Why does VISA serial support show transmit buffer echo to receive buffer if I'm NOT looped back on the port?`

[gjc]

Pardon my stupidity, but I'm new to this and trying to use basic VISA functionality (from NI-MAX) to verify serial port functionality.  I connect through COM1 to UUT.  When I boot UUT, and check receive buffer with VISA GUI tool, it looks fine.  I see proper data that is streaming from UUT.  The receive data ends at a boot prompt sent by UUT. 

But afterwards, when I transmit  ANY data, and follow that by looking at receive buffer, all I receive is exactly what I transmitted (minus line-feed character).  I know that the UUT is actually sending repeated login prompts if I do not send the proper login ID, but I NEVER see these once I've transmitted for the first time. 
What am I missing?  Thanks for any info....

[Adam W]

Hi gjc,

Are you using RS-232 or RS-485?  Also, I would double check and make sure you are using the correct cable.

Another thing you may want to try is to run a NI SPY capture while you communicate using the VISA Test Panel in MAX.  Try doing that and post the SPY capture when you get a chance.

Thanks,

Adam W
National Instruments

[gjc]

Adam,

Think I found the problem.  I was terminating the transmit string with the wrong character.  Changed from line-feed to carriage return and I seem to have normal communications now.   Still have no idea why I was seeing that loopback type functionality before, but I'll move on 'til I hit the next roadblock.  I may be using NI-spy at that time!

Thanks very much for your help.