The mode indicates the direction of the flow of data across
that port. This flow of data is defined with respect to the
component.
The five port modes available:
-
IN --
- data flows in this port and the device can only read from
this port
-
OUT --
-
data flows out this port and the device can write to this
port. Note that until 1076-1993, a component could not read a
signal it was itself driving through an OUT port.
-
BUFFER --
-
data can flow in either direction and is read/writable.
However, only one source at a time can drive a buffer; this
requires the ability to disconnect drivers and will be discussed
in the 'Behavioral VHDL' module.
-
INOUT --
-
data can flow in either direction and is read/writable.
Any number of sources are allowed to drive the inout port, but a
Bus Resolution Function is then required to determine what values
the signal will assume. Again, this issue will be covered in the
'Behavioral VHDL' module.
-
LINKAGE --
-
data flow direction is unknown. This mode indicates
only that a connection exists.
