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Point-to-point communication
6.9 Protocol Description
SEND Message Frame
When transmitting a SEND message frame, the CPU transmits an instruction frame including
user data. The communication partner responds with a response message frame that
contains no user data.
FETCH Message Frame
When transmitting a FETCH message frame, the CPU transmits an instruction frame that
contains no user data. The communication partner responds with a response message
frame that includes user data.
Sequential Message Frame
With SEND and FETCH message frames, sequential message frames are transmitted
automatically if the data length exceeds 128 bytes.
Message Frame Header
With RK 512 computer connection, every message frame is initialized with a header. It can
contain message frame IDs, information on data source and destination as well as an error
number.
The table below shows the structure of the instruction frame header:
Byte
Description
1
Message frame ID in instruction frames (00H),
for sequential instruction frames (FFH)
2
Message frame ID (00H)
3

'A' (41H): SEND request with target DB

'O' (4FH): SEND request with target DX

'E' (45H): FETCH request
4
5
6
Data to be transferred are fetched from a (only 'D' possible for sending):

'D' (44H): Data block 'X' (58H) = expanded data block

'E' (45H): Input bytes 'A' (41H) = Output bytes

'M' (4DH): Memory bytes 'T' (54H) = Time cells

'Z' (5AH): Counter cells
Data target with SEND job or data source with FETCH job, e. g. byte 5 = DB no., byte 6 =
DW no.*
7
High byte length: Length of data to be transmitted in bytes or word format depending on the
type
8
Low byte length: Length of data to be transmitted in bytes or word format depending on the
type
9
Byte number of the interprocessor communication flag; if you have not specified an
interprocessor communication flag, the value FFh is entered here.
CPU 31xC: Technological functions
Operating Instructions, 03/2011, A5E00105484-05
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