Page 366 - Programmable Logic Controllers, Fifth Edition - Mobile version

Tag-Based Addressing
is a digital input, 2 indicates that the limit switch is
connected to terminal 2 on the module, and C indi-
Logix 5000 controllers use a tag-based addressing struc- cates that it is a controller tag with global access.
ture. A tag is a text-based name for an area of the control-
ler where data is stored. An example of how a tag-based • The physical address for the tag High_limit_light is
address is implemented using a ControlLogix controller Local:2:O.Data.4(C). Local indicates that the module
is shown in Figure 15-30. Tag names use a meaningful is in the same rack as the processor, 2 indicates that
description of the variable. In this application when the the module is in slot 2 in the rack, O indicates that
normally closed high limit switch is activated the program the module is an output type, Data indicates that it is
will switch the high limit output light on. The addressing a digital input, 4 indicates that the high limit light is
format can be summarized as follows: connected to terminal 4 on the module, and C indicates
that it is a controller tag with global access.
• The physical address for the tag Limit_switch is
Local:1:I.Data.2(C). Local indicates that the module One advantage of the use of tag-based addressing is that
is in the same rack as the processor, 1 indicates that the allocation of variable names for program values is not
the module is in slot 1 in the rack, I indicates that tied to specific memory locations in the memory structure,
the module is an input type, Data indicates that it as is the case with rack/slot and rack/group type systems.

TAG EDIT
Tag Name Alias For Base Tag Type Style
Limit_switch Local:1:I.data.2(C) Local:1:I.data.2(C) BOOL Decimal
High_limit_light Local:2:O.data.4(C) Local:2:O.data.4(C) BOOL Decimal

0
1
2 8-point
High_limit_light discrete
0 3
1 Limit_switch 4 (digital)
onput
8-point 2 Light-OFF 5
discrete 3 Switch - closed 6 module
(digital)
input 4 7
module 5 Processor
6 module
7
0 1 2 3 4 5 6
7-slot
Power ControlLogix
supply chassis

Program

Limit_switch High_limit_light
<Local:1:I.data.2(C)> <Local:2:O.data.4(C)>

Tag Name Value Style Type
Limit_switch 1 Decimal BOOL
High_limit_light 0 Decimal BOOL
TAG MONITOR
Figure 15-30 Tag-based address implementation.

Bit-Level Programming Part 2 347

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