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| 7/11/2010 2:44 PM | |
Joined: 10/7/2005 Last visit: 3/20/2025 Posts: 3042 Rating:
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Hello mbm I have a slightly different take on things. To begin with a CP341 module occupies "only" 16 I/O Bytes in an ET200M (out of the max 244 Bytes that a DP Slave can have). This 16 I/O Bytes is a "data channel" that you needn't worry about though. The serial comms Blocks that you use in the S7 together with the data buffer in the CP341 "take care" of your Modbus related data transfer. It is the DONE and/or NDR bit from the the serial comms Blocks that you use in the S7 that will let you know when you can trigger another Modbus related Read or Write request. As such you do NOT need to worry about how many Modbus Registers you will want to Read or Write per Modbus Slave. Even if yourun your Profibus at a very low baudrate (e.g. 9.6 Kbps) and your Modbus at the highest that the CP341 supports (76.8 Kbps) and request a large amount of registers,the only thing you will notice is that it will take a loooooong time before you get your data from the Modbus Slave. Last but not least, as you'll communicate with only one Modbus Slave at any given time, italso won't matter how many Modbus Slaves you have allup (you'll process them sequentially in your logic). You will obviously however have a slower overall data refresh rate the moreModbus Slaves you have on one Modbus Network. I hope this helps |
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| 7/11/2010 5:26 PM | |
Joined: 7/2/2008 Last visit: 3/19/2025 Posts: 928 Rating:
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Hello fritz , I hope you would forgive my illiteracy of communication terms and mechanisms. I was waiting for someone to clarify the 16 I/O bytes in CP configuration and then you came, Does this mean that data received through Modbus is retransmitted through Profibus using these 16 bytes?! So, if we are using the maximum of Modbus255 bytes, it will be transferred cyclically over 16 bytes. If it works like this in Profibus then how does it work with CPs connected to back plane of CPUs? How bigwill bethe data channel in S7 communication? Isn't it 115.2 Kbps? It really does. Best regards. H-H |
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| 7/12/2010 4:05 AM | |
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Joined: 10/7/2005 Last visit: 3/20/2025 Posts: 3042 Rating:
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Hello H-H good on you for probing deeper and asking the "tough" question, I'll try to shed some more light on it.
It would have to be, the 16 I/O bytes "data channel" are essentially just a Profibus extension of the "peripheral bus" that the S7 300uses if the CP341 wouldbe located in the local CPU rack.
Correct again
It works in the same way. Any S7-300 CP occupiesan I/O area which is the "P-Bus" (Peripheral bus) which isusedby theS7 CPU to exchange data with the CP(Siemens does NOT make the exact mechanims behind the P-Bus comms known and it is really irrelevant for the user too). S7 400 PLC's on the other hand have a "real" backplane and use the "K-Bus" (Communication bus) rather than the P-Bus for CPU-CP related data exchange. As such, S7 400 CP's do NOT occupy an I/O area but will only occupya Diagnostic Input address in HWconfig. To make matters more confusing,300's also have a "K-Bus" but it is slooow and hence NOT used for CP-CPU reated data exchange a far asI'm aware (and 400's have of course also a P-Bus but do NOT need to use it for CPU-CP related data exchange because fo the available highspeed K-Bus on their backplane). Decdid some interesting research into the S7 300 "backplane"and it can be found in THIS thread.
It is always16 I/O Bytes irrespective of the protocolfor S7-300 CP's (be it an Ethernet CP, or Profibus CP or PtP CP).
not for a CP34x, 115.2 Kbps is only available for some S7 400 PtP CP's (which won't quite fit into an ET200M And one last thing for mbm: Thetechnical spec of the PtPCP Modbus Master manual contains example calculations about the expected update times which may come in handy for your evalution. I hope this helps andwelcome more opinions from others on this subject. |
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| 7/21/2010 10:04 AM | |
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Joined: 10/7/2005 Last visit: 3/20/2025 Posts: 3042 Rating:
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You spoiled the party dear J_Bell(good on you though), I was hopingfor someone to ask the question why the S7-300 has a K-bus at all Nevermind,here's my "best guess" on the K and P Bus usage forS7-300 CP's (even though thisis "surplus" knowledge and of no consequencefor the user, I am just too curious and perhaps someone can verify or correct this): S7-300 P Bus: SEND/RECEIVE,GET/PUT etc. user data "data channel" (for all CP's, be it Ethernet, Profibus, ASIor Serial). S7-300 K Bus: -Module status/diagnostic "data channel"(for all CP's). - Startup/Initial configuration data "data channel" (for all CP's) - NCM S7 diagnostic "data channel"(for Ethernet and Profibus CP's only). - Routing "data channel" (for Ethernet and Profibus CP's only). |
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