Vaisala WINDCAP WMT52 User Manual download pdf (Page 34)

34 Orion LT Weather Station

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CRC-16 Computation

The computation of the CRC is performed on the data response before parity is added. All operations are

assumed to be on 16 bit unsigned integers. The least significant bit is on the right. Numbers preceded by

0x are in hexadecimal. All shifts shift in a zero. The algorithm is:

Initialize the CRC to zero. For each character beginning with the address, up to but not including the

carriage return (<cr>), do as follows:

{

Set the CRC equal to the exclusive OR of the character and itself

for count =1 to 8

{

if the least significant bit of the CRC is one

{

right shift the CRC one bit

set CRC equal to the exclusive OR of 0xA001 and itself

}

else

{

right shift the CRC one bit

}

Encoding the CRC as ASCII Characters

The 16 bit CRC is encoded to three ASCII characters by using the following algorithm:

1 st character = 0x40 OR (CRC shifted right 12 bits)

2nd character = 0x40 OR ((CRC shifted right 6 bits) AND 0x3F)

3rd character = 0x40 OR (CRC AND 0x3F)

The three ASCII characters are placed between the data and <cr><lf>. Parity is applied to all three

characters, if selected for the character frame.

The CRC computation code is added to the end of the response, if the first letter of the command is sent by

using lower case.

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Vaisala WINDCAP WMT52 User Manual Page 34