Rockwell Automation T3420AF ICS Regent Analog Input Modules - Manual (Page 3)

Analog Input Modules (T3420A, AF)
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3
The processor modules send triplicated read data requests to
the analog input module over the I/O Safetybus. The
processors’ addressing data and data read requests are voted
by the module (preventing I/O Safetybus failures upstream
from the module from affecting module operations). The voted
result is then passed to the I/O bus interface logic.
After receiving the voted data read request, the I/O bus
interface logic retrieves the analog data values from the RAM
and places the data into the module’s three bus drivers. Each
of the three bus drivers is independently powered and
controlled (by the I/O transceiver modules) — preventing
failures in a single driver from propagating to the other two /O
busses. The bus drivers then transmit the data via the
backplane I/O Safetybus to the I/O transceiver modules which,
in turn, transmit the data to the processors.
The 12-bit analog input data is packaged as a 16-bit integer.
In this format the analog data is the least significant bits,
providing analog input data ranging from 0 to 4095. Two of
the remaining four bits are used to flag alarms as shown in
Figure 2.
Figure 2. Analog Input Data Format.
Testing and Diagnostics
Each module’s voter circuits are periodically tested by the
processor modules. Discrepant data are sent through one of
three legs of the I/O Safetybus to determine whether the
module’s voter is able to outvote the incorrect data. A failure
to return the correct majority-voted result to the processors