Yokogawa Data Acquisition with PID Control CX2000 - Manual (Page 384)

App-15
IM 04L31A01-01E
Appendix
App
Rules for Writing Equations (Common Items)
Follow the rules below when writing computing equations.
• Measurement equation can be entered using up to 40 characters.
• Control equation can be entered using up to 120 characters.
• The precedence of computing terms can be specified using parentheses.
• Specify the channels in the equation using channel numbers.
• One-digit numbers of channels, constants (K), communication input data (C), statuses
of remote inputs of the measurement alarm option terminal block (D), statuses of
contact inputs of the control output terminal block (DI), and statuses of contact inputs
of the control DIO expansion terminal block (RI) used in equations can be expressed
as “01” or “1”.
Examples: 01, 1, K01, K1, C01, C1, D01, D1
• The data value for the channel used in the equation, and for all channels greater in
number than that channel, are substituted with data from the previous scan.
• Do not use more than 16 stacks (channels, K01 to K30, W01 to W30, C01 to C30,
D01 to D08) in one equation. Otherwise, a computation error may occur. The
computed result displays “+*******” in this case.
Example: The number of stacks in the equation 01+K01*(03+04*K02) is five.
• Control measurement data used in PV/SP equations is the data prior to 1 skip.
Note
With control computation, you can compute using a value from the previous iteration of the
same computation. For example, given a PV for loop 1 of PV=101*W01, the previous value
for channel 101(PV of loop 1) can be used. On the other hand, a previous value cannot exist
in cases such as immediately after the power is turned ON. If no previous value exists, the
above computation result is treated as invalid data, and this invalid status continues in an
infinite loop. When computing using a value from the previous iteration of the computation,
be sure to use conditions and internal switches that do not result in infinite loops. When
computing using a value from the previous iteration of the computation for analog
retransmission, if a previous value does not exist, set the computed result to revert to the
range lower limit value.
Appendix 3 Meaning and Syntax of Equations