I am trying to use RS232 to TTL convertor much like this: https://www.amazon.com/MAX3232-Female-Serial-Converter-Module/dp/B00BXX2OU2 I am not sure if the chip is authentic MAXIM or some knock off.
When I connect the convertor to the rs232 device (a home alarm), the led on the convertor flashes on a regular interval. When I connect the (ttl) rx/tx leads from the convertor to my computer I see activity on the serial port, but it looks like gibberish. It does not follow the protocol of the system. If I attach the ground and +5 vcc to the ttl side, my led shines continuously, but I see NO activity on the serial port.
I have checked the RS232 converter via a loopback (crossing the tx/rx lines) and verify that I can see characters echoing back. I tested the connector by hooking it up to another one I had and verified that sending on one serial port results in the correct text arriving on the other. I can see that RS232 device (the alarm) is active and there is voltage on the TX pin when I put a voltmeter between TX and GND. The voltage is roughly the nominal 9V that is indicated in the manual.
I am not an electrical engineer, so I am out of ideas. What am I doing wrong? Do I need to solder in a resistor on the RS232 TX pin? Do I need a voltage divider?