4-20 Configuring the I/O
PowerFlex® 755 Drive Embedded EtherNet/IP Adapter User Manual
Publication 750COM-UM001A-EN-P
Limitations When Using
PLC-5, SLC 500, and
MicroLogix 1100
Controlling I/O with explicit messages is relatively complex compared
to normal implicit I/O control.
ControlLogix and CompactLogix controllers with EtherNet/IP provide
the easiest and most integrated form of implicit I/O control for a
PowerFlex drive. RSLogix 5000 v16 (and higher) programming
software for ControlLogix and CompactLogix controllers contains
integrated profiles for PowerFlex drives that, with a few clicks of the
mouse, automatically create all controller tags and an implicit
connection at the specified Requested Packet Interval to control the
drive. This connection is monitored at both ends to ensure that the
controller and drive are communicating. A watchdog will cause a drive
fault if the drive does not respond within approximately 100
milliseconds. Therefore, using a ControlLogix or CompactLogix
controller is by far the much preferred method of controlling drives on
EtherNet/IP.
If you are not using either of these type of controllers, then PowerFlex
drives on EtherNet/IP can be controlled with explicit messages using
PLC-5, SLC 500 or MicroLogix 1100 controllers with the following
limitations:
• An explicit message is a much slower form of control and is
non-deterministic. This means that you cannot guarantee how long
the drive will take to start up or stop when the command is given.
Therefore, all equipment used in this manner should be subject to a
risk assessment, taking into account the mechanical and electrical
implementation.
• A timeout value (in seconds) in the drive will issue a drive fault if a
message is not received from the controller within the specified time.
However, the controller has no way of detecting a loss of
communications to the drive until the next cycle of explicit
messages. This is another factor in the risk assessment.
• Any additional drives to be controlled will require additional explicit
messages for their control, and they need to be carefully sequenced.
Most controllers have small communication queues (refer to its User
Manual), which need to be carefully managed if messages are not to
be lost.
• Each controller has a limited number of communication connections
(refer to its User Manual for maximum connections), which will
limit the number of drives that can be connected.
In summary, unlike a ControlLogix or CompactLogix controller,
programming a PLC-5, SLC 500 or MicroLogix 1100 controller using
RSLogix 5 or RSLogix 500 software with explicit messages is a lot
more difficult, and produces a far more complex program.
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