Pofibus help needed

[Arlie]

Any Profibus gurus can help me on this one ?

I have two units using the same parameters to send info to the buss net. That part works ok, when read one the other is blank and so on. Problem is I also have a bit that I want to read as on or off and also send to it. Customer is telling me he reading only 8 words and should be reading 9. Dosn't make sense to me , this was my first profibus and hopefully last. any way anybody can help?

 

[mordred]

whats your word length is it the standard 16 bits/word it doesn't sound like you've exceeded any data length as your allowed I believe 244 bytes per transmition its actually 255 but 11 bytes is used in the header.

 

[mordred]

shot in the dark here as I'm not keen on profibus so this info is pulled from the PLC tutor articles from this site but I would check your length in your header here is the header breakdown

The full header consists of the following, each point being 1 byte:

SD-LE-LEr-SD-DA-SA-FC-DSAP-SSAP-DU-FCS-ED

Let's examine each item.

SD- Start Delimiter - This is used to determine the beginning of the

telegram and what format it will be in. There are 5 possibilities

for this value depending upon the purpose of the communication.

LE- LEngth - This byte is the 'net data' length which means it excludes

delimiters and error checking bytes.

LEr- LEngth Repeated - The length byte stated again for double checking

and verification.

DA - Destination Address - The address where our telegram will be

sent to. (ie. the destination)

SA - Source Address - The address from where the telegram is being

sent from (ie. the source of the telegram)

FC - Function Code - This frame specifies the type of message being

sent as well as the priority of the message.

DSAP - Destination Service Access Port - This is basically the

communication port of the receiving device.

SSAP - Source Service Access Port - This is the communication port of

the sending device.

DU - Data Unit - This is the actual data that will be transmitted.

Remember that we are allowed to send a maximum of 255 bytes of data

but 11 of these allowed bytes are taken up by the header. Therefore,

this data unit field can only be a maximum of 244 (ie. 255-11) bytes

in length.

FCS - Frame Check Sequence - This is how we check to verify that what

was sent was indeed received correctly. We've already seen this in

other protocols as you may recall from previous issues of this

update.

ED - End Delimiter - This signifies the end of the telegram.

 

[Arlie]

ok guys you both lost me

I will check the info above and see what I find then get back to you

 

[Freddie]

Have you checked the GSD files, that are required? They should tell you the behaviour of the units.

 

[mordred]

Thats OK as stated Profibus protocals are not something I've had much dealings with but your scenario indicates that the accepted data is being limited by some setting sa you have not exceeded max. I posted the above to help provide a direction to look into as the header defines the length of data accepted so ther is probably a setting on your devices that modifies that header. But as stated above thats just a guess

 

[mordred]

can you post any info you have on those two units

 

[Manglemender]

"You have nothing to fear except fear itself" Profibus isn't all bad... just a bit missunderstood.

Tell us more details of your problems and maybe we can help. What Slaves? What DP Master? Method of communication? etc.

Nick

 

[mordred]

Also what type of Profibus here is some info I have also here is a link to seimens download section
http://www.ad.siemens.de/fea/html_76/down_module.htm