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3-1 LR Area Data
PC Link Systems employ the LR area in the exchange of data. The content
of the LR areas in all PCs in the same PC Link Subsystem (or Single-level
System) is kept consistent. To achieve this, the LR area is divided among all
of the PCs in the Subsystem according to switch settings, and each PC
writes data only to the part of the LR area allocated to it. When a PC writes to
its LR area, the data is updated in the LR areas of all the other PCs in the PC
Link Subsystem during the next polling cycle. The other PCs can then read
this data and use it to coordinate activities with the PC that has written the
data. Each PC thus writes data to its write words and reads data from the
words written to by all of the other PC Link Units in the same Subsystem. Any
action that affects the contents of the LR area is reflected in the LR area in all
PCs. The data transfer is shown below in a Single-level System. Arrows indi-
cate data flow within the PC Link System.
“Write area” is the area written by that Unit. ”Read area” is an area read by
that Unit (i.e., written by another Unit). All unused portions of the LR area
may be used as work bits in programming.
Read
area
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Data Data Data
Data Data Data
Data Data Data
Data Data Data
What determines what part of the LR area is allocated to which PC is the unit
number assigned to each PC Link Unit. These numbers in turn determine which
PC Link Units are polling units and which are polled units.
When PC Link Unit 0 is set for the total number of LR bits used, and each of
the PC Link Units is assigned a unit number, the LR area is divided and as-
signed to each PC Link Unit automatically.
In a Multilevel System, all PCs have their LR areas divided in half, with one
half being assigned to each of two Subsystems. This is true regardless of
whether or not the PC is actually in two Subsystems, i.e., if only one PC Link
Unit is mounted to a PC in a Multilevel System, one have of the LR area is
not used by the PC Link System.
Each PC in two Subsystems (i.e.,with two PC Link Units mounted to it) thus
contains all the LR area data from both Subsystems and has a write data
area in assigned to each. Any PC with only one PC Link Unit contains only
the LR area data for the Subsystem it is in. The LR area of any PC with two
PC Link Units (i.e., the data-transfer PCs) can thus be used to transfer data
between two Subsystems by programming the data-transfer PC to move data
between its section of the first half and its section of the last half of its LR
area. See Section 6 Application Examples for specific LR area allocation ex-
amples for both Single-level and Multilevel Systems.
LR Area Data Section 3-1