While performing a loopback test, how do I determine wether the connection is RS-232 or RS-485?

[s.garcia]

I know that for the RS-232 loopback test I should short pins 2 to 3. For the RS-485, short the pins 4 to 8 and 5 to 9. I do this, but the only test that works is the RS-232 one. I was wondering if I had to specify for what connection I was performing the test. All help is appreciated, thanks. P.S. All of this is for the D-sub 9 pin port. I'm sorry for posting this message under the wrong topic. This probably has nothing to do with Field Point. Even so, if someone could help, I would really appreciate it. -Sergio

Message Edited by s.garcia on 07-06-2009 11:28 AM

[RavensFan]

For RS-485, there are no standards as to which pins are associated with transmit and which are associated with receive.  You'll have to look up that information in the manual of whatever RS-485 port you are using.

[s.garcia]

I found this page in NI.com (http://zone.ni.com/devzone/cda/tut/p/id/3450), where there is a diagram of the DB-9 pin definitions for both RS-232 and 485. It says that pins 4 and 5 are the recieving pins, and pins and 8 and 9 are the transmitting ones.

pin 4 -> Recieve +

pin 5 -> Recieve -

pin 8 -> Transmit +

pin 9 -> transmit - 

Therefore the loopback test should be done by connecting pins 4 to 8, and 5 to 9.

Hope this helps you understand my confusion.

Thanks.

[Sammy Z · Accepted solution]

Hi Sergio,
You don't need to pick the protocol.  The COM port of your computer is RS-232, so you can only do the test for RS-232 configuration.

Regards,

Sammy Z.

[s.garcia]

Oh ok....thanks Sammy.