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AG_Receive with variable lenght

Created by: aikona at: 5/26/2011 1:28 PM (12 Replies)

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5/26/2011 1:28 PM	Rate (0)
aikona Posts: 487 Rating: (36)	Hi In all comms we use FC5(0) and FC6(0) as well for profibus (FDL) as ethernet UDP and TCP . We now have a comms where the the messages are of variable lenght ?I have read a few forums and the all advise to put the comms on the receiving end to one byte and process per byte till the end of a message.But for our purpose its impossible because we receive a burst of messages and processing per byte floods the cp card. What I want to know .Is there a certain logic if you send messages of different lenght ? Example , buffer = 255 , mssge 1 = 20 byte , mssge 2 = 40 byte .Does that mean that 'with' an empty or clean buffer the first msse will be stored in 1 to 20 and the next from 21 to 60 etc till buffer full and then restart at beginning ? Is ther somewhere a pointer like MW200 or something that refers to the buffer position Eric
aikona Posts: 487 Rating: (36)	Last edited by: aikona at: 5/26/2011 1:29 PM corrected a few of the usual spelling mistakes
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5/26/2011 1:48 PM	Rate (0)
Niranjan Prasad Posts: 691 Rating: (37)	aikona Hi In all comms we use FC5(0) and FC6(0) as well for profibus (FDL) as ethernet UDP and TCP . We now have a comms where the the messages are of variable lenght ?I have read a few forums and the all advise to put the comms on the receiving end to one byte and process per byte till the end of a message.But for our purpose its impossible because we receive a burst of messages and processing per byte floods the cp card. What I want to know .Is there a certain logic if you send messages of different lenght ? Example , buffer = 255 , mssge 1 = 20 byte , mssge 2 = 40 byte .Does that mean that 'with' an empty or clean buffer the first msse will be stored in 1 to 20 and the next from 21 to 60 etc till buffer full and then restart at beginning ? Is ther somewhere a pointer like MW200 or something that refers to the buffer position Eric Dear I am not sure whether I understande your problem or not What I understand is that You want to send Datas of different lengths to different PLCs. You can do that, by triggering the Comm blocks one by one. Or by assigning the prameter values once done/error bit is active
Niranjan Prasad Posts: 691 Rating: (37)
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5/26/2011 1:56 PM	Rate (0)
Heksu Bronze Member Joined: 12/12/2005 Last visit: 3/17/2024 Posts: 180 Rating: (12)	Hi Aikona, If your message variating can´t you define max. length of messageand use this size for message in program? regards Heksu
	"Beer is proof that God loves us and wants us to be Happy"-Benjamin Franklin
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5/26/2011 1:58 PM	Rate (0)
aikona Posts: 487 Rating: (36)	This is a communication setup with a scanner and this scanner send various messages all different lenghts.The PLC has to decode these messages and take action , no problem decoding IF the informationin the receive buffer is alwaays at the same place but it is not. CALL FC 6 ID :=2 // tcp connection number LADDR :=W#16#160 // CP module adress RECV :=P#DB59.DBX0.0 BYTE 232 // DB,DW en lengte NDR :=M82.3 // New Data Recieved ERROR :=M82.4 //Error flag STATUS:=DB59.DBW252 // Status code LEN :=DB59.DBW254 // received lenght If RECV BYTE value = LEN value then your data starts at first byte and ends at last byte of your recv buffer But if LEN is smaller then the data is offset somehow .It gets worse when the LEN varies , for example receive 1 = 10 bytes ,then message 2 = 200 byte ,message 3 again 10 bytes or 12 bytes Eric
aikona Posts: 487 Rating: (36)
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5/26/2011 2:18 PM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 5/29/2025 Posts: 1561 Rating: (540)	This thread might help you.
	Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots. So far, the universe is winning...
