DL205 User Manual, 4th Edition, Rev. B
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Chapter 3: CPU Specifications and Operations
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During the Service Peripherals portion of the scan, the CPU analyzes the communications
request and responds as appropriate. The amount of time required to service the peripherals
depends on the content of the request.
CPU Bus Communication
Some specialty modules can also communicate directly with the CPU via the CPU bus.
During this portion of the cycle the CPU completes any CPU bus communications. The
actual time required depends on the type of modules installed and the type of request being
processed.
NOTE
: Some specialty modules can have a considerable impact on the CPU scan time. If timing is critical
in your application, consult the module documentation for any information concerning the impact on the
scan time.
Update Clock/Calendar, Special Relays, Special Registers
The clock, calendar, and special relays are updated and loaded into special V-memory
locations during this time. This update is performed during both Run and Program Modes.
Writing Outputs
The time required to write the output status for the local and expansion I/O modules
depends on which CPU you are using and the number of output points in the base. The
following table shows typical update times required by the CPU.
For example, the time required for a DL240 to write data for two 8-point output modules
would be calculated as follows (where NO is the total number of output points):
Formula
Time = 33 + (NO x 14.6µs)
Example
Time = 33 + (16 x 14.6µs)
Time = 266.6µs
Timing Factors
DL230
DL240
DL250–1
DL260
Overhead
66.0 µs
33.0 µs
28.1 µs
28.1 µs
Per output point
8.5 µs
14.6 µs
3.0 µs
3.0 µs
Modes
DL230
DL240
DL250–1
DL260
Program Mode
Minimum
8.0 µs fixed
35.0 µs
11.0 µs
11.0 µs
Maximum
8.0 µs fixed
48.0 µs
11.0 µs
11.0 µs
Run Mode
Minimum
20.0 µs
60.0 µs
19.0 µs
19.0 µs
Maximum
26.0 µs
85.0 µs
26.0 µs
26.0 µs
To Service Request
DL230
DL240
DL250–1
DL260
Minimum
260 µs
250 µs
8 µs
8 µs
Run Mode Max.
30 ms
20 ms
410 µs
410 µs
Program Mode Max.
3.5 Seconds
4 Seconds
2 Seconds
3.7 Seconds