CRC checksum algorithm

[Guest]

Hello,
I’m working with handheld device that communicates with windows service thru
the serial port.
Transaction between device and my application looks like this:

Handshaking:
Device sends ENQ
Service responds DLE 0
Device sends DLESOH4168840000SR01L01DLEETX4á

As you see at the end of the string, after DLE ETX (Data Link escape and end
of text) control codes, device sends two more characters 4á.
If I’m not mistaking, this would be the calculation of CRC checksum of this
string.

After each transaction, I need to calculate CRC and compare to the one that
device sends. Just to make sure that we didn’t miss any data.
I tried many different ways, but can’t come up with the correct algorithm to
get 4á on this string.
Is anybody can give me a hand on this one?

Thanks

 

[Dick Grier]

Hi,

This might be a CRC or, perhaps, a simpler checksum. Since it is only a
single byte, my guess is that it is not a CRC (though I have CRC-8 code in
my book, its use is quite uncommon).

You are printing the data as ASCII/ANSI (text), so the font used can confuse
things. If you print out the actual binary data received, I may be able to
run a quick check to see what algorithm has been used.

Dick

--
Richard Grier (Microsoft Visual Basic MVP)

See www.hardandsoftware.net for contact information.

Author of Visual Basic Programmer's Guide to Serial Communications, 4th
Edition ISBN 1-890422-28-2 (391 pages) published July 2004. See
www.mabry.com/vbpgser4 to order.

 

[Charles Law]

This looks similar to an Allen-Bradley protocol, so you might want to look
this up. As the last character is only 8-bit it is more likely to be a BCC
(binary check character) rather than a CRC, which is usually at least
16-bit.

The normal way to calculate a BCC is to sum the data MOD 256. Bear in mind,
though, that _data_ does not mean the entire binary string. BCC is usually
calculated on only a part of the string. In the case of an Allen-Bradley
protocol this would be all data between (but not including) the DLE SOH and
DLE ETX. Also, A-B actually takes the two's complement of the resulting BCC.

Ultimately, the only way to know for certain is to get some reference for
the equipment you are working with from the manufacturer.

HTH

Charles

 

[Guest]

Dick,
Here is the actual binary data of this string:

?bRead()
{Length=23}
(0): 16
(1): 1
(2): 52
(3): 49
(4): 54
(5): 56
(6): 56
(7): 52
(8): 48
(9): 48
(10): 48
(11): 48
(12): 83
(13): 82
(14): 48
(15): 49
(16): 76
(17): 48
(18): 49
(19): 16
(20): 3
(21): 52
(22): 225

 

[wg]

I am in the process of looking for code for a CRC 16H checksum algorithim.
If anyone has this or a reference to where to find one it would be greatly
appriciated. I am writing an interface to old old PLC (Programmable Logic
Controller).

Thanks

wg

 

[wg]

Dick,

I took a look at you book on your web site. As mentioned in a post I am
working on an older PLC interafce to OPC. I have written OPC servers before
so this is not a problem. I have never writtin a serial communications in
VB.NET until now. I have gotten an example rs232 class that seems to work
fine. I will have to write a CRC algoritham and since this interface is
attached to 10 PLC's on 10 serial ports I was thinking of multi-threading
the it. What books would you recommend I look into? I would really rather
not use the MSCOMM.OCX from VB6.

Thanks


wg

 

[Richard L Rosenheim]

I don't know if you are able to use a beta product or not, but just to let
you know, Visual Studio 2005 includes a serial communication control.

Richard Rosenheim

 

[wg]

Richard,

I have not downloaded it yet. I am worried about it being a beta version.
Will this affect anything?


wg

 

[Guest]

wg,
You have a few options:
You can get free Visual Basic Resource Kit from Microsoft. I has in it the
Sax Communications Community Edition .NET Serial control. Something like
MSComm control.

Also if you look at Dick Grier web site www.hardandsoftware.net , you can
download NETComm.ocx control.

