points that are found to have their timeout enable selections set to “enabled” will then have their configured timeout values automatically written to their assigned “source port” objects. This mechanism provides for a flexible set of device failsafe conditions to be established on a
11.3 Point Configuration
As mentioned in section 1, the Network Gateway Series concept revolves around a central “point database”, containing the value and access characteristics for each network. With respect to the Network Gateway Series, a “point” is simply an object that defines some sort of network access, mapping and configuration data, as well as a single “value” attribute that can be read from or written to by various communication ports or protocols.
The only restriction placed on this “central clearinghouse” concept is that only one port can autonomously update the point’s value, “mirroring” its designated object for other protocols to access. What this means is that although any protocol can read from or write to a point’s internal value, most of the time that point’s value will simply be mirroring a remote data object that resides on one of the gateway’s subnets. The selection of what a specific point is to mirror is performed via its “source port” selection.
For example, a point may be configured to contain Toshiba ASD parameter mapping and Modbus master ID and holding register mapping information. However, because both of these protocols act as “master” protocols, only one of them can be allowed to continuously update the point’s value. If both master protocols could simultaneously update the point’s internal value, it would erratically alternate back and forth between the values designated by the Toshiba parameter and Modbus holding register objects. Any “slave” protocol (Modbus RTU slave, Metasys N2 etc.) can read from or write to a point at any time, but only the protocol designated by the point’s “source port” 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 a Modbus RTU slave connection is used to write 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 an “Address” 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
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