RX TX signal General (No Arduino problem)

[caslor]

Hi to all.
i would like your knowledge about a problem i have.

I have two PCBs (same type) that are driving two machines . Because these machines doing the same functions they need to communicate in order not doing the same thing at the same time .
So they are connected with 0, RX, TX cables between the PCBs.
The Pcbs have a led matrix displays to show errors code.

for the last 12 years were working fine but recently both give me an error : "that are not able to establish communication" (both pcbs display the same error in screen.
So the machines still working but independently

i check the cables between them for continuity and are ok.
i checked also if someone has changed the rt,tx order but i find them ok (Rx from one pcb goes to Tx to other Pcb and Tx from Pcb1 to Rx to Pcb2)

The manufacture of the pcb was in a foreign country and after a long search i discovered that has been closed some years now. ( no option for help from there )

I want to check if the pcbs sending signal (spot which pcb stopped transmitting ) so i took my small portable DSO150 Digital Oscilloscope with my 10x probe and try to get a reading (not to decode the signal) but with no luck .

the manual i have says :

The Pcb Connectors with number 48, 49 in above table are the Group line that we are interesting for . (The other rx tx lines are for some pcb expansions they are using for digital imports/outputs and working fine)

So i connected my oscilloscope Ground to Common "0" and my Probe tip to Tx or RX .
i was not getting no reading , i used and the option to read the rise of a signal and display it but nothing was happen. (the machines were working during the tests as they should )

As i am an amateur on using oscilloscope , do you think i have used it wrong?

Thanks in advance

My Goal is to spot if they PCBs transmitting or not, which one is not and probably then detach it and find a specialized electronic repair shop to send it and check it why not transmitting (and hopefully repair it)

(The best option would be if someone of the shop could come and check it but no one doing this )

[camsysca]

I take it there are no schematics for the device? No RX/TX LEDs that might flash during communication?
Your description of your physical connection sounds ok, but I wonder if you're using the scope correctly. Serial communication is not continuous, and often can be hard to trigger on to get a stable signal, so you may interpret "no communication" when you're simply not catching it.
C

[camsysca]

Re-reading your description(it's early here!), I see you've self-implicated the scope setup. It can be tricky to help someone use a scope by giving instructions on the internet, truly. Five minutes in person would be preferable. Is there no technical staff at your space that would help with this?
C