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I have a program that sends packet data through a serial port to a device that requires CRC 16 checksum. I am not familiar with this. Can you suggest some beginner's resources to help me write my own in VB.Net.
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I was given code for a crc16 from C# that I converted to VB. Can anyone look at the code and answer some questions for me. I am new at this type of stuff.
Private Sub crc_check(ByVal dat As Byte)
Dim nextone As UInteger
Dim n As Integer
nextone = Integer.Parse(dat)
nextone <<= 8
For n = 8 To 1 Step -1
If (nextone Xor crc) And &H8000 Then
crc = (crc << 1) Xor &H1021
Else
crc <<= 1
End If
nextone <<= 1
Next n
End Sub
The dat, is that one byte at a time or all bytes of my frame?
What is crc? what value should that be?
I get a error when I run this stating Arithmetic operation overflow. Why?
Can anyone help me? Thank you.
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Hi,
I know C#, can read VB but dont speak it, and have done many CRCs
over the years.
There are thousands of CRC definitions; they vary in width, rotational
direction, polynomial value, and other details.
You will typically find two ways of implementing a CRC:
- the one you have, which is adding one item (a byte here) at a time by
iterating over its bits (that's the for loop inside); basically it says:
test the highest bit (that's H8000), if set rotate it back in at the
low end, not with value one, but with the selected polynomial value
(here H1021)
- the other standard implementation is by table lookup, where the for loop
is replaced by something that in the end does exactly the same, based
on a precalculated table (256 entries for a byte method). This way is
some ten times faster at the expense of higher complexity. Dont bother
for now.
Anyway, in the code at hand, clearly crc is a class member that needs to
be initialized once (depending
on the specific CRC that would most likely be to 0 or HFFFF), see C# code;
if no explicit initialization is present, it is an implicit zero, and
you can not reuse that class instance.
Then you feed all your data bytes to the method, one at a time, in the
intended order.
At the end, crc contains the CRC, except that for some CRC definitions
you may still have to invert it (i.e. exor with HFFFF).
The overflow occurs in one of the left shifts; the intention is to
ignore bits that "fall off"; when necessary they got noticed by the
overflow test (remember the H8000).
So either you see a way to turn them off in your code, or you must
modify your code to make sure they never happen.
I am not sure, it is likely though, that VB is defined such that
signed integers can cause overflow, and unsigned integers can not.
If so I suggest you declare both crc and nextone as unsigned.
Otherwise you must explicitly prevent overflow by first anding away
the most significant bit (and with H7FFF I guess), then do the shift left.
Some more comments:
1.
I dont see why you included nextone = Integer.Parse(dat)
I would expect nextone=dat to do exactly the same
2.
please publish code inside PRE tags, that will switch to a more
appropriate font and preserve indentation, making things easier to read.
3.
if starting from C# code it would have been wise to show that too.
One more advice:
as long as a single mistake is present in your code, the CRC will be wrong,
and so much so that you can not even start to guess what happened.
So my advice is make sure you have something else (an entire program on
your PC, on another machine, whatever) that does the same
thing correctly, and that you can feed any data you like; then
start with a single byte (try a couple of values, say H01, then H02, H04,
up to H80), and compare results from both implementations.
If not identical you must fix that first. Only then proceed to a two-byte
message (say H01 H80; or H02 H40; ...); and then a 9-byte message since
by then all integers have reached their 16-bit capacity and new bugs
]may pop up. If this also succeeds, you may be ready for real data of
arbitrary length.
Good luck with your CRC stuff.
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C# code:
<code>void crc_check(unsigned char dat)
{
unsigned int next;
unsigned char n;
next = (unsigned int)dat;
next <<= 8;
for(n=8;n>0;n--)
{
if((next ^ crc) & 0x8000)
{
crc = (crc << 1) ^ 0x1021;
}
else
{
crc <<= 1;
}
next <<= 1;
}
}</code>
VBcode:
Private Sub crc_check(ByVal dat As Byte)
Dim nextone As UShort
Dim n As Integer
nextone = dat
nextone = nextone << 8 And (&H7FFFS << n - 1)
For n = 8 To 0 Step -1
If (nextone Xor crc) And &H8000 Then
crc = (crc << 1) Xor &H1021
Else
crc = crc << 1
End If
nextone <<= 1
Next n
End Sub
Did I convert this right? When I run it it turns everything to 0.
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Cory Kimble wrote: Did I convert this right? When I run it it turns everything to 0
I think you have answered the question yourself.
BTW: The original code looks very much like C, not C#.
BTW: I read everything (everything in latin alphabet), but I don't speak VB
so I will not provide VB statements.
These are errors, the list may be incomplete:
Cory Kimble wrote: nextone = nextone << 8 And (&H7FFFS << n - 1)
1. n has not been assigned a value yet.
2. I don't get what you are trying to do; the C code was next=next<<8;
Together with 1. this probably explains all the zeroes you are getting.
