Rockwell Automation 1336-GM6 Enhanced DeviceNet Communications Module - Manual (Page 72)

7-2 Using DeviceNet Explicit Messaging
Messaging for the 1771-SDN
Scanner The PLC uses a 64-word Block Transfer Write (BTW) to copy an
Explicit Message into the 1771-SDN scanner. Ten explicit message
buffers are available within the 1771-SDN scanner. When the BTW
completes, the scanner executes the message. The PLC must then poll
the scanner by performing a 64-word Block Transfer Read (BTR) to
complete the message. (Note that the PLC can transfer two Explicit
Messages per BTW or BTR and the scanner can have up to 10
Explicit Messages active at any time.)
When the BTR completes, the data received by the PLC will contain
information about the status of the current Explicit Message being
processed by the scanner. If an Explicit Message has completed, the
STATUS code in the Explicit Message Response is set to 1 and the
response message contains the data requested.
If the message status indicates that it is not completed, the BTR
should be repeated until the message is complete.
The format of Request and Response messages is in Figure 7.1Figure
7.1.
Figure 7.1 Format of DeviceNet Messages
Format of 64-word Block Transfer
Write for Explicit Message Request
Format of 64-word Block Transfer
Read for Explicit Message Response
15 0 15 0
Transaction #1
Header (3 words) TXID COMMAND TXID STATUS word 0
PORT SIZE PORT SIZE
SERVICE MAC ID SERVICE MAC ID
CLASS SERVICE RESPONSE DATA
INSTANCE "
ATTRIBUTE "
SERVICE DATA "
" " word 31
Transaction #2
Header (3 words) TXID COMMAND TXID STATUS word 32
PORT SIZE PORT SIZE
SERVICE MAC ID SERVICE MAC ID
CLASS SERVICE RESPONSE DATA
INSTANCE "
ATTRIBUTE "
SERVICE DATA "
" " word 63