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echochange
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Echochange - Manual / echochange
2.6 Constraints of Protocol Conversion
Protocol-specific conditions, such as physical memory, S7 SSLs, PLC diagnostics or PLC circuits,
place constraints on protocol conversion. These elements are not supported and, consequently,
errors cannot be evaluated. Therefore, visualizations that are based on these functions (WinCC with
internal driver) cannot be used.
If requests do not match on both sides, no request is made on the destination side and an error is
immediately returned to the requester. This is the case, for example, with byte requests via word
protocols. Word/byte data is not converted. Byte swapping is processed, however.
Rockwell CLX uses little-endian, S7/S5 and Modbus usually use big-endian.
The S7 element types of the request are not evaluated because standard systems always request
bytes. The setting of the allocation table is used here. The byte setting prevents any swapping from
taking place. Bit requests are not possible. Bits are not included in any of the settings. If the
destination does not reply to the request, the requester waits for the result. The requester must not
make any further requests until it has received the response. To cancel a request, the connection
has to be reset. Standard request systems such as the OPC server or Visu driver behave like this.
PLC requesters sometimes do not know this behavior. They must wait or the PLC program must be
capable of a timeout. S5 protocols are capable of requests of any length and requests with undefined
lengths. The requests with undefined lengths (joker length) are not supported. Requests can have a
maximum length of 750 bytes.
Many protocols recognize detailed data types, but standard systems do not use them (particularly
with S7 and OPC servers). Applications' requests with variable offsets and lengths are combined into
one long request by these systems. Multiple offsets in the allocation table cannot be used in that
case. If necessary, the requester needs to adjust the optimization. Rockwell CLX is capable of arrays
and structure arrays. Arrays can only be processed from the beginning. Structure arrays are not
supported. When browsing, arrays and structure arrays are not resolved. For arrays, the browse
window displays the array length. Browsing of large PLC symbol lists is slow. After a maximum of 10
seconds, a browse request is canceled. This happens, in particular, with slow or overloaded
connections. Usually this behavior is caused by the short CLX frame lengths. The ping time has a
greater effect than the actual line speed. CLX structures that are completely specified in Modbus
register lists, for example, are not able to adapt the data individually.
When the source permits longer requests than the destination, the requests are processed by the
destination one after the other. If this processing exceeds the target length in the destination, the
transmission is aborted with an error message. During the write operation, part of the data has
already been transferred to the destination, however. The possible error signaling varies from protocol
to protocol. If an error occurs, protocol errors are converted although detail information may be lost.
Examples: S5 only recognizes “Data block does not exist” and “Data block is too short”, while S7
recognizes a large number of error codes. Rockwell CLX returns a general protocol error for non-
existent symbols or deviating data types. If both protocol sides support multiple requests in one
frame, the multiple requests are not always used on the destination side.
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