GE PMC423 Series Hardware Reference Manual page 18

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.
18 PMC423 Port Serial, Front/Rear I/O Hardware Reference Manual