Publication 1785-UM012D-EN-P - July 2005
C-6 Maximizing System Performance
Input and Output Modules
Delay
All input and output modules have a “delay time,” which is the time that it
takes the module to transfer information to/from the I/O backplane through
the I/O module to/from the field device.
Depending on the type of modules you are using, these delay times vary; but,
the times must be taken into account when calculating system throughput.
Choose modules that perform the function that you need with the lowest
possible delay times.
I/O Backplane Transfer
The I/O backplane transfer time is the time it takes for the 1771-ASB adapter
module to exchange data with the I/O modules in the same chassis, generally
1-2 ms for a full I/O rack.
This time is fairly insignificant compared to total system throughput, but can
be optimized in situations where empty slots or modules that use only
backplane power in the chassis exist. For example, if the last four slots of a
rack contain a 1785-KA module and power supply (with two empty slots), the
1771-ASB can be configured to ignore those last four slots.
For more information about configuring adapter modules, see the 1771
Remote I/O Adapter Module User Manual, publication 1771-UM001.
Remote I/O Scan Time
The remote I/O scan time is the time it takes for the scanner to communicate
with each device in the remote I/O system.
ab
AdapterAdapter Adapter
IOT (x)
IIN (y)
x
y
Rack 3
Rack 2
Rack 1
Housekeeping
Scan
Logic
Processor-
Resident
Rack
Data
Exchange
Immediate I/O
Data Exchange
I/O Image
Table
Remote I/O
Buffer
Program Scan LoopRemote I/O Scan Loop
Update
I/O image
a
write outputs
b
read inputs
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