What I write to buffer is still in buffer when I read

[jrdwight]

I'm trying to communicate with an instrument via serial. According to the instrument's manual, when I write "SP", the instrument should respond with with "Maximum Speed: [value]". When I try this in NI MAX, however, what I read is "SP \r \r Maximum Speed: [value]". It seems the command I write is still in the buffer when I read from the buffer. Is this typical for serial communication? Is there a way to avoid this? I can't clear the buffer after I write "SP" because the instrument responds immediately, so then I'd be clearing the information I'm trying to read.

[santo_13 · Accepted solution]

It may be typical for some instruments to echo the command along with the response. Check your instrument manual if that is expected.

[crossrulz]

Also, is this 2-wire RS-485?  I rarely deal with RS-485, but I remember reading what I sent before being able to read the response when I did.

You need to give a lot more information on your setup before we can really tell you if this is expected.  What is the instrument?  What is the protocol being used (RS-232, RS-422, RS-485, etc.)?

[jrdwight]

I determined this was the expected behavior for the instrument I was communicating with (which was via RS-422 by the way). I had it in a mode where it echoed every command it received. I was able to change the operating mode so that this echo is disabled.

[crossrulz]

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@jrdwight wrote:

I determined this was the expected behavior for the instrument I was communicating with (which was via RS-422 by the way). I had it in a mode where it echoed every command it received. I was able to change the operating mode so that this echo is disabled.

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That is common for instruments that want to interface with a terminal program such as Putty.  Thank you for letting us know what you found.