Multi-Protocol MasterOPC Server > Lua 5.1 Reference Manual > Examples and Other Useful Things > Examples of Calculations of Checksums

The function server.SendAndReciveDataByMask supports calculation of DCON checksum. To provide this, you should set the parameter of checksum calculation to 1. However, it is required the full correspondence to a frame of the DCON format in this mode (the last frame symbol is CR, carriage return). There are devices that use a protocol, which is similar to DCON but use the different last symbol - for example, two symbols, CR LF. In this case, the standard algorithm of calculation cannot be used, and you need to write calculation yourself.

The checksum of DCON is a 2-byte number, and the algorithm of its calculation is very simple: it is a sum of all bytes modulo 256. Below is a request example:

$012

Convert symbols to bytes (http://www.asciitable.com) and summarize; the checksum is 0xB7

0x24h + 0x30h + 0x31 + 0x32 = 0xB7

Example of DCON checksum calculation function:

function DCONcrc(str)

crc=0;

for i=1,string.len(str),1 do

local byte=string.byte(str,i);--get the byte required

crc=math.fmod((crc+byte),256); --divide modulo 256

end;

return string.format("%02X",crc); --convert to a string and return the sum calculated

end;

Example of the use

val="!01070600";

CRC=DCONcrc(val);

--CRC = AF

In order to integrate the function to your code, use the output argument number 4 (the buffer string received) of the function server.SendAndReceiveDataByMask. After that, you should prepare the string received, that is, check the end symbols, extract two bytes of the checksum, pass a string up to the cheksum to the function (use the function string.sub), and compare a result of the function with the checksum received from the device.

After that, you can start to analyze values received by the mask.