Rockwell Automation 1747-DCM,D17476.8 Direct Communication Module - Manual (Page 20)

Chapter 1
Overview
1–6
Information is transferred between the RIO scanner and the DCM every RIO
scan. RIO transmissions are asynchronous to the processor scans.
Important: The DCM provides word integrity on all words transferred via
the SLC backplane.
The DCM communicates with supervisory PLC or SLC controllers through
RIO scanners, as if they were addressing a logical rack. However, the DCM
does not scan the I/O in its local I/O chassis, rather it passes the supervisory
data to the distributed SLC processor.
In the DCM, outputs from the SLC output image table are inputs to the
supervisory processor input image table. Likewise, outputs from the
supervisory processor output image table are inputs to the SLC input image
table.
The diagram that follows depicts the communications flow between an RIO
scanner and the DCM.
Output Device
Input Device
Outputs from
Modules
Inputs to Modules
Outputs from PLC,
Input Data to DCM
Inputs to PLC, Output
Data from DCM
Scanner
RIO Scan
Processor Scan
DCM I/O Module
Distributed Processor Scan
SLC Chassis
SLC Expansion
Chassis
I/O Module
SLC Distributed
Processor
Supervisory PLC or SLC
What Is the Status Word?
The first word of the DCM input and output image is the status word. The
status word indicates the status of communication and data between the RIO
scanner and the DCM. For more information on status words and their
applications, see chapter 6, Programming.
Data Exchange Between RIO
Scanners and the DCM