Programming .01Second Timers
Appendix C
C5
Changing the timer presets in this manner also enables you to fine-tune the
system without physically adjusting the locations of detection devices.
When considering use of the 10-msec timer, you must consider other
timing factors, both within the programmable controller and in the
hardware devices. Several examples are:
Every input device requires a length of time to change state.
Photoelectric devices and electromagnetic proximity switches typically
operate in the range of 3 to 50 msec. Mechanical switches and magnetic
control relays can require longer times for operation.
Some input modules may provide a slight delay resulting from the input
filter time constant: typically 10-25 msec.
The execution of each program instruction requires a certain length
of time. Instruction execution times are discussed later, in relation to
program scan-time computation.
Scan time (I/O scan + program scan) depends on the number and type
of instructions, as discussed below. Incorporation of Immediate Input
and Immediate Output instructions can compensate for the length of
scan time.
DC Output modules typically require from 1 to 5 msec for response; AC
Output modules require 3 to 10 msec for response, depending on the
instantaneous value of the AC wave when the turn-on signal is applied.
Your output devices may take 50 to 100 msec or longer to operate after
current is applied. Inductive loads, or devices with substantial surge
suppression circuitry, may also have longer response time.
Each of the items discussed above will have an impact on the actual time
delay obtained from a programmable timer. Selection of fast-response
input devices is your responsibility and is beyond the scope of this
document. For selection of suitable I/O modules, contact your local
Allen-Bradley representative for further assistance.
The remainder of this Appendix discusses programming techniques which
you must use to effectively program 10-msec timers. The required
programming is based on two concepts:
Scan time
Sequential program scan
C.4
Hardware/Processor
Considerations
C.5
10Msec T
imers -
Programming Techniques
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