topic Re: RS485 Communication - Parity Error in LabVIEW

RS485 Communication - Parity Error

grimmy123 — Fri, 28 Oct 2016 12:16:48 GMT

I have an requirement to communicate with RS485 (in house power system).

I'm using an NI USB RS485 cable. (We made a custom connector since the pinout in
the power system is different).

The system uses 9 bit mode and they are using stop bit as their 9th bit. The
purpose of the 9th bit is to verify the address (0 0r 1).

In my program I'm using the parity bit as my 9th bit, this writes the command to the system and when I read the bytes in port i'm getting a parity error.

I've searched through all the related forums and tried their solution but nothing worked for me.

Re: RS485 Communication - Parity Error

RavensFan — Fri, 28 Oct 2016 13:05:07 GMT

Please don't attach 7z files, not everyone has 7zip nor wants to download a 3rd party app to view them when regular zip files are natively supported in windows.

Does your serial port have the possibility of setting 9 data bits? If this "in house power system" uses the "stop bit as their 9th bit", do they have an actual stop bit after that? (I don't know how a serial protocol would work if they didn't.

You are going to get a parity error if if the value of the parity bit doesn't match what the port expects based on the previous 8 data bits. There is no way around that. If you ignore the error, do the first 8 bits come through okay?

You might be able to get a serial port to work with this bit stream, but it sounds like a really hacked together protocol. If your serial port can be set for 9 data bits, that might solve the problem. If you can't, the I'd try to fix the device you are working with so that it doesn't behave that way or find another device that uses some standards on its data communication.

Other things you could do is set your serial port's partity to be "mark" or "space" rather than odd or even. Then you can use the presence or lack of an error as an indicator whether that 9th bit is a 1 or a 0. You might be able to set your port to 2 stop bits as well. But that is very hacky also.

Re: RS485 Communication - Parity Error

grimmy123 — Fri, 28 Oct 2016 13:17:18 GMT

Hi Raven,

Sorry about that, here is the zip file.

Yeah they Power System uses 2 Stop bit, 1 for 9th bit and other is actual stop bit.

I tried the program with all different possibility like sending the command as 9bit by enabling parity to Mark, and whil receiveing if the parity is None / space / odd don't receive anything in the buffer. If it's Mark / even Parity I get 15 bytes of data including the sent bytes with the parity error.

Actually this is my DUT to test the communcation, this is a part of my ATE system to validate the communication.

Re: RS485 Communication - Parity Error

RavensFan — Fri, 28 Oct 2016 13:52:21 GMT

Can your serial port be set for 9 data bits?

(I tried for the RS-232 port built into my laptop, and it gave an error.)

Can you purchase an RS-485 card that is capable of 9-bit data?

If this is your equipment you are trying to communicate with, why can't you get whoever created this equipment to standardize on an 8 bit protocol?

One of the tricky parts about a 9 bit serial port is that the data you send it is based on a standard computery byte of 8 bits. You send a string to VISA Write (or get one back from a VISA read) and that string is actually a series of bytes that are all 8-bit. The underlying datatypes don't have a provision for a 9th bit without doing some hacking before or after a VISA write or read. 7, 6, 5 bit protocols (7 still being somewhat common, I've never seen 6 or 5 in my life) don't have that same problem because they can all fit in a 8-bit byte. (ASCII character or U8).

When I googled "serial port with 9 bits", the top link took me to an NI article.

http://digital.ni.com/public.nsf/allkb/3BDC7FF03541F772862564990057F919

Read that, it seems to show some tweaks you can try to make 9 bit communication work with 8 bits and parity, and get it to return the data even with a parity error.

Re: RS485 Communication - Parity Error

grimmy123 — Fri, 28 Oct 2016 14:24:43 GMT

Hi Raven,

Since the system is responding for the command state that I write with Parity Mark, I assume that it's working as 9 bit communication.

The communication method is cannot be changed because the same method is used as a master / slave communication in legacy systems as well.

