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FX Series Programmable Controllers Devices in Detail 4
4-23
4.11.1 Basic High Speed Counter Operation
Although counters C235 to C255 (21 points) are all high speed counters, they share the same
range of high speed inputs. Therefore, if an input is already being used by a high speed
counter, it cannot be used for any other high speed counters or for any other purpose, i.e as an
interrupt input.
The selection of high speed counters are not free, they are directly dependent on the type of
counter required and which inputs are available.
Available counter types;
a) 1 phase with user start/reset: C235 to C240
b) 1 phase with assigned start/reset: C241 to C245
c) 2 phase bi-directional: C246 to C250
d) A/B phase type: C251 to C255
Please note ALL of these counters are 32bit devices.
High speed counters operate by the principle of interrupts. This means they are event
triggered and independent of cycle time. The coil of the selected counter should be driven
continuously to indicate that this counter and its associated inputs are reserved and that other
high speed processes must not coincide with them.
Example:
When X20 is ON, high speed counter C235 is
selected. The counter C235 corresponds to
count input X0. X20 is NOT the counted
signal. This is the continuous drive mentioned
earlier. X0 does not have to be included in the
program. The input assignment is hardware
related and cannot be changed by the user.
When X20 is OFF, coil C235 is turned OFF and coil C236 is turned ON. Counter C236 has an
assigned input of X1, again the input X20 is NOT the counted input.
The assignment of counters and input devices is dependent upon the PLC selected. This is
explained in the relevant, later sections.
Driving high speed counter coils:
• The counted inputs are NOT used to
drive the high speed counter coils.
This is because the counter coils
need to be continuously driven ON
to reserve the associated high speed
inputs.
Therefore, a normal non-high speed
drive contact should be used to drive
the high speed counter coil.
Ideally the special auxiliary contact M8000 should be used. However, this is not
compulsory.
X20
K4789
X20
D4
C235
C236
X0
K4789
X1
D4
C235
C236