Publication 1766-RM001A-EN-P - October 2008
350
Process Control Instruction
Analog I/O Scaling
To configure an analog input for use in a PID instruction, the analog data
must be scaled to match the PID instruction parameters. In the MicroLogix
1400, the process variable (PV) in the PID instruction is designed to work
with a data range of 0…16,383. The 1762 expansion I/O analog modules
((1762-IF4, 1762-OF4, 1762-IF2OF2, 1762-IT4 and 1762-IR4)) are capable
of on-board scaling. Scaling data is required to match the range of the
analog input to the input range of the PID instruction. The ability to
perform scaling in the I/O modules reduces the amount of programming
required in the system and makes PID setup much easier.
The example shows a 1762-IF4 module. The IF4 has 4 inputs, which are
individually configurable. In this example, analog input 0 is configured for
-10…10V and is scaled in engineering units. Channel 0 is not being used
in a PID instruction. Input 1 (channel 1) is configured for 4…20 mA
operation with scaling configured for a PID instruction. This configures
the analog data for the PID instruction.
The analog configuration screen is accessed from within RSLogix 500/
RSLogix Micro. Double click the I/O configuration item in the Controller
folder, and then double click on the specific I/O module.
PID Instruction Analog Data
Field Device Input Signal
Analog Register Scaled Data
> 20.0 mA
16,384…17,406
20.0 mA
16,383
4.0 mA
0
< 4.0 mA
-819…-1
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