ABB S800 I/O Manual page 64

Functional Description
Data Scanning Principles
The data transfer between PROFIBUS-DP/DPV1 and the ModuleBus is not
synchronized. Read and write operations are performed from and to a dual port
memory in the FCI.
The ModuleBus data is scanned (read or written) cyclically, depending on the I/O
module configuration. On one scan all digital modules, 1/4 of the analog modules
and 1/10 of the slow analog modules are scanned. It takes 4 scans to read all analog
modules and 10 scans to read all slow analog modules.
Redundant FCI
CI840 can work as a redundant pair where one FCI is primary and one is backup. 
As long as the primary works correct it will handle both the PROFIBUS and the
ModuleBus. Both FCIs supervise each other. If a fault occurs in the primary the
backup will automatically take over.
Hot Configuration In Run
CI801 and CI840 supports the function Hot Configuration In Run (HCIR).
HCIR is a function for modifying field device configuration without disturbing the
running system. The following actions can be done:
•
•
•
During the configuration the values are frozen for a short moment. The time is
dependent of the PROFIBUS master, communication speed, CI801, CI840, type of
configuration and type of changes. Typical 300 ms at a communication speed of 
1.5 MHz.
The configuration time is supervised by a watchdog. If the watchdog time elapse,
the outputs will go to OSP (Output Set as Predefined). The watchdog time consists
of two parts, one part calculated by the PROFIBUS master and one fix part for the
slave (CI801 and CI840 1200 ms).
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Delete modules
Insert modules
Parameter changes
Section 4 Operation
3BSE020926-600