Unable to communicate with Uno + RS232 shield

Hi all, i have this device which accepts serial commands over RS232 from either a PC or other devices. So, I got a RS232 shield for uno and using hardware serial code from arduino I have been trying to send RS232 command to this device but it does not respond. The device responds to same RS232 command when sent from a PC (teraterm). The RS232 communication is at baud rate of 11520. I also looked at the outputs of Uno+RS232 shield and PC on a scope and they look very close with some timing differences. I'm wondering if anyone here have used RS232 shield with uno and ran into similar problem?

Got a link to the RS232 shield?

Yes, I got it from cutedigi

http://www.cutedigi.com/product_info.php?products_id=4329

Okay, looking at the schematic, that shield just buffers the D0/D1 lines from the arduino - these are the same lines that are used for the USB interface. Thus you can only use one or the other.

That’s correct. So, I disconnected the RS232 shield from uno and connected USB to my PC to load the sketch. Once, it is loaded I disconnected the USB cable and re-connected the RS232 shield. I verified with the scope that TX pin on the shield is able to output the desired RS232 command however when I connect the device, nothing happens. Like I said in my earlier post, there are minor timing differences between two commands (PC and Uno) and I’m not sure if that is the culprit.

You may be fighting the simple DTE Vs DCE wiring requirement for RS-232 connectors. Are you sure that the RS232 connector is sending data on the pin that the external device is expecting? So the interconnection cable becomes one of the issues to be considered. An example would be if one tried to connect two PC computers that both have 9 pin RS232 connectors. As both PC connectors are wired as DTE devices they both send data out on pin 2 of the connector. Only if you wire the interconnected cable such that pin 2 on one end goes to pin 3 on the other and the same for pin 3 to pin 2. Search on null modems for better explanation as it also requires changes to the RS232 control pin numbers if they are wired up and being used.

http://en.wikipedia.org/wiki/Null_modem

Lefty

Yes, I'm aware of RS232 DTE and DCE requirement. So as you suggested, my connection from the shield to external device is not direct meaning that pin 2 (shield) goes to pin 3 (external device) and pin 3 (shield) goes to pin 2 (external device).