Presentation / operation
ABB Control - AC 31 Page 1-9
1SBC 260400 R1001-b 06.99
3.2. Program execution
The central unit microprocessor ensures the cyclic execution of the system as shown in
Figure 1-4.
The internal processing:
- PLC monitoring and control
- and processing requests from the terminal operator,
is executed in parallel with the previously described cycle.
The main program is processed sequentially. It may call up to a maximum of 12 sub-programs.
Each sub-program may be called numerous times in the main program.
Three types of interruptions may be executed parallel to the main program:
- A cyclic interruption
- A warning interruption triggered by an event on the I 62.03 input
- A warning interruption triggered by an event on the I 62.02 input
The interruptions have priority over the main program execution. If all three interruptions are
triggered simultaneously then the interruption triggered by I 62.03 has priority over the I 62.02
input interruption which in turn has priority over the cyclic interruption. Once an interruption has
been launched it cannot be interrupted by another (Figure 1-5).
The execution duration of a cycle (bus cycle + program cycle) is controlled by the central unit.
Any excess of the cycle time defined by the user in AC31GRAF is signaled by the ERR Led, at
the front of the central unit, as of the first program cycle.
3.3. Bus transmission
The master central unit manages the transmission of messages to the various slaves over a
RS485 serial liaison.
The messages are transmitted under the following format:
- Request from the master central unit:
address data CRC8
- Request from the remote units:
start data CRC8
The messages always end with an end of frame control: checksum CRC8.
The length of the exchanged frames depends on the type of unit. Frames exchanged with an
analog unit are the longest.
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