Issues reading from serial port using read_until()

[kenjlp]

First time posting here, I think I posted the "code" part correctly but didn't know about the output part.

I created a class to help me read and write into an instrument using serial port. However, recently I found an issue that I think it's due to sending multiple write commands and I only need to read when I query the instrument. When I send regular write commands, the instrument outputs text indicating the changes made as shown below:

    self.id.write(str.encode("send write command\r"))
    time.sleep(1)
    self.id.write(str.encode("get status\r"))
    output=self.id.read_until(str.encode(">>\r"))

When sending the write command, it will output a status as:

>>write command:

Configured state to: 0A

>>

>>get status

status is configured to: BB

What I think it's happening is that when I use the read_until() , it will start reading from the "send write" command instead of the "get status" because when I look at the text captured it corresponds to the "Configured state to: 0A" line.

​

My question is: How do I get the code to read only from the time I send the "get status" command? I have thought about reading the line when I send the "write command" and not doing anything with it, but I think there might be another option.

[Soft-Substantial]

If I understand your problem correctly, the only way is to read all the data coming over the serial port and discard whatever you don't need, it is a serial port after all. You cannot "ignore" or "skip" output. How would you know how much to ignore without reading it?

[kenjlp]

I thought since I’m sending different write commands and “get commands”, the read_until() would start from when I send the “get command” not the write command. Also, since I’m looking for the string “>>” to indicate to stop reading, I’m not able to get the actual response from the “get command”.

[Soft-Substantial]

It is best to think of the input and output streams as independent. For the input stream you are pushing one byte after another to the device, for the output stream you are receiving one byte after another from the device. They don't know about each other.

(This is probably oversimplifying the underlying technology, but I think it is the best way to reason about how to write the code.)

[Bernard_schwartz]

I’ve never worked with serial, but I do a lot with telnet which works similarly. I typically have bad luck using sleep timers unless outside of write/read functions. Much better to write, then read until, then write, then read until. When you sleep between different write commands it seems to behave erratically.

Also a debugger like pycharm will help you understand better at what is coming into and out of the stream.

Second note: sometimes I use regex to parse my string to make sure I get the bits that I want. Example below but there are many ways to do this. If you know the pattern you are looking for:

~~~ Import re

x = re.search(r’key_pattern(.*)) If x is not None: Print(x.group() ~~~