Val

 

[Dick Grier]

Hi,
I will have to write a CRC algoritham and since this interface is
attached to 10 PLC's on 10 serial ports I was thinking of multi-threading
the it. What books would you recommend I look into? I would really rather
not use the MSCOMM.OCX from VB6.
<<

Threading is OK.

Easiest my be to download DesktopSerialIO dll from my homepage. It already
implements threaded receive, so you don't really need anything more. It is
practical to simply use the OnComm events, which execute in the object's
worker thread context.

I have CRC examples in my book, and I suspect that the CRC-16 (CCITT) is the
one that you need.

Dick

--
Richard Grier (Microsoft Visual Basic MVP)

See www.hardandsoftware.net for contact information.

Author of Visual Basic Programmer's Guide to Serial Communications, 4th
Edition ISBN 1-890422-28-2 (391 pages) published July 2004. See
www.mabry.com/vbpgser4 to order.

 

[Dick Grier]

Actually, the DesktopSerialIO dll that I have placed on my site is better
(IMO) than NETComm.ocx for .NET applications, unless you are porting from
VB6 -- where it is just about a wash.

Dick

--
Richard Grier (Microsoft Visual Basic MVP)

See www.hardandsoftware.net for contact information.

Author of Visual Basic Programmer's Guide to Serial Communications, 4th
Edition ISBN 1-890422-28-2 (391 pages) published July 2004. See
www.mabry.com/vbpgser4 to order.

 

[Dick Grier]

Hi,

This looks like:
DLE-STX[data]DLE-ETX[2-byteCS]

Is that your understanding? If so, then this probably is a 16-bit CRC, not
a simple checksum. The trick is to determine what CRC (or, perhaps some
other form of 2-byte checksum). Do you have any description from the vendor
on what it is, or how it is calculated?

I've tried your data with a standard CRC-16, and the result is D066, not
34FF (hex format), so my guess it that this CS is different.

Can you get any more information on what or how this CS is used?

Dick

--
Richard Grier (Microsoft Visual Basic MVP)

See www.hardandsoftware.net for contact information.

Author of Visual Basic Programmer's Guide to Serial Communications, 4th
Edition ISBN 1-890422-28-2 (391 pages) published July 2004. See
www.mabry.com/vbpgser4 to order.

 

[Dick Grier]

Hi,

I have two different CRC-16 algorithms in my book. You also can download
from my homepage. However, there are lots of possible algorithms, so you
need to know for sure which one is being used. Probably the CCITT CRC-16,
but...

Dick

--
Richard Grier (Microsoft Visual Basic MVP)

See www.hardandsoftware.net for contact information.

Author of Visual Basic Programmer's Guide to Serial Communications, 4th
Edition ISBN 1-890422-28-2 (391 pages) published July 2004. See
www.mabry.com/vbpgser4 to order.

 

[wg]

Dick,

I just ordered you book to help out. I also downloaded the DestopIO dll to
ty. The algorithm is CRC-16H or CCITTA.

Thanks,


wg

 

[Dick Grier]

Hi,

What protocol? Some protocols calculate the CRC over only the data (not the
packet header and trailer), and some calculate the CRC over the entire
packet.

Dick

--
Richard Grier (Microsoft Visual Basic MVP)

See www.hardandsoftware.net for contact information.

Author of Visual Basic Programmer's Guide to Serial Communications, 4th
Edition ISBN 1-890422-28-2 (391 pages) published July 2004. See
www.mabry.com/vbpgser4 to order.

 

[wg]

Dick,

Not sure the protocal, just rs232 serial string. It calculates it over the
data only, not using the header, not sure if there is a trailer. From the
PLC manual I am to send a request and the PLC responses with the data. Both
the send a receive have a checksum of CITTA.

I had sent you an email asking about the DesktopIO form you site. How would
you use this control and look for a delimeter? Specifically if the buffer is
of size catch a string message and keep the rest.

Thanks

wg