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Entry type: FAQ Entry ID: 58378237, Entry date: 03/13/2019
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Which blocks for Modbus/TCP can you rename or re-wire when the block numbers of the Modbus blocks are already being used in the user program?

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The blocks that are available for Modbus/TCP communication with SIMATIC S7 are delivered with a unique block number.

If there are already blocks in the user program that have these block numbers, there is a block number conflict.

Please proceed as follows to avoid this conflict.

  • You can rename the Modbus blocks that are called directly in the user program.
  • You cannot rename the blocks that are called internally in the Modbus blocks. You have to re-wire those blocks. When re-wiring all the internally called blocks must be in the STEP 7 project, this means that you do not re-wire in the library, but you use the sample project for re-wiring.

The following table shows the Modbus blocks that you can rename or re-wire.
 

ProductArticle numberModbus block  > RenameBlock called internally from the Modbus library > Re-wireBlock called internally from the Standard / SIMATIC NET library > Re-wire
Modbus/TCP CP
up to V5
2XV9450-1MB00FB108FB106, FB107AG_(L)SEND
AG_(L)RECV
EQ_STRNG(*)
AG_CNTRL(**)
Modbus/TCP CP V66AV6676-6MB00-6AX0
6AV6676-6MB00-6AD0
 FB905FB number cannot be changed:
FB906, FB908
AG_(L)SEND
AG_(L)RECV
AG_CNTRL
Modbus/TCP PN CPU up to V2.6.12XV9450-1MB02FB102FB103, FB104, FB105TSEND
TRCV
TCON
TDISCON
EQ_STRNG(*)
Modbus/TCP PN CPU V36AV6676-6MB20-3AX0
6AV6676-6MB20-3AD0
FB900FB number cannot be changed:
FB901, FB903

TSEND
TRCV

TCON

TDISCON

Modbus/TCP CP Redundant V22XV9450-1MB11FB909, FB907FB908, FB906AG_(L)SEND
AG_(L)RECV
AG_CNTRL
Modbus/TCP CP Redundant V3

6AV6676-6MB30-3AX0
6AV6676-6MB30-3AD0
6AV6676-6MB30-4AX0
6AV6676-6MB30-4AD0

FB909, FB907FB908, FB906AG_(L)SEND
AG_(L)RECV
AG_CNTRL
Modbus/TCP PN CPU Redundant6AV6676-6MB10-0AX0
6AV6676-6MB10-0AD0
FB917, FB915FB913, FB916, FB914TSEND
TRCV
TCON
TDISCON
Modbus/TCP SENTRON PAC6AV6676-6MA30-0AX0
6AV6676-6MA30-1AX0
6AV6676-6MA30-2AX0
FB109, FB110, FB111FB99, FB103, FC99TSEND
TRCV
TCON
TDISCON
TUSEN

Table 1

(*) The EQ_STRNG block is only in older versions of the product.
(**) If both EQ_STRNG and AG_CNTRL are used by the Modbus blocks, then AG_CNTRL is already re-wired in the Modbus library. More information about this is available in the relevant product manual.

You must follow a specific order when re-wiring the blocks that are called internally in the Modbus blocks. Re-wire the blocks - if necessary - in the order listed.

  1. EQ_STRNG, AG_CNTRL, AG_(L)SEND, AG_(L)RECV, TSEND, TRCV, TCON, TDISCON
  2. Internally called blocks (FB106, FB107, FB1734, FB908, FB906, FB103, FB104, FB105, FB913, FB914, FB915, FB99, FB103, FC99)
  3. Remaining blocks

You do not have to re-wire all the blocks. Even if you re-wire only a few internally called blocks, you must nevertheless keep to the order given above. In this case, you leave out the blocks that are not re-wired.

Note on the Modbus/TCP PN CPU
If the T blocks are re-wired, the MODBUSPN block must have a higher number than the lower-level blocks.

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