What happends if you connect Rs to Tx in the GPIO connector of RPI ?
I mean, leave everything else out and use a DIP shortcut.
Use 'picocom /dev/ttyAMA0 -b 1200'
( Check that local echo if OFF )
Send some characters. They should come straight back to the terminal.
If they do, the problem is in the MAX3232 card or between the MAX and external device.
If something comes back, check my previous note about etty@ttyAMA0.service
If nothing comes back, you have a real problem.
connect the TTL Rx to TTL Tx in the MAX3232 card. You can use a short piece of copper here. Let the card use the 3.3V and GND of RPI. You must also connect the GND to the GND of the RS232 connector
Now connect the card to your laptop with your cables.
Use some terminal program in your laptop.
Send some characters. They should come straight back.
If something comes back, check Rx-Rx and Tx-Tx connections between MAX and GPIO.
If nothing comes back, check the terminal program settings and the cable.
If something comes back, check the cable.
You said "Pi to scale the Rx suddenly goes to 0V and Tx keeps 3.3V"
Add a 1kOhm resistor in series in this Rx line between GPIO and MAX card.
When the Rx goes to 0V, you can measure the voltage on both sides of the resistor against GND. Now you know which draws the line down, GPIO or MAX.
The same thing can be done with the Tx line.
The signals do not care about this 1kOhm, the inputs are so high impedance that it does not matter. In truth I'm using these resistors in cases, when I'm not sure.
In any case I'd recommend a simple logic analyser to check where the signal is going where. Just connect red and green LED side-by-side anode to cathode and cathode to anode and connect this in series with a 1 kOhm resistance. The other end you can solder to the GND with a cord.
You can use this to check both TTL and RS232 lines.
When you are testing RS232 with this 'analyzer', use as low speed as possible. For example
"picocom /dev/ttyAMA0 -b 110".
The slow speed makes it easier to see when the bits are running.
I'm using a D9 shortcut adapter to test RS232. Take a female D9 and connect the pins like this: 2-3, 6-7-8.
Those lines 6,7 and 8 are used for handshaking when you connect it to your laptop. This adapter works if the device has a XON-XOFF handshaking. If the device has DTS-DTR handshaking, you got to invent something else. In case of slow interaction the adapter should work still, because the handshaking is mostly there to tell that the buffer is full.
Some laboratory devices need similar shortcuts in their connectors. The device does not say or listen anything, if there is no handshaking. RTFM of the device