Read MODBUS Registers

[buickgn]

It's a USB/Serial converter.

[Wayne.C · Accepted solution]

buickgn,

This may be a problem with the USB converter/driver.  Suggest you modify your vi to use a VISA Write to write out your known good query string, wait and then read the expected number of bytes and see what happens.

[buickgn]

Wayne,

It works fine with the Modbus Poll program which leads me to believe it's not a driver issue. I tried your suggestion here:

It sends the right command, however, in the "Read Buffer" it just repeats the command I sent.

[Wayne.C]

buickgn,

I don't think you need the carrige return on the end of the write string.

[buickgn]

You're right, I got rid of that. Still repeating the request. Here's a screenshot of the trace:

[Wayne.C]

Is the written command being echo'd by the serial adapter?  What happens if you try to read more characters with the simple read/write string vi?

[buickgn]

If I set it to 17, I get: 0203 0002 0002 65F8 0203 0439 0F7F 7394 79. Anything larger than 17 and it throws a timeout.

[buickgn]

Wayne,

That was the issue, it's sending back the correct response after it echos the request. I guess the Modbus library doesn't play nice with this particular usb/serial converter!

[Wayne.C]

Can you disable the echo?  Any driver settings?

[buickgn]

It doesn't look like it. I'm in device manager looking at all the settings and don't see anything to enable/disable the echo.