Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 39)

Publication 1757-UM006A-EN-P - May 2002
The Fieldbus Communication Model 1-25
Ratio Block
Figure 1.13 Functional schematic for Ratio function block.
Table 1.M Ratio Block Specifications
Description The Ratio function block set point is the ratio of its output to its input. A ratio set point of 0.5 produces an output that
is one half of its input. The input (IN_1) is either a wild flow or the output of a blend-pacing controller. The output can
be used as the set point for a secondary flow controller. An input (IN) from the secondary measurement is used to
calculate the actual ratio, which is displayed as the PV. A functional schematic of the block is shown in Figure 1.13 for
reference.
Function Notes • Supports Out of Service (OOS), Initialization Manual (IMan), Local Override (LO), Manual (Man), Automatic (Auto),
Cascade (Cas), and Remote Cascade (RCas) modes.
• The input 1 (IN_1) value to be ratioed passes through a filter with a time constant of RA_FTIME. The filtered value
is multiplied by the Set point (SP) and GAIN to become the target output. The GAIN controls the number of zeros in
the SP display.
• The input (IN) value is the actual value of the ratioed variable and it passes through a filter with a time constant of
PV_FTIME. The filtered IN value is divided by the filtered IN_1 value and the GAIN to become the PV. The units of IN
are not PV, but OUT. The units of IN_1 are OUT units divided by PV units.
RA OUT
Output
OUT_HI_LIM
OUT_LO_LIM
BAL_TIME
OUT
Mode
SHED_OPT
SP
CAS_IN
RCAS_IN
BKCAL_OUT
RCAS_OUT
Setpoint
SP_RATE_DN
SP_RATE_UP
SP_HI_LIM
SP_LO_LIM
Output Track
TRK_SCALE
BKCAL_OUT
RCAS_OUT
CAS_IN
RCAS_IN
BKCAL_IN
TRK_IN_D
TRK_VAL
IN
BKCAL_IN
TRK_VAL
IN_1
TRK_IN_D
Ratio
GAIN
IN
Filter
PV_FTIME
Calc PV
GAIN
Alarm
HI/LO
DEV
PV
IN_1
Filter
RA_FTIME