This is one of the most maddening debugging problems I can remember.
I had my laptop talking to a motor controller over a RS232 serial connection to COM1, in a program I wrote. Then I unplugged and moved everything to a different location. Now there doesn’t seem to be any communication at all.
Everything was 9600,8,N,1 which is the default for the motor controller.
Neither my own program or a serial communications program seems to do anything with the motor control.
The serial communications program still works normally with a precision thermometer that also communicates with those settings. I can have it send data, and they scroll by on the laptop screen.
I have alternates for the motor controller and removed one from service to try it here, but there is still nothing.
The serial cable still tests good with an ohmmeter.
If I put an AC voltmeter between pins 2 (Rx) and 5 (ground) on the PC end of the cable, I see a few volts AC when the motor controller is supposed to be transmitting, and I see about the same thing when I do this to the cable from the precision thermometer. So, while pins 2 and 5 seem to have similar signals as seen by a voltmeter, the terminal program does not register if the source is the motor controller.
I have tried wiggling each component I can think of while data should be flowing from the motor controller to the laptop, but don’t see anything.
I just verified the settings of 9600,8,N,1 for probably the 8th time on the terminal software, the thermometer, and both the motor drives I have available.
There’s no gibberish, no lights showing RX or TX changing state, nothing.
I don’t have any serial diagnostic equipment but am going to go try to find an oscilloscope…