Rockwell Automation 61C351 2-In/2-Out 4-20 mA Analog Rail Module - Manual (Page 23)

4Ć1
4.0 PROGRAMMING
This section describes how the data is organized in the module and
provides examples of how the module is accessed by application
programs.
When creating application programs, the programmer must estimate
the magnitude of input signals and output data because both must
be in the specified range of the Analog Rail module (4Ć20mA). Input
signals greater than 20mA will be clamped at 4095. Signals greater
than approximately 22.6mA will also cause the overĆrange bit (12
decimal; 14 octal) to be set to 1. Input signals less than 4mA are
clamped at zero (0). Signals less than approximately 2.5mA will also
cause the underĆrange bit (13 decimal; 15 octal) to be set to 1. See
figure 4.1.
Counts
4095
0 4 20
<2.5 = underĆrange
>22.6 = overĆrange
mA
Figure 4.1 Ć Input Signal Conversion
Recall that at a digital output = 4096, the output rolls over to zero
again. The programmer must include limits in the application
software to ensure that the data sent to output channels is always in
the specified range (allowable range = 0 to 4095).
4.1 Analog Rail Module in AutoMate Systems
This section describes how the Analog Rail is used with AutoMate
systems. Local Head mode allows all four channels on the module
to be updated at the end of the scan (normal I/O update rate in this
configuration), or during the scan using AIN and AOUT blocks (see
section 4.1.4). Rail mode allows only one channel to be updated at
the end of the scan (normal I/O update rate in this configuration), or
all four channels to be updated during the scan if AIN and AOUT
blocks are used.