Rockwell Automation 2755-SNx Adaptascan Bar Code Readers - Manual (Page 245)

12–24 Communicating with a PLC-5 Processor on a DeviceNet Network using Explicit Messaging
Publication 2755-6.8
The scanner module recognizes I/O data and control as higher
priorities over explicit messaging on DeviceNet.
Message lengths and slave device types impact transaction message
completion times. If the processor has queued multiple Explicit
Message Transactions to the scanner module for multiple slave
devices, the transactions with the slaves may not complete in the
order in which the requests were received. The slave responses are
queued to the 64 word Block Transfer Read in the order in which
they are received. As response transaction blocks are uploaded, the
processor’s program matches the responses to the requests using the
TXID field.
Request
Transaction
Block
Response
Transaction
Block
Ladder Scans
Process Requests
and Responses
Request
Transaction
Blocks
Response
Transaction
Blocks
Scanner
Response
Queue
Scanner
Request
Queue
Execute
Done or
Error-detected
BTR
BTW
Processor Scanner Module
DeviceNet
Explicit Message
Requests and
Responses
DeviceNet Network
Slave
Device
Explicit Message Program Control Limitations
•
The processor is always the DeviceNet client and the slave is
always the DeviceNet server.
•
A maximum of 10 Explicit Message Request Transaction Blocks
with the execute command can be queued to the scanner module
at any time. For example, 5 Block Transfer Write files containing
two transactions each, can be performed at any time. The scanner
module receives and deletes any additional client/server requests
with the execute command over the maximum of 10.