Alstom GEM80-400 Series, Техническое руководство

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2. Controller Architecture and Program Operation GEM80-400
Page 2-10 ALSPA GEM80-400 SERIES CONTROLLERS TECHNICAL MANUAL Issue 3 04/99
2.6.2. Preset Cycle Time
If the cycle time is preset to a fixed value, the processor inserts a waiting period at
the end of each ladder program cycle and any serial communication activity
time is subtracted from this wait period to give a substantially constant cycle time.
The preset cycle time, which is stored in a P-table, must of course be longer than
the free running time. This is the way to run the controller wherever time
dependent GEM
80 Special Functions are used, for instance, if smooth ramp
staircase functions are required. The smooth ramp is dependent upon the scan
time being less than the interval defined in the P-table.
2.6.3. Calculating the Program Scanning Interval
The scanning interval of the controller depends on many factors, making it difficult
to derive an accurate estimate of the scan interval. For most applications, a
reasonable estimate of the scanning interval can be made by adopting the
method detailed below.
The scanning interval can be considered to be the sum of nine components:
Scan Interval = Tb + Tw + To + Ti + Tf + Tc + Tfip + Ts, where:
(1) Tb = The time taken to execute all of the bit logic instructions (e.g. --
]/[--, --] [-- and --( )--) in the ladder program. To calculate Tb, count
the number of bit logic instructions in the ladder program and then
use the relationships given below to calculate the value.
(i)
For a 20K system, each bit logic instruction takes 1.35
µ
s
(ii)
For a 36K system, each bit logic instruction takes 1.5
µ
s
(2) Tw = The time taken to execute all of the word logic instructions (e.g.
AND, OR, XOR and INV) in the ladder program. To calculate Tw,
count the number of word logic instructions in the ladder program
and then use the relationships given below to calculate the value.
(i)
For a 20K system, each word logic instruction takes 0.9
µ
s
(ii)
For a 36K system, each word logic instruction takes 1
µ
s.
(3)&(4
) Ti & To = The time taken to scan all plant inputs and outputs. Ti and To
are dependent upon the number and type of I/O being scanned and
can be calculated by using the formulae given below.
(i)
Ti = Nvi x 200 + Nbi x 120 + Nei x 400
µ
s
(ii)
To = Nvo x 200 + Nbo x 170 + Neo x 400
µ
s, where:
Nvi
Nbi
Nei =
=
= number of words of Verification Inputs to be scanned.
number of words of Basic Inputs to be scanned directly.
number of words of Basic Inputs to be scanned via an 8191
Expander Module.