Network Variable Handling; Error Handling; Overall Function - Danfoss VLT 5000 Operating Instructions Manual

Phase 2:
Is the interface watchdog function performed by
the option card as the first operational action. After
completion of this phase, data can be passed between
the VLT frequency converter and the option card
through the command channel in the dual port RAM.
Phase 3:
The VLT frequency converter will write to the Initialization
Channel of the dual port RAM which will set the
displacement values of the various channels (Control
Channel, Status Channel, Command Channel 1,
Command Channel 2, Command Channel 3, Warning
Channel, and Spontan Channel), and the DeviceNet
option card software will read these displacements
and return a response to the VLT frequency converter
notifying that the initialization was successful or an
error was obtained in the process.
During operation (after initialization) the dual port RAM
interface is supervised by a watch-dog in the VLT
frequency converter. The VLT frequency converter will
be kept "alive" by cyclic writing to a channel in the dual
port RAM by the watch-dog handler of the option card.

Network Variable Handling

The parameter access network variable is a structure
containing parameter number, data, value, and a
function code containing a read or write command.
The application program shall receive the data,
validate the data, convert the data to the format of
the VLT frequency converter, process the read or
write of the parameter, then send the information
back in an output network variable of the same
structure. This output variable would contain the
parameter number, error code information in the event
of an exception, and parameter value.

Error Handling

The option card will stop sending a watchdog signal
to the VLT frequency converter when:
1. The node has been disconnected from the network
2. A network management tool has the
Option Card go Off-line
3. A hardware fault on the DeviceNet Option Card
PCB prevents reliable operation
By withholding the watchdog message, the VLT
frequency converter can detect a "Bus Time-out"
and enter secure, pre-defined state.
For recoverable errors such as [1] and [2] the option
card shall re-send the watchdog message when
MG.50.H4.02 - VLT is a registered Danfoss trademark
®
VLT 5000/ 5000 FLUX/ 6000 HVAC/
8000 AQUA DeviceNet
the fault or condition has disappeared such as after
re-connection of the node to the network.
For unrecoverable errors such as [3] the application
program shall permit logging of network errors
without resetting or restarting the microprocessor.
This will send information to a network management
tool that will allow appropriate actions to be taken.
The only way to recover from a serious error is to
re-initialize the VLT frequency converter DeviceNet
option card by cycling power.
In case of serious VLT frequency converter fault, the
DeviceNet processor shall hold the CAN chip reset,
thereby disabling any communication to the node until
the VLT frequency converter has been re-initialized.

Overall Function

DeviceNet is a low-level network that standardizes
communications between industrial devices (sensors,
limit switches) and high level devices (controllers).
The communication network can be peer to peer or
master/slave. DeviceNet uses CAN technology for
Media Access Control and Physical Signaling and it
supports up to 64 nodes. DeviceNet also defines
device profiles for devices belonging to specific classes.
For other devices, a custom class must be defined in
order to make it DeviceNet compatible. This further
enhances the interchangeability and interoperability of
the network. Each node on the network has its own
unique media access control identifier (MAC ID) to
distinguish it on the network. The MAC ID is stored
in the header of the message which is split into four
different message groups. However, DeviceNet only
makes use of three message groups keeping the fourth
one for future use. If two nodes attempt to get control
of the network bus simultaneously, the CAN protocol
resolves the issue by arbitration. A dominant bit (0)
will win arbitration over a recessive bit (1).
For the VLT frequency converter, the option card
will be slave node on the DeviceNet network. The
option card will be a message server that will request
and set parameters in the VLT via a dual ported
RAM interface. The option card has an embedded
CAN controller that will screen all network messages
with a Mask and Match register. This feature
allows the option card to filter out any unwanted
messages from the network. The option card will
receive group 2 and group 3 messages. Group 2
messages allows the option card to use the predefined
Master/Slave connection set which identifies a use
for all group 2 message identifiers. The ability to
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