Rockwell Automation 1771-SDN DeviceNet Scanner Module Installation Instructions - Manual (Page 32)

Publication 1771-IN014B-EN-P - September 2001
32 DeviceNet Scanner Module Catalog Number 1771-SDN/C
Depending on the network load, the scanner could take a few
seconds to complete the request. When a response is loaded, bit 15 of
the module status register is set to 1. The program may have to poll
the scanner module a number of times before the scanner returns
a Response Transaction Block.
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