Regarding the ini file, that's the first step I tried when I got that error, but still no improvement. Even making the ini file to disable error replacement to 1 I was getting parity error.

Re: RS485 Communication - Parity Error

billko — Fri, 28 Oct 2016 14:30:03 GMT

You're sure it's not a real parity error induced by the house-built cable?

Re: RS485 Communication - Parity Error

RavensFan — Fri, 28 Oct 2016 14:31:40 GMT

When you disable the parity error replacement, do you get a byte that makes sense for the first 8 bits?

You can expect to get parity errors. That is normal based on the fact that the device is not trying to make that bit a parity bit. The presence of the error or lack there of will tell you whether the 9th bit (the 9th data bit on the device side, the parity bit on your computer side) is a 1 or a 0. As long as you get the first 8 bits through okay even if there is a parity error, you can then join the U8 that is either a 1 or a 0, with the U8 that you get from the VISA read of 1 byte. (you'll have to typeast the 1 string character to a U8 byte.) Now you'll have a U16 to contain that contains your 9 bits of data.

Re: RS485 Communication - Parity Error

grimmy123 — Fri, 28 Oct 2016 14:44:45 GMT

Billko,

I hope it's not, The cable connection are as follows,

Re: RS485 Communication - Parity Error

grimmy123 — Fri, 28 Oct 2016 14:51:35 GMT

Raven,

I'm getting the bytes at port if the party is mark, with/ without updating the ini file. But the parity error is still there?

The system enginner says receive is only 8 byte (only sending the data is 8+1 bit for 8 bytes), so I tried to make the parity property to none before reading. If I make it to none I don't get the data. Only if the party is Mark I'm getting the data at port.

Re: RS485 Communication - Parity Error

RavensFan — Fri, 28 Oct 2016 15:51:44 GMT

I suggest you read up on some resources on serial communication. I feel like you don't understand what a parity error is.

Parity has nothing to do with your cable, jus the pattern of 1's and 0's that make up a byte. (That image is too small to read anyway.)

"The system engineer says receive is only 8 byte (only sending the data is 8+1 bit for 8 bytes),".

Be careful on your wording. You are talking about bytes and bits simultaneously. A given message can be X bytes long. Are you saying that each message is 8 bytes?

If your parity is none, you get a start bit, 8 data bits, and a stop bit. If the device is sending 9 data bits, and you set your parity to none, you won't get a parity error. You will probably get a framing error. And you won't get any data.

Let's say you have a pattern of 8 data bits of 1010 1010. If your parity is even, you'll get a 0 for the parity bit (to keep an even number of ones). If your parity is odd, you'll get a 1 fo rthe parity bit (to make an odd number of ones.) (This gets calculated for each group of 8 data bits.)

If your parity is mark, you're looking for a 1 no matter what the data pattenr. If your parity is space, you're looking for a 0 no matter what the data pattern.

If the incoming parity bit doesn't match up with the expected pattern, then you'll get a parity error. With normal devices, this would be a true error indicating something is wrong with the data pattern. With your "legacy 9-bit system" (which should be taken out back and shot), you need to hack the protocol to use the parity bit as a placeholder for the 9th data bit. If that bit doesn't match what is expected, you get a parity error. But in your case it is not a real communication error.

If you get a parity error, and parity replacement is enabled, then whenever you get an error, the 8 data bits are discarded and replaced with something else like the null character 0x00. You'll get an error message. If parity replacment is disabled, but the parity doesn't match, the 8 data bits come through okay, but you'll still get an error that you can discard as an error, but use the presence of that error to indicate whether that parity bit (the "9th data bit" on your device) is a 1 or a 0.

Re: RS485 Communication - Parity Error

billko — Fri, 28 Oct 2016 16:19:34 GMT

@RavensFan wrote:
I suggest you read up on some resources on serial communication. I feel like you don't understand what a parity error is.

Parity has nothing to do with your cable, jus the pattern of 1's and 0's that make up a byte. (That image is too small to read anyway.)

