(Engine-Level Function)
| Description | Returns a buffer containing a CRC table. |
| Returns | Buffer |
| Usage | Script or steady state. |
| Function Groups | String and Buffer |
| Related to: | CRC |
| Format | CRCTable(Polynomial, Length, Shift) |
| Parameters |
| Polynomial |
| Required. A long expression that gives the bit pattern of the generating polynomial. For Allen-Bradley® and Modicon®, Polynomial = 0b1000000000000101, which corresponds to the expression: x^16 + x^15 + x^2 + 1 To determine the value of Polynomial for a given generator expression, drop the highest order term, and represent each term present by a 1 in the bit position equal to its exponent. For example, x^16 + x^12 + x^5 + 1 would have a Polynomial value of 0b0001000000100001. The first 1 corresponds to x^12, the second to x^5, and the last to 1 (which is x^0). This polynomial is used by the XMODEM protocol. |
| Length |
| Required. Any numeric expression giving the byte length of the CRC accumulator register (usually 2 or 4). The CRCTable buffer returned is 256 * Length + 1 bytes long. |
| Shift |
| Any numeric expression that is 0 for right-shifted CRC calculations (such as Allen-Bradley® or Modicon®), and 1 for left-shifted CRC calculations (such as XMODEM). |
| Comments |
This function is part of the driver toolkit. |
Example:
CheckTable = CRCTable(0b1000000000000101
{ Polynomial: x^16 + x^15 + x^2 + 1 },
2 { 16 bit CRC register },
0 { Right-shifted CRC calculations });
This generates a CRC look-up table for use with the CRC function.
