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From: John Devereux on 1 Feb 2007 17:59
Jonathan Kirwan <jkirwan(a)easystreet.com> writes:

> On Thu, 01 Feb 2007 21:06:53 +0100, janka vietzen <janvi(a)t-online.de>
> wrote:
>
>>for the moment I struggle with the modbus protocol checksum what seems to
>>use a quite odd 16 bit CRC polynom 0xA001. I have no idea if my ATmega
>>assembly code works correct becouse its responsible for both directions
>>generation and test same time and for the moment I do not have further
>>modbus certificated devices.
>>
>>Is there any CRC calculator known what allows to comapare reference values?
>
> Isn't there sample code for calculating modbus CRCs included in the
> documentation on modbus? I seem to recall seeing it near the end of
> one particular manual.

I'm sure I saw that too.

(checks...)

Yes, it's in the "Modbus over serial line specification and
implementation guide V1.0". I downloaded it from the modbus web site.

--

John Devereux
From: Arlet on 2 Feb 2007 02:06
On Feb 1, 9:06 pm, janka vietzen <j...(a)t-online.de> wrote:
> for the moment I struggle with the modbus protocol checksum what seems to
> use a quite odd 16 bit CRC polynom 0xA001. I have no idea if my ATmega
> assembly code works correct becouse its responsible for both directions
> generation and test same time and for the moment I do not have further
> modbus certificated devices.
>
> Is there any CRC calculator known what allows to comapare reference values?

Here's my modbus CRC code for AVR. It calculates CRC over data in
'rx_buf' with length in r24. The result has first CRC byte in r25,
second CRC byte in r24

crc_update:
ldi r31, hi8(rx_buf)//
ldi r30, lo8(rx_buf)//
ldi r26, 0xff // crc_high
ldi r27, 0xff // crc_low
ldi r18, 0xa0 // xor_high
ldi r19, 0x01 // xor_low
crc_byte:
ld r20, Z+ // get next data byte
eor r27, r20 // crc ^= data
ldi r20, 8 // 8 bits
crc_bit:
lsr r26 //
ror r27 //
brcc crc_no_xor //
eor r26, r18 // crc ^= 0xa001
eor r27, r19 //
crc_no_xor:
dec r20 //
brne crc_bit //
dec r24 //
brne crc_byte //
movw r24, r26 // r25:r24 = CRC16

A table based solution will be faster, but this worked fine for me,
and I didn't have room for a table.

From: janka vietzen on 2 Feb 2007 08:06
Arlet wrote:
many thanx, i am going to test your code.

here is my first try with atMega

;** R0 tempory use
;** R16 counts Bits of a Byte
;** R17 counts Bytes of txbuf
;** R18:R19 contains polynom
;** R20:R21 CRC data
;** R30:R31 Z Register points to txbuf

crcgen: ldi r17,low(txend-txbuf+1) ;r16 rxbuf length
ldi zl,low(txbuf) ; Z points to txbuf start
ldi zh,high(txbuf)
ldi r18,0x01
ldi r19,0xa0 ;r18:r19 = CRC Polynom = 0xa001
ldi r20,0xff
ldi r21,0xff ;r20:r21 is CRC register 1)
ld r0,z+ ;get first Byte from txbuf

bytlop: eor r20,r0 ;exor crc lowbyte with next data byte
ldi r16,8+1 ;count 8 bits per byte
bitlop: dec r16 ;one bit has been processed
breq nbyte ;-> byte is complete
lsl r20 ;shift 16 bit CRC Register
rol r21
brcc bitlop ;-> shift continius if C=0
eor r20,r18 ;if Carry = 1
eor r21,r19 ;exor with 16 bit Polynom
rjmp bitlop ;repeat for all 8 bits

nbyte: ld r0,z+ ;fetch next byte from rxbuf
dec r17 ;one byte has been processed
brne bytlop ;-> until all Bytes processed
ret ;CRC should be in R20:R21 now


Assume txbuf contains the 6 byte modbus rtu message: 1,3,0,0,0,2
the crc should be 0xc40b (what I can see in a old trace record)
but my code calculates 0x98e8 for crc result. Any bugs obvious?

From: David Kelly on 2 Feb 2007 14:38
janka vietzen wrote:
> for the moment I struggle with the modbus protocol checksum what seems to
> use a quite odd 16 bit CRC polynom 0xA001. I have no idea if my ATmega
> assembly code works correct becouse its responsible for both directions
> generation and test same time and for the moment I do not have further
> modbus certificated devices.
>
> Is there any CRC calculator known what allows to comapare reference values?

A year or so ago I used a Windows Modbus application downloadable from
England as a reference for verifying correct Modbus operation with my
AVR-gcc based product. The Modbus app I am thinking of would work for
free for 5 or 10 minutes each time it was launched. Otherwise $60 or so.
Couldn't write anything like it for only $60, money well spent.

You need something for your Modbus design to talk to, else you don't
know that it works.

From: Alexander Baranov on 2 Feb 2007 17:16

"David Kelly" <n4hhe(a)Yahoo.com> wrote in message
news:12s74p1c5ea08fd(a)corp.supernews.com...
> janka vietzen wrote:
>> for the moment I struggle with the modbus protocol checksum what seems to
>> use a quite odd 16 bit CRC polynom 0xA001. I have no idea if my ATmega
>> assembly code works correct becouse its responsible for both directions
>> generation and test same time and for the moment I do not have further
>> modbus certificated devices.
>>
>> Is there any CRC calculator known what allows to comapare reference
>> values?
>
> A year or so ago I used a Windows Modbus application downloadable from
> England as a reference for verifying correct Modbus operation with my
> AVR-gcc based product. The Modbus app I am thinking of would work for free
> for 5 or 10 minutes each time it was launched. Otherwise $60 or so.
> Couldn't write anything like it for only $60, money well spent.
>
> You need something for your Modbus design to talk to, else you don't know
> that it works.
>
This one worked with Siemens power meter:
int crc16_siemens9200(unsigned char *ptr, int count)
{
unsigned int crc;
char i;

crc = 0xffff;
while (--count >= 0)
{
crc = crc ^ (unsigned int) *ptr++;
i = 8;
do
{
if (crc & 0x0001)
crc = (crc >> 1) ^ 0xA001;
else
crc = (crc >> 1);
} while(--i);
}
return (crc);
}
Alex.


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