Omron CQM1-ARM21, Operation Manual

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Performance
Section 3-3
20
3-3-1 AS-Interface Cycle Time
The maximum cycle time of an AS-Interface system is governed by the AS-
Interface Complete Specification. If the number of slaves on the AS-Interface
is less than the maximum of 31, the manufacturer is free to either fill the cycle
with specific additional commands, or reduce the bus cycle time. The CQM1-
ARM21 is designed to always provide the shortest possible bus cycle time.
In Normal Operation, without retransmission of messages due to
disturbances, the bus cycle time of the CQM1-ARM21 depends on the
number of detected slaves n as follows:
Formula T
AS-I
= 0.168 + n x 0.154 ms
With the maximum of 31 slaves this results in a cycle time of slightly under 5.0
ms.
The 0.168 ms is the time required in each bus cycle to execute the Inclusion
Phase. In this phase the master searches for new slaves on the AS-Interface
line. If no response is received, a time-out will cause the master to continue
with the next data exchange phase 0.168 ms after the start of the inclusion
phase.
The 0.154 ms is the time taken by each master request, slave response, and
the pauses in between. The exact value of course depends on the delay with
which each slave responds to a master request. The AS-Interface
specification allows a margin of 3 bit times, or 18
µ
s. All current slave
implementations will give approximately the same response time.
While executing a command, e.g. to read or write the slave’s parameters, the
cycle time will be increased by an additional 0.154 ms.
Retransmissions
Electrical disturbances can cause the master to retransmit data exchange
telegrams within an AS-Interface cycle. This can increase the AS-Interface
cycle time up to double the normal cycle time.
3-3-2 PLC Cycle Time
The PLC’s cycle time depends on:
- The PLC program to be executed.
- The number and types of I/O units in the system.
- Whether or not peripherals are connected to the CPU.
The CQM1-ARM21 will increase the PLC’s cycle time by 0.01 ms times the
number of allocated I/O words, i.e. a maximum of 0.16 ms.
See the CQM1/CPM1 Programming manual (W228-E1), for details on the
PLC cycle time.