BECKHOFF FC5101 - Manual (Page 51)

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Service Data Objects (SDO)
The parameters listed in the object directory are read and written by means of service data objects. These
SDOs are multiplexed domains, i.e. structures of any size that have a multiplexer (address). The multiplexer
consists of a 16-bit index and an 8-bit sub-index that address the corresponding entries in the object directory.
SDO protocol: access to the object directory
The CANopen bus couplers are servers for the SDO, which means that at the request of a client (e.g. of the
IPC or the PLC) they make data available (upload), or they receive data from the client (download). This in-
volves a handshake between the client and the server.
When the size of the parameter to be transferred is not more than 4 bytes, a single handshake is sufficient (one
telegram pair): For a download, the client sends the data together with its index and sub-index, and the server
confirms reception. For an upload, the client requests the data by transmitting the index and sub-index of the
desired parameter, and the server sends the parameter (including index and sub-index) in its answer telegram.
The same pair of identifiers is used for both upload and download. The telegrams, which are always 8 bytes
long, encode the various services in the first data byte. All parameters with the exception of objects 1008h,
1009h and 100Ah (device name, hardware and software versions) are only at most 4 bytes long, so this de-
scription is restricted to transmission in expedited transfer.
Protocol
The structure of the SDO telegrams is described below.
Client -> Server, Upload Request
11 bit identifier 8 byte of user data
0x600 (=1536dez) + node ID 0x40 Index0 Index1 SubIdx 0x00 0x00 0x00 0x00
Parameters Explanation
Index0 Index low byte (Unsigned16, LSB)
Index1 Index high byte (Unsigned16, MSB)
SubIdx Subindex (Unsigned8)