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?
CRC calculation
Started by ●February 1, 2007
Reply by ●February 1, 20072007-02-01
On Thu, 01 Feb 2007 21:06:53 +0100, janka vietzen <janvi@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. Jon
Reply by ●February 1, 20072007-02-01
> Isn't there sample code for calculating modbus CRCs included in theI dont have many modbus manuals but there are 2 diffrent code samples for crc written in C. One uses 2 bested byte - bit loops and the other uses 255 byte lookup tables for low and highbyte each. Unfortunately I dont tried any Compilers for Atmels ATmega128 (probably there is a gcc available) and even the cpu family is new for me why i decided to start with assembler
Reply by ●February 1, 20072007-02-01
janka vietzen wrote:>>Isn't there sample code for calculating modbus CRCs included in the > > > I dont have many modbus manuals but there are 2 diffrent code samples for > crc written in C. One uses 2 bested byte - bit loops and the other uses 255 > byte lookup tables for low and highbyte each. Unfortunately I dont tried > any Compilers for Atmels ATmega128 (probably there is a gcc available) and > even the cpu family is new for me why i decided to start with assemblerThere is GCC available for AVRs (including your chip). I suppose that you mean the binary Modbus encoding, also called Modbus/RTU. (There's a hexadecimal encoding resembling Intel's hex object code, but it does not use CRC at all, but a straight checksum). The Modbus CRC is the old IBM's CRC-16, used e.g. in the early disks and in the Bisync data communication protocol. For eight-bit processors, the simplest way of calculating the CRC is the use of two 256 (not 255) byte tables, if you do not want to do it bit-by-bit. The table-driven method calculates the CRC for each incoming byte, not each bit separately. -- Tauno Voipio tauno voipio (at) iki fi
Reply by ●February 1, 20072007-02-01
"Jonathan Kirwan" <jkirwan@easystreet.com> wrote in message news:5mi4s25krt4qes5j25j5mr2k06sih0utqc@4ax.com...> On Thu, 01 Feb 2007 21:06:53 +0100, janka vietzen <janvi@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. > > JonIt is probably 10 years since I used Modbus, and hopefully the manuals have been updated, but I recall the code sample in my Modicon manual being incorrect :-( I still have some hair left though. Regards, Richard. + http://www.FreeRTOS.org + http://www.SafeRTOS.com for Cortex-M3, ARM7, ARM9, HCS12, H8S, MSP430 Microblaze, Coldfire, AVR, x86, 8051, PIC24 & dsPIC .... and soon AVR32
Reply by ●February 1, 20072007-02-01
Jonathan Kirwan <jkirwan@easystreet.com> writes:> On Thu, 01 Feb 2007 21:06:53 +0100, janka vietzen <janvi@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
Reply by ●February 2, 20072007-02-02
On Feb 1, 9:06 pm, janka vietzen <j...@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.
Reply by ●February 2, 20072007-02-02
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?
Reply by ●February 2, 20072007-02-02
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.
Reply by ●February 2, 20072007-02-02
"David Kelly" <n4hhe@Yahoo.com> wrote in message news:12s74p1c5ea08fd@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.
