Siemens SIMATIC S7 Manual page 46

Cyclical Operation
S7 is Server
MODBUS / TCP communication via the integrated PN interface of the CPU
2XV9450-1MB02; Manual edition 2.1
Verification in the response telegram:
•
Received transaction identifier is equal to the sent one.
•
Protocol identifier = 0
•
Length is between 3 and 253
Additionally, the length in the header of the response telegram is
checked for plausibility regarding the request.
•
Sent UNIT is equal to the received one
•
Sent FC is equal to the received one
•
Response is an exception code telegram
•
For write requests, the start address and number of registers/bits
have to match with the request telegram.
•
FC 5 or FC6:
Echo in the respond is equal to the request
•
Receipt of the second part of the request telegram within the
monitoring time
The monitoring time can also elapse if less data than specified in the
MODBUS telegram header is received. Subsequent errors with loss
of telegrams can occur.
•
Protocol Identifier = 0
•
Length between 6 and 207
•
Received function code is verified. If the function code is not equal to
1, 2, 3, 4, 5, 6, 15 or 16 an exception telegram is sent.
•
For write requests, the length in the header, the number of registers
or bits and the byte count in the telegram must match.
•
The number of registers or bits is verified. If the number is too large,
an exception telegram is sent.
•
During the execution of a request, there is also a check whether the
data block specified by the register or bit address is available and has
the necessary length. The DB number must not be 0 or identical to
the number of MODBUS_PARAM or the instance DB. In case of an
error, an exception telegram is sent.
Diagnosis
7-4