Buffer to tag sems doesn't work

[ORSO2001]

Dear All,

I am having a strange issue...I try to explain, I am checking the serial communication between a master and a slave (modbus rtu...like a sniffer) and I am using the Unistream to catch the exchanged data...at the beginning, as first test, I record every 10 bytes and save the buffer in a DTI...and all has works fine...now I want to take the exact request/answer between master and slave....then the first record is checking the 8 bytes...the data will be, for example, 1-3-0-216-0-3-134-56 (the CRC is not calculated...I wrote only as example)...then I want check the position 5 in the buffer (3) to know how many bytes will be the answer...in this case will be (3 *2)+2 =11 bytes...I am using the function block buffer to tag to store the buffer[5] and do the calculation...I tried to save the specific buffer position to a single tag and all the buffer in an array...both UINT8 and UINT16 but...but this never works....in the position C of the function block is wrote that if the end tag has not enough space the function will not execute...but I don't think that is the problem...what cna be the cause? What I am wrong?

[Ausman]

Hi Orso,  have you got a small delay in between the various buffer reads you are doing?  Given that you are working with modbus, you will likely need a few scans at least, if not more, for the buffer to be correct for what you want to do.

cheers, Aus

[ORSO2001]

Hi Ausman,

no delay ...what I done is check the amount of incomming bytes and every 8 bytes, that are in the RS485, using an EQUAL block,  pass at an buffer array using the COM RX function...store this last buffer array in a struct that has the same buffer and store in a DTI...increase the row where I store and restart; this works (the communication between master and slave start and stop on my command)...I repeat that if I check, passed, store this every 8 byte all works fine...but I want to split row by row the request and the answer...what is not working is the "store" of a part of buffer in a tag...I have had an idea about the next test that I can do...later I will try and I will update the topic.

 

[ORSO2001]

ok..what I were doing was a control of the "expected bytes in input"  in the same row using to function block "equal" and "not equal" then a ramification of the row...if equal i checked the buffer[5] if not equal I did a store as 8 (because 8 byte is alwayse the lenght of the function 0x03)....I splited these in two individual rows...and now works.