Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 74)

5–2 Data-link Layer Message Frames
Publication 1770Ć6.5.16 - October 1996
Half-duplex protocol uses three types of transmissions:
•
polling frame
•
master message frame
•
slave message frame
The master node transmits both polling frames and master message
frames, and slave nodes transmit slave message frames.
The following figure illustrates the formats of these frames.
The slave message frame has the same format as the full-duplex
message frame. The master message frame is the same as the slave
message frame except that it is prefixed with DLE SOH and an
address to specify a slave station address (STN).
Important: Even if you set your module to use CRC, when you
send a polling frame, it uses a single BCC byte.
BCC ENQ DLE STN
CMD STS SRC TNS DLE DST STX DLE SOH STN DLE ETX
BCC/
CRC
Data
(command data)
From common application routines
CMD STS SRC TNS DLE DST STX DLE ETX
BCC/
CRC
Data
(command data)
From common application routines
Polling Frame
Master Message Frame
Slave Message Frame
The BCC field contains the 2's complement of the 8Ćbit sum (moduleĆ256 arithmetic sum) of the slave station address
(STN) and all the data bytes in the frame. For polling frames, the BCC is simply the 2's complement of STN. The BCC
does not include any other message frame symbols or response symbols.
HalfĆduplex Protocol
Message Frames