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4-5
H Using Scan Lists
D Enabling/Disabling a Scan List
To enable a scan list, turn ON the Enable Scan List software switch or use the Configurator. To disable a
scan list, turn ON the Clear Scan List software switch.
Note 1. Scan lists cannot be disabled from the Configurator. The scan list will always be enabled when
created on the Configurator and registered in a Master Unit.
Note 2. Always enable the scan list during actual system operation. Although operation is possible
with the scan list disabled when the fixed allocations are used, communications will continue
even if Slaves fail, creating the possibility of undetected errors in system operation.
D Scan List Enabled
When a scan list is enabled, communications are performed according to the scan list stored in the Mas-
ter Unit’s non-volatile memory. Either fixed allocation or free allocation of remote I/O is possible.
The Master will communicate only with the Slaves registered in the scan list. A verification error will
occur if a Slave registered in the scan list does not exist in the Network or is not started when I/O commu-
nications begin. Even if it is properly connected in the Network, a Slave which is not registered in the
scan list will not participate in communications and will not be checked for errors.
D Scan List Disabled
When a scan list is disabled, communications are performed without using the scan list. The scan list is
disabled when the scan list has not been created or it has been cleared. Do not disable the scan list
during actual system operation.
All of the Slaves in the Network can communicate with the scan list disabled, so Slaves can be added to
the Network while communications are in progress. Without a scan list, however, there is no way to
check for Slaves that have not started or have failed, and errors can thus go undetected. The commu-
nications cycle time will be much longer that the computational value when the scan list is disabled.
When a scan list is disabled, operation is possible only by using the fixed I/O allocations. Free allocation
is not possible.
Note 1. The scan list is automatically enabled when the Configurator is used to allocate remote I/O. If
the software switch is used to clear the scan list, remote I/O communications will operate un-
der the default allocations with the scan list disabled. Always confirm that the system is
stopped before disabling the scan list in a Master Unit. This is particularly important if there is
more than one Master on the same Network. If the scan list is disabled for one of the Masters,
normally communications will not be possible.
Note 2. The scan list data in a Master Unit is deleted whenever the scan list is disabled.
4-1-3 Fixed Word Allocations
H Allocation Areas for Different PCs
If fixed allocations are used, the node addresses and the words allocated to them are determined ac-
cording to the PC used. The remote I/O allocation area consists of the input area that receives input from
the Slaves, and the output area that writes output data to the Slaves.
CompoBus/D System Startup Chapter 4