Advantech RS-485 user manual Appendix D

Appendix D

Example 2

This example explains how to calculate the checksum value of a Read High alarm limit command string:

Case 1. (If the Checksum feature is disabled)

Command: $07S1RH(cr)

Response: !07+2.0500(cr) when the command is valid.

Case 2. (If the Checksum feature is enabled)

Command: $07S1RHA9(cr)

Response: !07+2.0500D8(cr)

where:

A9 represents the checksum of this command, and<R>D8 represents the checksum of the response.

The checksum of the command string is derived as shown below:

A9h = (24h+ 30h + 37h + 53h + 31h + 52h + 48h) MOD 100h

The hexadecimal ASCII codes for $, 0, 7, S, 1, R and H are 24h, 30h, 37h, 53h, 31h, 52h and 48h respectively. The sum of these ASCII codes is 1A9h. The modulo-256(100h) - of 1A9h is A9h.

ADAM-5000 D-3