9a c9 fe 0a ad d0 1e 02  |.......;........|
00000020  b7 82 7f 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000050  00 00 00 00 4a 2e 0d 0a  40 40 45 61 0a 0e 07 e8  |....J...@@Ea....|
00000060  09 1b 10 3b 9a c9 fe 0a  ad d0 1e 02 b7 82 7f 00  |...;............|
00000070  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000000a0  4a 30 0d 0a 40 40 45 61  0a 0e 07 e8 09 1b 12 3b  |J0..@@Ea.......;|
000000b0  9a c9 fe 0a ad d0 1e 02  b7 82 7f 00 00 00 00 00  |................|
000000c0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000000e0  00 00 00 00 00 00 00 00  00 00 00 00 4a 32 0d 0a  |............J2..|

I get good Zdiac Binary protocol frames.

I read somewhere that I could get same behaviour with this commands this way:

netbsd-raspa4$ stty -f /dev/ttyU0 9600
netbsd-raspa4$ cat /dev/ttyU0

But I do get nothing from the cat command.

Probably something stupid but I have been strugling with this with no solution.

Thanks.
Ramiro.