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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!

Greg S
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  • 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. – Michael Karas Feb 01 '18 at 01:01
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    Are both ends running at the same baud rate? – Ian Bland Feb 01 '18 at 01:26
  • Yes, and I have run the receiving end at several baud rates to isolate this thing. – Greg S Feb 01 '18 at 01:37
  • Same number of bits, stop bits etc? Handshaking? – Ian Bland Feb 01 '18 at 01:39
  • @MichaelKaras The RS232 I feed into a prolific TTL to USB dongle. Again, I ran a loop, PC <-> PROLIFIC <-> RS232 || crossover and and test of PC <-> PROLIFIC <-> RS232 <-> RS232 <-> PROLIFIC <-> PC. In both cases (loopback and null modem) I get the expected result. – Greg S Feb 01 '18 at 01:40
  • @IanBland, yes I have a python script and tried that. I am using Python3's readline function in serial. I see readline timing out and returning an empty buffer. – Greg S Feb 01 '18 at 01:41
  • I have also attempted to log any raw activity using: (stty raw; cat > received.log) < /dev/ttyUSB0 and the file is literally empty. – Greg S Feb 01 '18 at 02:48
  • You described the connections with loopback where you get expected results – Michael Karas Feb 01 '18 at 03:26
  • That makes sense. But you did not really describe the computer connection where things do not work. Please elaborate. – Michael Karas Feb 01 '18 at 03:28
  • @MichaelKaras I see no activity on /dev/ttyUSB0. The funny thing is that if I leave the vcc and gnd unplugged on the TTL side there is something happening, so physically I think the connection is okay. Not sure if this is what you were asking. – Greg S Feb 01 '18 at 04:37
  • One other thing which I wanted to ask is the function of the shield. Should the shield on the db9 connector be grounded? If so, which ground? The ttl side or the other device. – Greg S Feb 01 '18 at 04:53
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    @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. – Michael Karas Feb 01 '18 at 06:51
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    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. – user131342 Feb 01 '18 at 09:33
  • Not familiar with this device but does it invert the signal? – RoyC Feb 01 '18 at 15:12
  • I notice the following blog post which describes problems with some of these chips: http://blog.bogpeople.com/2015/01/fake-max3232-rs232-line-drivers.html – Greg S Feb 03 '18 at 06:13
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    @RoyC the MAX3232 chip both reduces voltage and inverts the signal so that the raw rs232 signal is useable by a UART. – Greg S Feb 07 '18 at 16:48

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