Rockwell Automation 1771-DCM USER MANUAL 1771-DCM - Manual (Page 7)

Overview of Direct
Communication Module
Chapter 2
2Ć2
The 1771-DCM passes data table values such as command bits, status
bits, and data blocks between supervisory and local processors. The
supervisory processor typically writes commands and/or data table values
to the local processor, and reads resulting status, diagnostic data, and data
values from the local processor (or vice versa).
The supervisory processor communicates serially with the 1771-DCM as
though it were a 1771-ASB Adapter Module, and addresses the
1771-DCM as though it were addressing a separate I/O chassis. You
select the mode of transfer between the supervisory processor and
1771-DCM: discrete transfer (up to eight words) or block transfer (up to
64 words). Either mode includes one status word.
The local processor communicates with the 1771-DCM over the
backplane using block transfers and addresses the 1771-DCM as an
intelligent I/O module. When the local processor is remotely configured,
it communicates with the 1771-DCM via block transfer through its
scanner-adapter link.
The 1771-DCM and data highway allow communication between PC
processors. Consider the following factors when determining the
appropriate choice. Use the 1771-DCM to:
Transfer data table words
Obtain predictable transfer times
Transfer up to eight words when transfer time over a remote I/O
channel is adequate, in addition to the transfer time of the local
processor and 1771-DCM
Transfer up to 64 words when transfer time for block transfers over a
remote I/O channel is adequate, in addition to the transfer time of the
local processor and 1771-DCM
Decrease data highway traffic
At least one processor must have a remote I/O scanner.
You cannot transfer ladder diagram programs nor messages stored in the
message area of memory using the 1771-DCM.
You can select one or more of the following options depending on your
application requirements using switches on the module:
Choosing 1771ĆDCM or Data
Highway
Selecting Options