RS485 LabVIEW interface not working with a constant

Question

I need to control an RS 485 interface (not a National Instruments device).
I found a vi written by a colleague which I have attached.
This works perfectly, so I used a probe to identify the value being generated, and used this to replace the variables with a constant.
Now the vi doesn’t work. It runs, but the device it is connected to no longer activates. Then I found that by replacing only one variable with the constant, and keeping the other as a variable kept the vi working.
Can anyone offer any suggestions please ?
Of course, where I really want to get to is a vi that just reads and writes to a non-NI RS485 interface.
Many thanks.
Attachments:
RS485_usb.vi ‏55 KB

Answer 1

Using NI Measurement and Automation Explorer, I sent vset50\n and the instrument responded correctly by going to position 50.
Using the 485 example http://zone.ni.com/devzone/cda/epd/p/id/2674      202314.vi    The documentation states that this example will only work with National Instruments RS-485 interfaces. I don’t have and NI 485 and not sure what difference it will make.
In 202314.vi    I enabled the termination character  since  RS485_usb.vi ‏55 KB uses this and it works. Not sure what to do about Assert DTR and transceiver mode so I tried all combinations but kept getting error code 1073807330.
I looked at (Help>>Find Examples..>>search term "485"), but couldn’t pick anything out that I could apply here.
Yes, the randome numbers are now string constants in the block diagram. The string constant, now including \n, is placed at the VISA write input.
I’ve used Measurement and Automation Explorer to send the same command repetitively and there is no problem with this, the instrument jiggles back and forth and rests at the original position.
When the instrument doesn’t work, the vi completes without errors, but there is no instrument activity.
Any thoughts would be appreciated.