Allen-Bradley ControlNet PLC-5 User Manual page 43

PLC-5 Programmable Controller
Logic Scan
In scheduled I/O data
transfer, updates occur
Data
between logic scans
Table
(i.e. during
Files
"housekeeping")
Housekeeping
Program Scan
3
Input data from the private memory buffer is put
into the input image file. New input data
may stay in the private memory buffer up to 1
program scan or until the next housekeeping.
What Happens During Scheduled Input Data Transfer
Private
Scheduled Data
Memory
Data Update
Transfer
Buffers
Scheduled data transfer
occurs between the PLC-5
and the adapter module in
the I/O chassis. Data may
stay in the private memory
buffer for up to 1 API.
The frequency of I/O chassis backplane updates depends on the
adapter, the type of scheduled data transfer (rack or module
connection) and the number of different modules being updated.
Actual I/O throughput times must be determined with real products to
determine if the I/O throughput times meet the requirements of the
application.
AB Drives
Planning to Use Your ControlNet PLC-5 Processor
I/O Chassis
Private
Memory
Data Update
Buffers
Adapter Module
2
Input data is written from the I/O module
to a private memory buffer on the adapter.
The new input data may stay in
the private memory buffer for as long
as it takes to complete the I/O chassis
backplane update.
In the same manner,
the I/O adapter has
scheduled I/O data
transfer with I/O
modules
I/O Modules
1
1785-UM022B-EN-P - February 2002
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