CS8C © Stäubli 2009 – D28070504A
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Chapter 5 - Integration
Overrun error
If execution of a PLC cycle takes longer than the specified cycle time, the current cycle ends normally, and then
the following PLC cycle starts immediately. An Overrun error is generated (on the dOverrun digital output by
default) to enable a reaction to the PLC or VAL3 program.
The dOverrun digital output thus shows during each PLC cycle whether the previous cycle was carried out within
the specified time lapse or not. The name of the output can be changed (see"Configuration"). The value 1 shows
the Overrun error.
Input/Output error
If an Input/Output reading / writing error is detected, a RwError error is generated by the default dRwError digital
output to enable a reaction by the PLC or VAL3 program. The name of the output can be changed
(see"Configuration").
Interaction with the VAL3 program
When the PLC cycle time is not null:
Execution of the PLC program takes priority over the VAL3 program. The VAL3 program is only executed once
the PLC program is waiting, between two PLC cycles.
Execution of a VAL3 program can be interrupted at any time by a repeat of a PLC cycle. If synchronization is
required between the PLC and the VAL3, it has to be programmed using the Inputs/Outputs.
When the PLC cycle time is null:
Execution of the VAL3 program takes priority over the PLC program. The PLC program is only executed once the
VAL3 program is waiting, between two VAL3 cycles.
Inputs/Outputs are refreshed with each PLC cycle, each VAL3 synchronous cycle and each VAL3 asynchronous
cycle.
PLC utility for the MCP
The main menu of the CS8C includes a "PLC" utility that displays the main characteristics of the PLC program
being executed:
• Requested PLC cycle time (as defined in the PLC programming environment)
• Measurement of the execution period for the last PLC cycle (this period includes the execution time for
the critical system tasks)
• Maximum execution time measured for the PLC cycles since the previous PLC start
• TCP port for communication with the PLC programming environment
CPU load
The ratio between the execution period for the previous PLC cycle and the requested PLC cycle time gives the
current CPU load for the PLC program. The ratio between the maximum execution period for the PLC cycle and
the requested PLC cycle time gives the maximum CPU load for the PLC program.
The unused CPU load is allocated as a priority to the VAL3, and then to non-critical system tasks (screen
refreshing, Ethernet communication). As a rough guide, it is necessary to leave about 30% of the CPU load to
obtain correct execution of a VAL3 program and the system. For a very simple VAL3 program, we can let the PLC
use 90% of the CPU load. For a complex VAL3 program, the PLC program should not exceed 50% of the CPU
load.
CAUTION:
If the delay between two PLC cycles is not long enough, execution of the VAL3 program can
be slowed down considerably. It is then necessary to increase the PLC cycle time to leave
more time for the VAL3, or to specify a null cycle time for the PLC (see "CPU load").
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