Publication No. RX-URMH 006 Rev. E.0
Theory of Operation 19
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.
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