Programming the Local Host
Chapter 5
540
There may be other instances where program monitoring of commands is
useful. In some cases, you might program a timeout to monitor the
execution time of critical commands. An application may require that a
time-critical message, such as a command to read an immediate-access
block, be sent within a certain limited amount of time. You can program a
TON instruction for this purpose; here, however, its preset interval will
generally be shorter than the interval entered as the timeout preset for the
module. You can also use the second timeout allowed in the header rung
with a smaller timeout preset to more closely control your time-critical
commands.
Before transmitting write commands to a remote node, you should
become aware of what areas of that node’s data table is accessible to you.
This information is available either in the programming manual for that
processor or the manual for its node interface. Further general memory
access limitations are discussed in the following sections.
Memory Access Rungs
A remote node may have memory access rungs, giving memory access of
parts of its data table to certain nodes. If your address is not specified in
the remote communication zone, your protected write commands will be
rejected.
Writes Switch Disabled
A node interface will not accept writes to its data table unless it has a
switch set to the proper position to accept writes to its data table.
Write Bit Access
You can use a write bit command to control any accessible data table bit.
However, do not use this to control:
Any bit whose status is controlled by an output instruction in the
ladder-diagram program.
Any bit in a byte which also contains program bits in the PLC-2 family
processor.
The first restriction simply states that you must not attempt to directly
control a bit by both an output instruction in the local ladder-diagram
program and a write bit command from a remote node. This rule applies
to all PLC processors.
Memory Access Limitations
