To understand the use of External Ports, it is probably better to read
the sections on Ports and Subnets first. External
Ports indicate
the ports of a subnet, and basically just have a name, which must match
a subnet port name at the next higher level.
An External Port can be one of three types: input, output and
output/input.
The output/input external port is really an output type linked tightly
to an input type, and therefore
must have two names associated with it.
Right clicking on an External Port brings up a popup menu showing
operations that can be performed on it. These are the following:
An External Port can be connected by arrows to any block type - see
the section on Arrows.
However, arrows in some directions are prevented by the diagramming
tool, e.g. routing an arrow from
a block to an input-type
external port is forbidden.
In the case of the output/input type, the two associated names are
entered together, but separated by a colon, as follows:
The user will be prompted to enter the subnet port name.
Substream sensitivity is related to the idea that in FBP a subnet can be set up to process a substream as if it were a complete stream, closing down at end of substream, rather than having to wait for the whole stream to be processed.
In this case, this subnet port will cause the subnet to see end of stream, resulting in the usual wave of close-downs through the network. This is the only situation in FBP where processes can be closed down, and then restarted.
Subnet port symbols with substream sensitivity enabled will be marked with an "SS".
Self-explanatory.
Self-explanatory.