"The system engineer says receive is only 8 byte (only sending the data is 8+1 bit for 8 bytes),".
Be careful on your wording. You are talking about bytes and bits simultaneously. A given message can be X bytes long. Are you saying that each message is 8 bytes?
If your parity is none, you get a start bit, 8 data bits, and a stop bit. If the device is sending 9 data bits, and you set your parity to none, you won't get a parity error. You will probably get a framing error. And you won't get any data.

Let's say you have a pattern of 8 data bits of 1010 1010. If your parity is even, you'll get a 0 for the parity bit (to keep an even number of ones). If your parity is odd, you'll get a 1 fo rthe parity bit (to make an odd number of ones.) (This gets calculated for each group of 8 data bits.)

If your parity is mark, you're looking for a 1 no matter what the data pattenr. If your parity is space, you're looking for a 0 no matter what the data pattern.

If the incoming parity bit doesn't match up with the expected pattern, then you'll get a parity error. With normal devices, this would be a true error indicating something is wrong with the data pattern. With your "legacy 9-bit system" (which should be taken out back and shot), you need to hack the protocol to use the parity bit as a placeholder for the 9th data bit. If that bit doesn't match what is expected, you get a parity error. But in your case it is not a real communication error.

If you get a parity error, and parity replacement is enabled, then whenever you get an error, the 8 data bits are discarded and replaced with something else like the null character 0x00. You'll get an error message. If parity replacment is disabled, but the parity doesn't match, the 8 data bits come through okay, but you'll still get an error that you can discard as an error, but use the presence of that error to indicate whether that parity bit (the "9th data bit" on your device) is a 1 or a 0.

Well, parity does have something to do with the cable. How else do you end up with a different pattern of bits than was expected? Of course, this is assuming that the data was interpreted correctly in the first place, which, as you point out, may be suspect.*

Edit:

* The cable would have to be really bad, like poor shielding and/or not twisted pair.

Re: RS485 Communication - Parity Error

RavensFan — Fri, 28 Oct 2016 16:24:52 GMT

@billko wrote:
Well, parity does have something to do with the cable. How else do you end up with a different pattern of bits than was expected? Of course, this is assuming that the data was interpreted correctly in the first place, which, as you point out, may be suspect.*

Edit:
* The cable would have to be really bad, like poor shielding and/or not twisted pair.

True. But a wiring diagram of the cable is going to prove it is a a problem. (Especially in this case where the problem as stated was a device that doesn't adhere to common RS-xxx standards.) It would either be a correctly wired cable and basically works unless there are other problems. Or an incorrectly wired cable which won't work at all. You would only get a parity error based on a cable if it is otherwise wired correctly, but is poorly constructed, i.e. flaky.

Re: RS485 Communication - Parity Error

grimmy123 — Fri, 28 Oct 2016 16:28:57 GMT

Raven & Bill,

I had a KB of what parity is when I start having this error, anyway I'll go through again.

Here's what I'm doing,

Sending this 8 byte by U8 to String function, with Parity Mark,

Byte0 0x01 0b 1 0000 0001

Byte1 0x00 0b 0 0000 0000

Byte2 0x00 0b 0 0000 0000

Byte3 0x02 0b 0 0000 0010

Byte4 0x03 0b 0 0000 0011

Byte5 0x21 0b 0 0010 0001

Byte6 0x00 0b 0 0000 0000

Byte7 0x00 0b 0 0010 0001

The unit respond with 8 byte of data with Parity Error.

Attached the Pin diagram

Re: RS485 Communication - Parity Error

billko — Fri, 28 Oct 2016 17:03:23 GMT

@RavensFan wrote:
@billko wrote:
Well, parity does have something to do with the cable. How else do you end up with a different pattern of bits than was expected? Of course, this is assuming that the data was interpreted correctly in the first place, which, as you point out, may be suspect.*

Edit:
* The cable would have to be really bad, like poor shielding and/or not twisted pair.
True. But a wiring diagram of the cable is going to prove it is a a problem. (Especially in this case where the problem as stated was a device that doesn't adhere to common RS-xxx standards.) It would either be a correctly wired cable and basically works unless there are other problems. Or an incorrectly wired cable which won't work at all. You would only get a parity error based on a cable if it is otherwise wired correctly, but is poorly constructed, i.e. flaky.

