IBM BSC CRC?

[Nigel Johnson]

True, but in my case it was a typo! But yes we digital types like 
saturation.  Still have the funny thing with LTSpice though. I used it a 
lot for electricity 1 and 2 demos for students when the college wouldn't 
support me running electronics workbench under a VM on linux!

Works just fine with 100k!  Maybe I will just go back to the hardware, 
but it is a pain changing resistors in that dense board!  I think I must 
have blown the second transistor with too high a base current, burnt 
fingers tomorrow!

73

id


On 26/01/2020 18:12, Bob Smith via cctalk wrote:
> I ma rusty on this, been almost 50 years since I worked on the DP8EP
> aka the KG83. then the KG11, and the Autodin 2 CRC32 designs in
> hardware.
> I don't recall whether bisync, aka bsc used LRC8, 12, 16, or crc16 as
> the error detection algorithm.
> I don't think it used VRC. I did find a refresher that might help, but
> I don't think the polynomial you have for crc 16 has enough terms.
> BUT I could be misremembering.
>
> https://www.automatas.org/modbus/crc7.html
> bob
>
> On Sun, Jan 26, 2020 at 2:59 PM Mattis Lind via cctalk
> <cctalk at classiccmp.org> wrote:
>> Hello IBM BSC Experts!
>>
>> I am trying to figure out the CRC algorithm used by IBM BSC. I have tried a
>> lot of different settings in crcreveng but not getting a match.
>>
>> I am pretty convinced that the CRC-16 used by IBM was
>>    16          15       2
>> x      +   x     +   x     +  1
>> This would give the polynomial 8005.
>> Anyone against this statement?
>>
>> But what was the initial value?
>>
>> I have two actual messages from equipment employing IBM BSC:
>> 32016CD90240404070032688
>> and
>> 32016CD90240C84050030D28
>>
>>  From this document (
>> http://bitsavers.trailing-edge.com/pdf/ibm/datacomm/GA27-3004-2_General_Information_Binary_Synchronous_Communications_Oct70.pdf
>> )
>> I get that the CRC calculation is reset on SOH (01h) or STX (02h) and
>> accumulates until and including the ETX (03h). (excluding any SYN (32h)
>> characters).
>>
>> I have tried crcreveng back and forth and I am not getting the CRC bytes
>> right.
>> I think I have tried most things, different bit order, different initial
>> values. But nothing.
>>
>> I also tried the mode in crcreveng where it searches for matches but it
>> always says "no models found". Maybe I am doing something wrong when using
>> crcreveng?
>>
>> Any clues? Surely there are someone out there that has been around for some
>> time and knows this, right?
>>
>> On the topic of crc reveng I tried to verify how it works by using some
>> kind of known value: This article
>> https://stackoverflow.com/questions/23638939/crc-16-ibm-reverse-lookup-in-c
>>
>> has a specific example where a certain data in (75h) with initial value
>> 90f1h gives output 6390h. I tried to get crc reveng to do the same, but
>> failed. There has to be some option I simply do not understand. I tried
>> most combinations.
>>
>> /Mattis

  

-- 
Nigel Johnson
MSc., MIEEE
VE3ID/G4AJQ/VA3MCU

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