Communications Timeouts - Control Techniques unidrive sp Advanced User's Manual

Parameter Keypad and Parameter
structure display
message but forwards the message for processing by the SM-
Applications in slot (port) Y.
SM-Applications notices that the CMP Destination Sub-Node
Address field is 0 and so processes the message and returns a
response.
Figure 7-1 Example message routing from a PC through Unidrive SP to a SM-Applications in a Unidrive SP option slot
PC sets Slave Node
Address to X, CMP
Destination Port to Y,
CMP Destination Sub-
Node Address to 0
PC
2. Routing of a CT Modbus RTU by Unidrive SP to a SM-Applications
in one of the Unidrive SP options slots and then re-routing by the
SM-Applications onto a CTNet network.
In this example, a PC wishes to communicate with node Z on a
CTNET network connected to a SM-Applications option located in
option slot Y (where Y = 1, 2 or 3) in a Unidrive SP with node
address X on an RS485 network.
The PC sets the Slave Node Address to X, the CMP Destination Port
to Y and the CMP Destination Sub-Node Address field to Z in the CT
Modbus RTU message and transmits the message over a RS485
network connecting the PC and Unidrive SP.
Figure 7-2 Example message routing from a PC through Unidrive SP and SM-Applications to a CTNet node
Unidrive SP receives message over
RS485. Since the CMP Destination
Port has the value Y, Unidrive SP
makes the message available to
slot (port) Y for processing.
PC
RS485
network
7.2.8

Communications timeouts

When a CT Modbus RTU master sends a message to a slave, the
master should use a timeout to detect a missing response from a slave.
Ideally, a variable timeout will be used based on the number of hops a
CT Modbus RTU message makes between the master and its eventual
destination. For example, in the Unidrive SP/ SM-Applications re-routing
scenarios shown in the previous section, when a PC sends a message
to SM-Applications one hop is performed (through Unidrive SP) to reach
Unidrive SP Advanced User Guide
Issue Number: 7
Parameter Advanced parameter
x.00 description format descriptions
Unidrive SP receives message over
RS485. Since the CMP
Destination Port has the value Y,
Unidrive SP makes the message
available to slot (port) Y for
processing
Unidrive SP
has node
address X on
the RS485
network
RS485
network
More
Unidrive SPs
network
Unidrive SP
has node
address X on
Dual-port
the RS485
RAM
network
connection
More
Unidrive SPs
network
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Serial comms
Macros
protocol
SM-Applications
in slot Y (simulated port Y)
processes message.
SM-Applications
in slot Y
on the
Dual-port
drive
RAM
connection
The Unidrive SP receives the message and notices that the CMP
Destination Port has the value Y - Unidrive SP does not process the
message but forwards the message for processing by the SM-
Applications in slot (port) Y.
SM-Applications notices that the CMP Destination Sub-Node
Address field has the value Z and so transmits the CMP portion of
the message over its CTNET connection to node Z.
Node Z receives and processes the message and returns a CMP
response to the SM-Applications. SM-Applications constructs a
suitable CT Modbus RTU response using the CMP response and
sends the reconstructed RTU message back to the PC via Unidrive
SP.
Node Z process
the message
Node Z
SM-Applications
in slot Y
on the
CTNET
DB9
connection
the SM-Applications; when a PC sends a message via through Unidrive
SP and then SM-Applications onto CTNet, two hops are performed.
In practice a master may not be able to handle variable timeouts in such
a fashion. If this is the case a single timeout should be used which is
large enough to cater for the longest route to a destination. The
recommended timeouts for use with a specific product are given in the
specific product user guides.
Electronic
Performance
nameplate
on
CTNet
Feature look-
up table
365