Rockwell Automation 1760-xxxx Pico Controller User Manual - Manual (Page 169)

1 Publication 1760-UM001D-EN-P - September 2005
Chapter
8
Inside Pico
Circuit Diagram Cycle In conventional control systems, relay control processes all the circuit
connections in parallel. The speed with which a relay switches is thus
dependent on the components used, and ranges from 15 to 40 ms for
relay pick-up and drop-out.
With Pico, the circuit diagram is processed with a microprocessor that
simulates the contacts and coils of the relay logic and thus processes
all switching operations considerably faster. Depending on its size, the
Pico circuit diagram is processed cyclically every 2 to 40 ms.
During this time, Pico passes through five segments in succession.
How Pico Evaluates the Circuit Diagram
In the first three segments, Pico evaluates the contact fields in
succession. As it does so, Pico also checks whether contacts are
connected in parallel or in series and stores the switching states of all
the contact fields.
In the fourth segment, Pico assigns the new switching states to all the
coils in one pass.
The fifth segment is outside the circuit diagram. Pico uses this to make
contact with the ’outside world’: Output relays Q1 to Q… are
switched and inputs I1 to I… are read. Pico also copies all the new
switching states to the status image register.
Pico only uses this status image for one cycle. This ensures that each
circuit connection is evaluated with the same switching state for one
cycle, even if the input signals I1 to I12 change their status several
times within a cycle.
T2--------{Q1
I2-I3-----RT2
1 2 3 4 5
1
2
3
4
.. ...
I1-I4- 1-TT2
P1-
.. ...
I1-Q1----{Q8
Range
Circuit Connections