Operation Manual

Appendix C LRC and CRC calculation

LRC generation

The Longitudinal Redundancy Checking (LRC) field is one byte,

containing an 8–bit binary value. The LRC value is calculated by the

transmitting device, which appends the LRC to the message. The device

that receives recalculates an LRC during receipt of the message, and

compares the calculated value to the actual value it received in the LRC

field. If the two values are not equal, an error results.

The LRC is calculated by adding together successive 8–bit bytes in the

message, discarding any carries, and then two’s complementing the

result. The LRC is an 8–bit field, therefore each new addition of a

character that would result in a value higher than 255 decimal simply

‘rolls over’ the field’s value through zero. Because there is no ninth bit,

the carry is discarded automatically.

A procedure for generating an LRC is:

1. Add all bytes in the message, excluding the starting ‘colon’ and

ending CRLF. Add them into an 8–bit field, so that carries will be

discarded.

2. Subtract the final field value from FF hex (all 1’s) to produce the

ones–complement.

3. Add 1 to produce the twos–complement.

Placing the LRC into the Message

When the 8–bit LRC (2 ASCII characters) is transmitted in the message,

the high–order character will be transmitted first, followed by the low–

order character. For example, if the LRC value is 61 hex (0110 0001):

Colon Addr Func

Data

Count

Data Data Data Data

LRC

CR LF

“6”

0x36

“1”

0x31

Example: an example of a C language function performing LRC

generation is shown below.

The function takes two arguments:

unsigned char *auchMsg; /* A pointer to the message buffer containing binary data */

/* to be used for generating the LRC, */

unsigned short usDataLen; /* The quantity of bytes in the message buffer. */

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