Bradley Smoker PLC-5 user manual Input and Output Modules Delay, Remote I/O Scan Time

C-6Maximizing System Performance

Input and Output Modules Delay

I/O Backplane Transfer

Remote I/O Scan Time

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.

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.

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 Scan Loop Program Scan Loop

Rack 3

Adapter

x y

a b

Immediate I/O

IOT (x)

IIN (y)

Rack 2

Rack 1

AdapterAdapter

Remote I/O Buffer

Data Exchange

x y

I/O Image	Update
Table	I/O image

Data

Exchange

Processor-

Resident

Rack

Housekeeping

a

write outputs

b

read inputs

Logic

Scan

Publication 1785-UM012D-EN-P - July 2005