Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 134)

Chapter 10
Maximizing System Performance
10-2
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 there are empty slots or modules that
only use backplane power in the chassis. For example, if the last four slots
of a rack contain a 1785-KA module and power supply (with 2 empty
slots), the 1771-ASB can be configured to ignore those last four slots.
For more information on configuring your ASB module, refer to the 1771
Remote I/O Adapter Module User Manual, publication 1771-6.5.83.
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.
Remote I/O
Buffer
Remote I/O Scan
x y
Adapter
Adapter
Adapter
Rack 3
Rack 2
Rack 1
Program Scan
Read Inputs
Write Outputs
Immediate I/O
IIN (y)
ProcessorĆ
Rack
Data Exchange
I/O Image
Table
x
y
Resident
Housekeeping Data
Exchange
IOT (x)
Logic
Scan
Three factors that affect the remote I/O scan time are:
communication rate
number of chassis
block transfers
Communication Rate
The baud rate determines the time it takes for the scanner to communicate
with each individual entry in its scan list. Table 10.A lists the amount of
time required to communicate to a device at each communication rate.
I/O Backplane Transfer
Remote I/O Scan Time