Plc Programming Considerations; Blocking Network Data; Writing To Integer Registers; Writing To Discrete Points - Siemens SIMATIC TI500 User Manual

D.2

PLC Programming Considerations

Blocking Network
Data
Writing to Integer
Registers
Writing to Discrete
Points

PLC Status Register

TIWAY I Gateway User Manual
The restrictions on data types and numbers of contiguous data blocks that
can be accessed by a PCIU influence the design of the PLC programs to be
used in a PROVOX environment. To increase the number of secondaries
that can be supported by a single PCIU, you should group together the data
to be read by the network into a single contiguous block. Similarly, data
supplied to the PLC from the network should be written into a single
contiguous block. In most cases, data is read from and written to the same
block of memory. This puts the burden of blocking and unblocking the
network data on the PLC.
Box functions in Series 500, Series 505, and PM550 PLCs make this
blocking and unblocking of data a relatively easy task. Refer to the
appropriate user manuals for more information about this function.
Another important consideration is that the network writes values into the
PLC memory; it does not force the memory to the given value. This
distinction implies that if, after a value has been written into a memory
location, the PLC program writes a value to the same location, the network
data will be overwritten.
NOTE: The PLC ladder logic program must be carefully designed to avoid
this problem.
The PROVOX system allows you to write discrete values to any Y point that
is defined in a discrete register. However, the PCIU expects the contents of
the discrete register to accurately reflect the status of the Y points defined
in it. The ladder logic program must be designed to ensure that this is the
case. This can be accomplished by moving the contents of the Y points to the
discrete register.
The first word of each contiguous V-memory block serves as the PLC status
register. This register is used to pass information about the state of the PLC
and the process it is controlling to the PROVOX system. The exact contents
of this register are user-definable.
Section D.3 provides an example of network and PLC program design for a
Fisher PROVOX system.
Configuring the PLC for Fisher PROVOX D-3