Rockwell Automation 2100-GK61 DeviceNet to SCANport - Manual (Page 95)

Publication 2100-UM001B-EN-P – January 2001
Using DeviceNet Explicit Messaging 6-3
The scanner module requires a precisely-formatted block transfer
read and write size of 64 words. The Explicit Message Control table
in the scanner module is 64 words. The scanner module uses the
block transfer size as an indicator that the content is a client/server
request.
Message Translations The communications adapter provides electronic translations of
DeviceNet explicit messages into SCANport messages and back. The
format of all DeviceNet explicit messages supported by the adapter is
8:16: within an explicit message, the class field is 8 bits long, and the
instance field is 16 bits long.
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: both the BTW and BTR commands are
of module type “1771-SDN DeviceNet Scanner Module” and of
length “64.” The PLC can transfer two Explicit Messages per BTW or
BTR and the scanner can have up to ten 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 Data File messages is in Figure
6.2.