I pretty much agree with what you said. In fact, your post is pretty much a re-iteration of what I said. Which is fine, because the more different ways of saying the same thing, the better chance of everyone understanding everything. To me "flaky" would include a cable constructed without twisted pair for an RS-485 cable. And the diagram doesn't call out for twisted pair, so I can only guess. That's why satin RJ-45 (flat ribbon) is improper for a network connection.

But now that that is out of the way, I agree that it looks like the problem is not with the cable. I'm not sure how you can set up a serial port to have unique parameters for input and output.

Re: RS485 Communication - Parity Error

RavensFan — Fri, 28 Oct 2016 19:01:53 GMT

With those 8 bytes that you receive, do you care at all that the first byte as a 1 in the 9th bit and that the other 7 bytes have a zero?

First, make sure you have parity error replacement DISABLED.

If you do not care about that 9th bit, then just read 8 bytes (do not use bytes at port), and ignore the parity error that you are guaranteed to get. Somewhere there is a function called Clear Specific Error. You could also use the general error handler. Or unbundle the error code and feed it to a case structure that sends no error if it is a parity error, and passes through all other errors and warnings.

If you do care about the value of the 9th bit, then you need to read that serial port 1 byte at a time, determine if that byte is a parity error. I would set the parity to Space right before the VISA Read. With the data stream you have, you'd only have an error with the first byte which has a 1 as the 9th bit.

Re: RS485 Communication - Parity Error

grimmy123 — Fri, 28 Oct 2016 19:25:01 GMT

Raven,

I suspect the receiver has 8 bytes with 8 bit of data / byte and not (9 bit). I need to probe in the signal to make it sure.

Actually i'm just clearing the error as of now, but I read in some forums that if there is parity error clearing error is not a good way. So I posted here to upgrate if something I'm missing.

There is no error when writing the bytes, the error is generated only while reading the bytes. I just tried space / none / even / odd parity setting before read (as you can see in the code) but i'm not getting the data when I read it. So I stick with Parity Mark where I atleast receive the data correctly.

I'll also try the other solutions and will update you.

Re: RS485 Communication - Parity Error

RavensFan — Fri, 28 Oct 2016 19:42:46 GMT

Actually i'm just clearing the error as of now, but I read in some forums that if there is parity error clearing error is not a good way. So I posted here to upgrate if something I'm missing.

I'd have to see those particular messages myself. I believe that statement would be correct if you were getting parity errors because of the wiring or noise problems corrupting data, but otherwise would not have any parity errors.

In your case, you can expect a parity error has normal part of operation because of the abnormal 9-bit protocol the device implemented. So just you can just safely clear the parity error you do get.

Re: RS485 Communication - Parity Error

grimmy123 — Wed, 02 Nov 2016 12:28:36 GMT

Hi Raven,

I have analyzed the data, it's the same 9bit data / byte with first byte is Mark (1) and remaining 7 byte is space (1).

I'm gonna read the port byte by byte and check for any error's.

I'm gonna use for loop with parity changing from mark for first byte and space for the second byte. I'll post the findinds soon.

Re: RS485 Communication - Parity Error

SSPTest — Thu, 03 Nov 2016 19:13:44 GMT

Hi Grimmy,

Whenever you get a chance to post your findings we will be able to troubleshoot further. Any additional information you can think to share is always encouraged!

Re: RS485 Communication - Parity Error

grimmy123 — Tue, 08 Nov 2016 20:36:51 GMT

Hi SSPTest,

I hope I should clear the parity error. I didn't find any options, I have modified all the possibilities like writing into bytes or reading in bytes nothing worked.

Appreciate all your support...