1-5
Section
Functions of the DeviceNet (CompoBus/D) Interface Unit
11
Transferring data with explicit message communications takes more time than
transferring data with the input and output areas, but all of PT memory can be
accessed.
If explicit message communications are used as the main method to transfer
data, the DeviceNet Interface Unit’s input and output areas can be reduced to
their minimum required sizes and more input and output area capacity will be
available for other Slaves.
Note Observe the following precautions in order to minimize the communications time
required for explicit message communications.
•
Limit the amount of data transmitted in each message communications event.
•
Consider setting the ladder program in the host that controls the DeviceNet In-
terface Unit so that it transfers just the data in the displayed screen. (In this
case, it will be necessary to write a ladder program section for each screen.)
PT
Remote I/O
communications
Master Unit
CPU Unit
PT status
control area
PT status
notify area
Touch switch
Numeral
memory
table
Direct access
PT memory
Input
area
Output
area
DeviceNet Network
Explicit message com-
munications (Transfers
data in any area.)
Lamp
1-5-2 Contrast with the Regular Memory Link Method
The following operations are different with the DeviceNet Interface Unit and with
regular Memory Link method.
•
The transmission direction of the input area is always PT
→
Host and that of
the output area is always Host
→
PT, so two-way functions cannot be used
when using the input and output areas.
•
Except for remote I/O communications, no locally-initiated communications
functions are supported by the DeviceNet Interface Unit. With explicit mes-
sage communications, communications are always executed by commands
sent from the host and corresponding responses from the PT.
•
Since the DeviceNet Interface Unit does not support locally-initiated commu-
nications functions other than remote I/O communications, it is necessary to
set the strobe ON interval to match system requirements and always read the
status of the strobe at the Master. (Set the interval in the system menu.)
The following table lists the operational differences between the regular Memory
Link and the DeviceNet Interface Unit’s Memory Link that arise due to differ-
