I am trying to hook up my Jackson Labs' LC_XO GPS disciplined oscillator (GPSDO) to my computer motherboard COM/serial port so I can talk to it and for it to send back NMEA messages. Basically, I am trying to connect a serial TTL CMOS device to a computer motherboard RS232 10-pin IDC header.
The LC_XO uses +3.3V serial TTL communication interface. Therefore, to connect it to a COM/serial port on a modern computer motherboard, I need to convert the serial TTL voltages from the LC_XO to computer RS232 compatible voltage levels, correct?
Attached is a wiring diagram using two MAX3232 (from SparkFun) that I am using to convert the PPS, RX, and TX coming from the LX_CO into RS232 voltage levels.
The issue that I am having is that using an oscilloscope, the voltage levels coming out of the MAX3232 are not -12V to +12V, but -3V to +3V which I believe is too weak and get lost in the background noise of the RS232 port voltage levels, correct ? Don't modern COM ports expect -12 to +12V and anything weaker than that get lost in the noise?
The LC_XO is able to stream NMEA messages and I can see those using Putty (115200, 8N1), but I cannot send any commands to the LC_XO.
Any suggestions on what I am doing wrong?
I also tried swapping the TX/RX connectors, but that didn't help (I lost the NMEA message stream.)
The attached PNG has the specs for the LC_XO.
Here is the spec sheet for LC_XO:
UPDATE: Sep 30, 2021:
As Peter Bennett suggested below, I rewired the "TXD --> LX_CO ser_in" channel and here is my final configuation. As DoxyLover suggested, I also added common Grounds. I am getting NMEA messages so I will leave it at that...still cannot send any commands though...I occasionally get a "Command Error" if I send the command a bunch of times using Putty, 115200, 8N1, Local Line Editing and Local Echo set to Forced On. Thank you all for assistance!