Cory Kimble wrote: For n = 8 To 0 Step -1
3. the loop must execute 8 times, not 9
Cory Kimble wrote: If (nextone Xor crc) And &H8000
4. Does this compile ? the C code
if((next ^ crc) & 0x8000)
is shorthand for
if(((next ^ crc) & 0x8000)!=0)
I don't know whether VB accepts it without a zero-compare.
It is proof the original is not C#
That's it for now.
If you have to publish again, please do it again with original and yours, in
PRE tags, as you did; but then include the definition of crc variable, and
the calling code (initializing crc, looping over the data, etc).
Good luck!
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hi all,
how can i remove icon from any form. i already try the property ShowIcon=false ,but it show .NET default icon in MDI child form
thanks in advance
Rupesh Kumar Swami
Software Engineer,
Integrated Solution,
Bikaner (India)
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I'm assuming you're form is maximized in the mdi form
if that is the case it's very difficult to remove the form icon
the only way I have found untill now is to set the controlbox property to false, downside you lose alle buttons (minimize,maximize,close) so you'll have to add those yourself
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My form has a TabControl consisting of several TabPages, one of which is called processingTabPage. When the forms Load event fires I remove the processingTabPage for the TabControl's TabPages with :-
TabControl.TabPages.Remove(processingTabPage)
When a particular event occurs I add the tabPage back onto TabControl
TabControl.TabPages.Add(processingTabPage)
Then I would like the processingTabPage to show to the user automatically.
TabControl.TabPages(TabControl.TabPages.IndexOf(processingTabPage)).Show()
All works ok accept the showing of the TabPage to the user. Why?
Steve Jowett
-------------------------
Sometimes a man who deserves to be looked down upon because he is a fool, is only despised only because he is an 'I.T. Consultant'
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I believe you want to set the TabControl.SelectedIndex or the TabControl.SelectedTab. If you are using .net 2.0 there is also a new method in the TabControl called SelectTab().
Hope that helps.
Ben
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Thanks Ben, that sorted it.
Steve Jowett
-------------------------
Sometimes a man who deserves to be looked down upon because he is a fool, is only despised only because he is an 'I.T. Consultant'
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hi!
please help me how can i use VScrollBar to Scroll a long page.... using vb.net of vb6
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hi
i am developing an ide for java,iam facing problem with
getting column number(or char no at line being edited).can anyone help.
also can anyone guide me how to handle syntax highlighting.
Thanks
Sourabhj
sourabh
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Hi Guys
Does anybody know how i can obtain the number of pages from a crystal report (version 8.5) using VB 6 before i call the print function. My current code is:
Dim rep As Report<br />
Set rep = CreateReport("Report.rpt")<br />
rep.PrintOut False
The PrintOut property supports optional properties to specify pages from and to. Basically i want to specify these before calling printout as only the first 200 pages are printing even though there is 500 pages in the report.
If i could get the number of pages i can call the printout method a few times passing it different page numbers each time.
Thanks in Advance
Keith
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Hello,
I am using a tab control with each tab created at run-time.
I am using the SelectedIndexChanged event so I can process some data that has been input on the selected tab. However, I need to know which tab the user has just left, so I can process some data that is needed before that tab loses focus.
The selectedIndexChanged event is good for finding the tab that has been selected, but not the one that has just lost focus.
I could proberly loop through all the tabs and find the one that has lost focus, but I am wondering if there is a better method.
Many thanks,
Steve
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one possible way would be to store the selectedindex from the tab in a variable
then in the selectedindexchanged event you process your data and after that you set you're variable to the new selectedindex
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If you are using .net 2.0 there is a deselected event where the sender being passed in is the tabcontrol and the selectedindex of that tabcontrol is the tabpage that is losing focus.
Hope that helps.
Ben
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Yes, I am using VS 2005
Thanks
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So I think the deselected event should work pretty good for you then. The SelectedIndex and SelectedTab should still be the previous TabPage.
Ben
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hi all
i need a ruler to set the indentation of richtextbox in vb.net
can some one help me to get one
ash
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Q1/
If this is the path to save an open Excel file running in the background
xlSheet.SaveAs("c:\My Path\tk" & Format (Me.MonthCalender1.SelectionStart, "yyyyMMdd") & ".xls")
How would I open a file using the MonthCalender1 (Change-Date) as the trigger to determine the name? e.g. "tk20070727.xls" ?
Q2/ Having opened the file (running invisable in the background), how can I pull the data from a cell to a variable?
Q3/ How do I close the invisable running .xls file in runtime?
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how to identify status of mscomm event when a reciever attend the phone
thank you
-RisKhan-
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There is no event if the phone is picked up... Whatever you have the phone connected to, I'm assuming it's a modem, has to be able to send a message to the PC. AFAIK, there is no modem out there that will tell you if the phone goes "off-hook".
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i m trying to use the filesave-dialogbox, it works ok bt 1 problem i m facing is when the filename is even nothing the "Save" botton is enable and clicking it does nothing except the dialog reappears.... Plzz tell me how can i disable the "Save" botton when the filename is "", r nothing.
(Jameel)
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