Rockwell Automation 1747-PT1, D1747NP002 Hand-Held Terminal - Manual (Page 468)

Appendix D
Estimating Scan Time
D–2
Worksheets A, B, and C on the following pages are for use with SLC 500
systems as follows:
•
Worksheet A – Fixed controllers
•
Worksheet B – 1747-L511 or 1747-L514 processor
•
Worksheet C – 1747-L524 processor
These worksheets are intended to assist you in estimating scan time for your
application. Refer to appendix C for instruction execution times. Refer to
the SLC 500 System Overview, publication 1747–2.30, for I/O module part
numbers and sizes.
An example scan time calculation appears on page D–6.
Defining Worksheet Terminology
When you work through the worksheets, you will come across the following
terms:
Background Communications – Occurs when your processor is attached to
an active DH–485 network. During this event the processor accepts
characters from the network and places them into a packet buffer.
Foreground Communications – Occurs only when another node is
attached, or when another processor sends an MSG instruction to your
processor. During this event the processor performs the communication
commands contained in completed packets built during background
communications.
Forced Input Overhead – This value is included in your scan time
whenever input forces are Enabled in your program.
Forced Output Overhead – This value is included in your scan time
whenever output forces are Enabled in your program.
Single Step – When using this function with a 5/02 processor, you can
execute your program one rung or section at a time. This function is used for
debugging purposes.
Multi–Word Module – Example of multi–word modules are DCM, analog,
and DSN.
Scan Time Worksheets