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This contribution was helpful to 2 thankful Users fritz Krish

5/27/2011 9:16 AM	Rate (0)
aikona Posts: 487 Rating: (36)	smiffy This thread might help you. Yes and no , more confused than ever. example ID :=1 // tcp connection number LADDR :=W#16#100 // CP module adress RECV :=P#DB64.DBX0.0 BYTE 6 // DB,DW and Length NDR :=M81.3 // New Data Recieved ERROR :=M81.4 //Error flag STATUS:=DB64.DBW252 // Status code LEN :=DB64.DBW254 // received lenght two questions 1:What happens if RECV lenght bigger than LEN ? does two messages of say 2 byte and 3 byte fille the RECV buffer up to byte 5 2 What happen when LEN bigger than RECV.? does it send the first amount and then wait for 'something and then send the second amount ? Say I send 10 byte , get the first 6 , ok then what must I do to receive the next 4 byte ? Eric
aikona Posts: 487 Rating: (36)
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5/27/2011 9:41 AM	Rate (0)
Kaulquappe Posts: 3093 Rating: (324)	Hello Aikona, to receive telegrams with variable length with TCP native and AG_RECVproperly, it is necessary that the telegram of the communication partner includes any length information. Can you give us an example how the telegram is set up? When you call AG_RECV with e.g. length 6, the CP will wait until it has received 6 byte. These 6 bytes are transferred to the data destination. When the first telegram is 4 bytes long, the CP won't transfer any data to the CPU. It waits for the next telegram and then the 4 bytes of the first telegram and the first 2 bytes of the second telegram are transferred to the CPU. When the communication partner sends a telegram with 12 byte, you will get two data transfers CP -> CPU, each one 6 byte long (and the second will overwrite the first one). Regards, Kaulquappe I found a FAQregarding this matter.
Kaulquappe Posts: 3093 Rating: (324)	Last edited by: Kaulquappe at: 5/27/2011 10:21 AM
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5/27/2011 11:17 AM	Rate (1)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 5/29/2025 Posts: 1561 Rating: (540)	When receiveing, you do not specify the length, the AG_RECV call returns the ammount of data transferred to the data area in the LEN variable. You specify how much data of that packet you want to transfer to the user program in your ANY pointer (the BYTE 6 bit of your RECV parameter). You will only get NDR set when at least the ammount of data you specified in your ANY pointer has arrived (regardless of how much data actually arrived). aikona 1:What happens if RECV lenght bigger than LEN ? does two messages of say 2 byte and 3 byte fille the RECV buffer up to byte 5 If less data arrives than you want (6 bytes in your case), the LEN parameter will remain at 0 and NDR will remain FALSE. You would then have to keep calling AG_RECV until you get at least an ammount of data that you specified in your ANY pointer, at which point the NDR will be set and LEN set to 6 and 6 bytes transferred to the data area. aikona 2 What happen when LEN bigger than RECV.? does it send the first amount and then wait for 'something and then send the second amount ? Say I send 10 byte , get the first 6 , ok then what must I do to receive the next 4 byte ? If more data arrives than you want (6 bytes in your case), the LEN parameter will be set to the 6 and 6 bytes of data transferred to the data area. You would then have to perform another AG_RECV call for the remaining data with the ANY pointer setup to the ammount of data you want to receive. The issue with using an ANY pointer to get the data into a specific location is that you have to know how much data you want to receive in advance, which is kind of hard if you are trying to receive variable length packets. Are you sure there is no way of identifing how big the data packets are in advance? Is there not some form of packet header you can receive that has the packet size in it? (TCP packets have this feature). Are there any packet demarkcation characters that mark the start/end of the data packets?
	Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots. So far, the universe is winning...
