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assignment will autonomously update the point’s value independent of any
other protocol traffic.
The “source port” designation also determines where a new point value will be
written to when a “slave” protocol writes a new value to the point. For example,
if an Ethernet/IP connection consumes new data that changes the value of a
point, how do we know where this new value will exit the gateway to arrive at its
final destination? The answer is that any new point values written by “slave”
protocols will generate “write” transactions only on the “source port”.
This concept may best be further explained by way of a representative
scenario. For example, let’s assume that the gateway’s RS485A port has been
designated to be a Modbus Master. Let’s further assume that the “Modbus
Master” portion of point #5 indicates a “Source ID” value of 8 and “Register”
value of 14, and that point #5’s “Source Port” selection is set to “RS485A”.
What this means is that independent of any other gateway traffic, point #5 will
continuously attempt to update its internal value by making requests to the
RS485A port. And, because the RS485A port has been designated as a
Modbus Master, then the “Modbus Master” portion of point #5’s configuration
will be referenced by the update task, and point #5’s value will therefore always
be mirroring the value of (holding or input, depending on the configuration)
register #14 of remote Modbus station address #8 connected to the Modbus
subnet attached to the gateway’s RS485A port. Perhaps register #14 of
Modbus station address #8 is a monitor item, indicating the pressure in
compressor tank. Whenever the tank’s pressure changes, therefore, the value
of point #5 will automatically update to reflect the new value read from the
remote device. Once the tank’s pressure reading has been brought into the
gateway, it can then be retrieved by any protocol (or ALL the protocols)
currently assigned to the gateway’s other communication ports.
As a modification to the previous example, let’s assume this time that register
#14 of Modbus remote station address #8 is the speed command of a conveyor
belt. In this case, point #5 of the gateway will be mirroring the current speed
command of the conveyor, in a similar fashion to how it previously mirrored the
compressor tank’s pressure. This time, however, the speed command
represents something that can also be written to. Therefore, let’s assume that
point #5 has been included in the output assembly member list of the
Ethernet/IP protocol, and that a new data value is consumed by an Ethernet/IP
connection object that causes the value of point #5 to be changed. In this case,
this new point value will automatically cause a “write holding register”
transaction to occur on the RS485A Modbus master port, updating the value of
register #14 on remote Modbus station #8, causing the conveyor to accelerate
(or decelerate) to the new speed.
Note that it is also perfectly acceptable to have a point’s “source port” assigned
to “no source”. All this means that this point will not be autonomously updated
(i.e. that it will not automatically mirror anything.) In a sense, it will simply be
“scratchpad memory” that the various ports and protocols can use to exchange
information among themselves. For example, a Modbus TCP/IP write
transaction could update the value of such a point, which then can be inserted
Summary of Contents for ETH-200
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