Allen-Bradley controllogix 1756-L55M12, User Manual

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Publication 1756-UM001G-EN-P - January 2007
Place, Configure, and Monitor I/O Modules 55
Configure I/O
To communicate with an I/O module in your system, add the module
to the I/O Configuration folder of the controller in RSLogix 5000
programming software.
Addition of I/O Modules
When you add a module, you also define a specific configuration for
the module. While the configuration options vary from module to
module, there are some common options that you typically configure.
Configuration Options
Add I/O modules to the
1756 backplane.
Configuration Option Description
Requested Packet
Interval (RPI) The RPI specifies the period at which data updates over a connection. For example, an input module sends data
to a controller at the RPI that you assign to the module.
•
Typically, you configure an RPI in milliseconds (ms). The range is 0.2...750 ms.
•
If a ControlNet network connects the devices, the RPI reserves a slot in the stream of data flowing across
the ControlNet network. The timing of this slot may not coincide with the exact value of the RPI, but the
control system guarantees that the data transfers at least as often as the RPI.
Change of State (COS)
Digital I/O modules use COS to determine when to send data to the controller. If a COS does not occur within
the RPI, the module multicasts data at the rate specified by the RPI.
Because the RPI and COS functions are asynchronous to the logic scan, it is possible for an input to change
state during program scan execution. If this is a concern, buffer input data so your logic has a stable copy of
data during its scan. Use the Synchronous Copy (CPS) instruction to copy the input data from your input tags to
another structure and use the data from that structure.
Communication Format Many I/O modules support different formats. The communication format that you choose also determines:
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data structure of tags.
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connections.
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network usage.
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ownership.
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whether the module returns diagnostic information.
Electronic Keying When you configure a module, you specify the slot number for the module. However, it is possible to purposely
or accidentally place a different module in that slot. Electronic keying lets you protect your system against the
accidental placement of the wrong module in a slot. The chosen keying option determines how closely any
module in a slot must match the configuration for that slot before the controller opens a connection to the
module. Keying options differ depending on your application needs.