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Dual Length Cyclic Redundancy Check Circuit

IP.com Disclosure Number: IPCOM000084699D
Original Publication Date: 1975-Dec-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 2 page(s) / 36K

Publishing Venue

IBM

Related People

Powers, JT: AUTHOR

Abstract

This is a circuit for generating and checking both an 8-bit and a 16-bit polynomial checking code, using substantially the same hardware, under external control. The 16-bit generating polynomial is of the standard Cyclic Redundancy Check Code (CRCC)-16 format; the shorter is an 8-bit adaptation thereof.

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Dual Length Cyclic Redundancy Check Circuit

This is a circuit for generating and checking both an 8-bit and a 16-bit polynomial checking code, using substantially the same hardware, under external control. The 16-bit generating polynomial is of the standard Cyclic Redundancy Check Code (CRCC)-16 format; the shorter is an 8-bit adaptation thereof.

The standard logic for generating and checking the IBM CRCC-16 cyclic redundancy check code is modified by the addition of steering network 16 to produce and check an alternate cyclic code which is 8 bits in length, rather than the usual 16.

In the usual CRCC-16 implementation, output 17 of shift register position 15 is connected directly to the input of exclusive Or gate 18. Steering network 16, under control of select long line 19, selectively supplies an alternate input to XOR 18 which is the output 20 of shift register position 7, effectively bypassing 8 positions (stages 8-15) of the shift register and using most of the same hardware to produce an 8-bit check code.

The length of the check code is selected for generating a polynomia whose length is matched to that of the message being transmitted. When the input signal +Select Long 19 is true, a conventional CRCC-16 check code generating polynomial of the form x/16/ + x/15/ + x/2/ + 1 is produced or checked. When signal 19 is false, the check code has a generating polynomial of the form x/8/+X/7/+X/2/+1. The format of the check code may be altered by altering that po...