Cyclic Poll Area; Data Set Download Release (Bit 'Ddf') - Omron NT4S Operation Manual

Standard Mode

4.21.5 Data Set Download Release (Bit 'DDF')

4.22

Cyclic Poll Area

The Data Set Download Release (DDF) bit allows the controller to determine the
point of time from which transfer of a data set to the controller can begin.
In addition to a random read and write access to controller variables, a 23-byte
memory area is defined in the user description as the cyclic poll area.
The cyclic poll area consists of:
- 1 byte Write coordination byte
- 2 bytes serial message channel (low and high byte)
- a terminal-specific number of control bytes for the status LEDs in the function
keys
The poll area is written to by the controller and polled by the operating terminal at
cyclic intervals. During this process, the controller can trigger, control, enable or
inhibit actions that are connected with the terminal. In addition, the liveness flag of
the Write coordination byte can be used to verify whether or not the connection with
the terminal is still established. Actions are triggered and serial messages are
passed to the operating terminal via the serial message channel (for example,
transfer of data sets, change of masks). The bits of the control bytes can be used to
activate or deactivate the status LEDs in the function keys or to place them to the
flashing mode.
The marginal conditions regarding the memory area for the poll area are:
- the PLC accesses the Write coordination byte and status LEDs in bit mode and
the serial message channel in byte mode or word mode
- the terminal accesses in byte or word mode
- the memory area must be contiguous.
The symbolic name for the cyclic poll area is specified in the parameters for the poll
area using the programming system. The assignment of the starting address of this
area is defined in the variable list.
Note that access to byte and word structures is not identical. Once an access meth-
od is selected, it should be applied throughout.
Section 4-22
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