topic Re: Overwrite byte array in LabVIEW

Overwrite byte array

lvrichard — Wed, 27 Jul 2016 15:11:49 GMT

Hi All,

Please see me attached code, where i am receiving serial data, converting it into a byte array and will be using a state machine to process it.

I am writing the data into an array and also displaying it via a string indicator. In order for the data to remain on the screen, I have used a feedback node. However, this means the data is appended everytime it is sent (at 10 second intervals). How can i get the data to remain on the screen when sent, and then be over written once new data is received, both in the indicator and the array??

Thanks,

Richard.

Re: Overwrite byte array

RavensFan — Wed, 27 Jul 2016 15:35:56 GMT

First, don't use bytes at port. You have enabled a termination character of a line feed, so rely on that. Just read a large number of bytes.

Second, in your code, put the concatenation and building of an array in a case structure. If the number of bytes read is zero, do nothing. If >0, then do the concatenation.

Third, check spelling. "Received" not "recieved".

Re: Overwrite byte array

lvrichard — Wed, 27 Jul 2016 17:25:22 GMT

Hi,

Thanks very much for your reply..

I dont fully understand the theory behind placing the concat strings + building array in a case structure? What will this achieve ?

Thanks,

Richard

Re: Overwrite byte array

Bob_Schor — Wed, 27 Jul 2016 17:42:51 GMT

Did you understand the bit about reading, say, 1000 characters? If you (a) specify Enable Termination Character (which you did), (b) specify the correct termination character for your device, and (c) have a "reasonable" Timeout (which you do), then if your expected VISA "message" is less than 1000 characters, you will "pause" in the VISA Read until the entire however-many-characters have been read, or no Termination Character was received (and nothing is returned).

Assume you got a legitimate String (read the Help to see how to tell if this is so). If so, you have ... a String. You can put it in a String indicator (no need for a feedback loop), and can use String to Byte Array to convert the entire String to Bytes. You'll then loop, waiting for the next VISA read to complete.

Bob Schor

Re: Overwrite byte array

crossrulz — Thu, 28 Jul 2016 01:35:18 GMT

Let's take a step back before we progress any further down this rabbit hole. What are you trying to communicate with and what is the protocol of the messages being sent back and forth? This does make a huge difference in how you should approach the problem.

Re: Overwrite byte array

lvrichard — Thu, 28 Jul 2016 05:06:40 GMT

Thanks for the repays so far everyone!!

I'm communicating with a power supply unit which sends out a very basic stream of data (consisting of mainly flags). It's a burst of data which occurs at 10 second intervals consisting of one start bit, one stop bit and a varie data length between.. Usually 8 bits. It's all in house protocol which has been used for years

Re: Overwrite byte array

lvrichard — Thu, 28 Jul 2016 06:58:55 GMT

Please ignore my spelling... it was early.

replies*

varying*

Re: Overwrite byte array

lvrichard — Thu, 28 Jul 2016 07:29:00 GMT

Okay this seems to be displaying as i would like, but i can't stop the program!!

help please.

thanks,

Richard.

Re: Overwrite byte array

lvrichard — Thu, 28 Jul 2016 10:51:32 GMT

Anyone konw how to get round this? I have tried a few more methods but cannot get ther result I need. As stated, the data is now being displayed as i would like i.e. updating each time it is sent. However, I cannot stop the loop as it seems to be stuck in the read state ?

once again help appreciated

Re: Overwrite byte array

RavensFan — Thu, 28 Jul 2016 13:42:39 GMT

Not sure what you are talking about here. There is no "read state". You have a while loop. Inside of that it does the reading. The way to stop the while loop is to press the stop button like you have it wired up.

So where are you stuck?

Re: Overwrite byte array

lvrichard — Thu, 28 Jul 2016 14:27:34 GMT

The problem is.. when i press stop, the program does not stop!!

Re: Overwrite byte array

johnsold — Thu, 28 Jul 2016 14:41:26 GMT

Have you waited ten seconds for the next Read to complete? There does not appear to be anything in your VI that would prevent it from stopping. Dataflow will not let it stop until the VISA Read completes or times out. The Byte to Double subVI is missing but that seems unlikely to be the problem.

Lynn

Re: Overwrite byte array

lvrichard — Thu, 28 Jul 2016 14:56:23 GMT

Hi,

If i hold down the stop button for ten seconds (it pops back out on its own accord before this time).. then yes the program does indeed stop.. so this is the problem. How can i get around this ? Furthermore, i get the time out error message when the progam does finally stop!

Thanks,

Richard.

Re: Overwrite byte array

johnsold — Thu, 28 Jul 2016 15:13:52 GMT

Richard,

You need to learn how the LabVIEW dataflow paradigm works.

Within your while loop anything which does not depend on anything else may run in parallel. So the Stop button and all the other code are parallel. This effectively means that the Stop button will be read at the very beginning of each iteration. Because the mechanical action for the Stop button is set to Latch When Released, it will pop back out as soon as it is read. But the rest of the code, particularly the VISA Read will take about 10 seconds. The while loop will not decide whether to stop or not until everything inside has completed. So, even though the Stop button may have been read seconds ago, the loop will not stop until after the Read. Holding the button down should not make any difference. Once it has been read the value will remain on the wire going to the loop termination terminal until the next iteration.

How can you get around it? You need to make everything else in the loop take less time. But shortening the timeout on the VISA Read may cause problems with getting your data since the device only sends data once every 10 seconds. There are ways to work around that but they can get complicated and the details depend on how the data coming on the serial port is formatted and how you will handle partial messages.

Lynn

Re: Overwrite byte array

lvrichard — Thu, 28 Jul 2016 15:28:37 GMT

Thanks again for your reply,

I will continue tomorrow. Should i be going down a producer/ consumer route ?

Thanks,

Richard.

Re: Overwrite byte array

johnsold — Thu, 28 Jul 2016 15:36:25 GMT

A producer/consumer architecture might be useful in decoding the partial messages.

Lynn

Re: Overwrite byte array

crossrulz — Fri, 29 Jul 2016 02:27:14 GMT

Since you are apparently dealing with binary data instead of ASCII, you want to turn off the termination character. I also recommend using the Bytes At Port to detect if a message has started. If it hasn't, wait for something like 100ms. This will make your stop button more responsive and avoid those timeout issues. And since it looks like you are expecting 10 bytes in a message, just read the 10 bytes.

Re: Overwrite byte array

RJ_15 — Fri, 29 Jul 2016 03:31:27 GMT

You can also use 'Enable Event' with event type being your Serial TermChar and then 'Wait for Event' in your while loop with a small timeout.

So, you will read the available bytes only when your event is generated that would prevent unnecessary polling.

Runjhun

Re: Overwrite byte array

crossrulz — Fri, 29 Jul 2016 03:34:33 GMT

@RJ_15 wrote:
So, you will read the available bytes only when your event is generated that would prevent unnecessary polling.

That is still technically polling (wait for event, timed out, try again, timed out, etc.) and I have found it to add unneccessary complication.

Re: Overwrite byte array

lvrichard — Fri, 29 Jul 2016 08:10:34 GMT

Thanks everyone for their input in this.. Im learning lots and really appreciate it.

Crossrulz thanks very much for taking the time to make that program. Perfect thankyou!!