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Reference Manual
00809-0100-4696, Rev AA
September 2004
Rosemount 848L
3-10
The 848L can force the outputs to a predetermined state in the event of a
device malfunction.
The following procedure can be used to set the fail safe condition for each
output:
1. Set MODE_BLK.TARGET to OOS
2. For each Output "n" select the parameter
OUT_n_CONFIG.FAIL_SAFE
3. Select False, True or Last Good Value
4. Set MODE_BLK.TARGET to AUTO
Latching
Most often the inputs are scanned and the logic equations processed at a rate
greater than the macrocycle frequency. To be certain that positive or negative
transitions are communicated, the inputs, equation results, and output values
can be latched until read by the function blocks.
The following procedure is used to set the latching state of each input,
equation or output:
1. Set MODE_BLK.TARGET to OOS
2. Select the appropriate parameter for either inputs, equations, or
outputs:
a. Inputs use parameter MACRO_IN_LATCH
b. Outputs use parameter MACRO_OUT_LATCH
c. Equations use parameter MACRO_EQ_LATCH
For each input, output or equation select either "Latch Positive Pulses" or
"Latch Negative Pulses" or “disabled”.
LOGIC TRANSDUCER
BLOCK
Logic Equations
The 848L provides for 16 Logic Equations and 4 Output Equations. The
Output Equations drive the hardware outputs. Each logic equation consists of
up to 80 characters with a semicolon as the last character. The equations are
evaluated at a nominal rate of 100msec. However this will vary based on the
number and complexity of the equations used. The logic block consists of
variables that are connected to the hardware I/O, obtain values or send
values over the bus and internally calculated variables as shown in
Figure 3-1.
The value or state of the logic block variables can be communicated on the
bus by assigning the appropriate channel number of a DI or MDI block. The
DO variables can be set externally by assigning the appropriate channel
number in a DO or MDO function block. The DO function blocks do not drive
the outputs directly. The DO function block can drive the output by referencing
the appropriate DO variable in the output equations.