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5/27/2011 1:53 PM	Rate (0)
aikona Posts: 487 Rating: (36)	smiffy aikona 1:What happens if RECV lenght bigger than LEN ? If less data arrives than you want (6 bytes in your case), the LEN parameter will remain at 0 and NDR will remain FALSE. You would then have to keep calling AG_RECV until you get at least an ammount of data that you specified in your ANY pointer, at which point the NDR will be set and LEN set to 6 and 6 bytes transferred to the data area. Ok so my any pointer has to be the smallerst possible message smiffy aikona 2 What happen when LEN bigger than RECV.? If more data arrives than you want (6 bytes in your case), the LEN parameter will be set to the 6 and 6 bytes of data transferred to the data area. You would then have to perform another AG_RECV call for the remaining data with the ANY pointer setup to the ammount of data you want to receive. The issue with using an ANY pointer to get the data into a specific location is that you have to know how much data you want to receive in advance, which is kind of hard if you are trying to receive variable length packets. That is just the point smiffy Are you sure there is no way of identifing how big the data packets are in advance? Is there not some form of packet header you can receive that has the packet size in it? (TCP packets have this feature). Are there any packet demarkcation characters that mark the start/end of the data packets? Well we are using TCP but I will give you an example of the kind of messages we receive from a sick scanner this is from the DB we use to decode the strings SICK620.PARAM_OPEN STRING [ 17 ] 'sMI 0 03 F4724744' SICK620.PARAM_CLOSE STRING [ 5 ] 'sMI 2' Leave parameter mode SICK620.ECHO_OPEN STRING [ 7 ] 'sAI 0 1' Echo after entering parameter mode SICK620.ECHO_TEXT STRING [ 21 ] 'sAN mEVSetMatchCode 1' Echo after sending text SICK620.ECHO_CLOSE STRING [ 7 ] 'sAI 2 1' Echo after leaving parameter mode SICK620.ECHO_WRONG STRING [ 5 ] 'sFA 2' Echo unknown SICK620.ERROR STRING [ 3 ] 'ERR' Error message received from scanner The integrator that wrote the software reads the message byte wise and decodes the message in subsequent reads .This works but ... ( htere is allways a but).... We now need to log the scanned products .Basically all the good reads are counted up and the bad reads are logged in a file with Date/timetagging.Taking into account a 50msec cyle time ,17 reads for a 17 byte string add to that a burst of 12 scanned product in about 400 milliseconds .The CP needs about 10 seconds to process this and by then the second burst of scanned items are there and the CP just overflows. So my problem remains and no elegeant way to solve it yet ... Still have to read a feq FAQ's
aikona Posts: 487 Rating: (36)
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5/27/2011 2:45 PM	Rate (0)
Krish Silver Member Joined: 5/24/2007 Last visit: 4/24/2024 Posts: 284 Rating: (27)	Hi Aikona Thought I will just give a small heads-on on this subject. Well receiving telegram of variable length is not so easy . One could design that you receive the telegram byte wise and thensort them out in a seperate program. But this is a veryslow process. Every cycle the receive program is executedyou get one byte.You can imagine if you wantto receive 6kb of data (limit of the CPcard) how long you need. Worst problem is that once the buffer in the CP card is full (because you are not picking it fast enough) the connection gets dropped and the sender is forced to abort the sending sequence.If it is really important information , then it needs to be sent again. On the other side you could pick alarge size buffer and start receiving all telegramsas fixed size and then sorting them out. But the whole program becomes complicated. As mentioned before you could use the first 2 bytes to identy the length of the telegram and then in the next cycle you could extend the buffer to the size inidentified in the previous cycle.But the problemwith this methog is that if there issome error or data loss, howdo we recognise which 2 numbers is "the garbage" being received isthe length of the next telegram. So you need to identify that the telegram begins with a header and then comes the length. The header could a set offixednumbers and so when you receive this correct set (hope fully no data is exactly same as these numbers). As a summary, if you can possibly change how the sender behaves, design the telegram such that it has a header and length and then come the data. When you receive you first look for the header and then pick the next2 as length and then collect the data. Then start liiking for the header again. Hope this helps you..just in case of a chance to redesign how it working now.The tips given by SMIFFY could be used for the actual implementation. Krish Krish

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