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?

Thank you!


closed as unclear what you're asking by Andy aka, Voltage Spike, Sparky256, JRE, Dmitry Grigoryev Feb 13 '18 at 12:58

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  • \$\begingroup\$ When you say that you connect the TTL side of that converter to your computer can you be more explicit with just what that means? What kind of computer? It may be a good idea if you could draw out a bit of a detailed block diagram of what is connecting to what and post that as a picture in your question. \$\endgroup\$ – Michael Karas Feb 1 '18 at 1:01
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    \$\begingroup\$ Are both ends running at the same baud rate? \$\endgroup\$ – Ian Bland Feb 1 '18 at 1:26
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    \$\begingroup\$ @GregS - Step back and get a grip here. Go back to basics and prepare a diagram that details what the hookup is for the configuration that does not work including exactly how the TTL lines are attached to the "computer" and then describe the computer and its ports. \$\endgroup\$ – Michael Karas Feb 1 '18 at 6:51
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    \$\begingroup\$ Is this a case of needing a cross-connect on the TTL side? It's common that the target device has an output marked Tx which needs to connect to the Rx of the source, and vice versa. This is particularly evident with USB to TTL converter modules. \$\endgroup\$ – henros Feb 1 '18 at 9:33
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    \$\begingroup\$ @RoyC the MAX3232 chip both reduces voltage and inverts the signal so that the raw rs232 signal is useable by a UART. \$\endgroup\$ – Greg S Feb 7 '18 at 16:48

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