DATA SENT BY RS-485 ECHOED BACK

[Dirk_Steel]

I'm using a pci-485/2 board to communicate with a multidrop network of 4 West 6701 limit controllers. Whenever I send data the same string is echoed back. I intially checked out the port using a vi I got on the NI website to write and read from the same port. This worked but I can't seem to communicate with the instruments. I even get the string sent echoed back if I turn off the chassis containing the limit controllers. When I did the serial loopback test I tied together pins 4 and 8 and 5 and 9 on the DB-9 connector per the vi instructions. I notice that the way my duplex network is set up (per the NI Serial manual) I'm making the same connection at that port. Any ideas?

[Chris_Rake]

Are you using a full-duplex or half-duplex network (are TX+ and RX+ the same line?). If so, you probably need to operate in a different transceiver (wire) mode. I suggest using "485 Auto 2-Wire" which can be set either through a VISA property or the device manager. This will enable the transceiver during transmission while disabling the receiver, thus preventing echos.

Regarding failure to communicate with your device, this may be a separate issue - how long is your cable, are you using termination, what baud rate are you using, etc.