18 PMC423 Port Serial, Front/Rear I/O Hardware Reference Manual
The
PMC423
is
a
flexible
solution
for
integrating
multiple
serial
I/O
channels
onto
a
Single
Board
Computer
(SBC).
Efficiently
packaging
sixteen
channels
onto
a
single
PMC
makes
effective
use
of
the
few
PMC
locations
available.
Routing
I/O
to
the
rear
(via
the
‘J4’
connector
to
the
host
card)
enhances
flexibility
in
developing
economical,
and
non
‐
intrusive
cabling
solutions.
Each
serial
port
is
implemented
with
an
independent
UART
(Asynchronous
Receiver
and
Transmitter)
with
a
separate
programmable
baud
rate.
Accessing
the
UART
controllers
via
PCI,
integrates
more
effectively
to
modern
SBC
implementations
than
proprietary
or
legacy
devices.
Traditional
UART
controllers
are
limited
to
8
‐
bit
access,
incurring
all
bus
protocol
overhead
for
every
character
transferred
(and
control
operations).
The
Octal
UART
utilized
on
the
PMC423
supports
32
‐
bit
transfers,
allowing
four
characters
to
be
transferred
in
a
single
bus
operation.
Reducing
programmed
I/O
operations
is
critical
to
achieving
good
performance
in
PCI
systems.
While
the
extended
transfer
options
offer
an
opportunity
to
maximize
performance,
in
many
cases,
the
first
(and
sometimes
only)
criteria
are
software
development
and
integration.
The
16C550
register
set
definition
is
in
common
usage
for
serial
I/O
devices
in
all
environments.
Maintaining
compatibility
with
this
de
facto
standard
can
markedly
reduce
the
time
required
to
integrate
the
PMC423
into
existing
software
environments.
Having
the
software
integration
based
upon
earlier
drives
reduces
the
risk
of
unexpected
(and
undesirable)
changes
in
application
interfaces,
which
can
be
critical
when
creating
an
evolutionary
improvement
of
existing
systems.
