Advantech ADAM-5511 Manual page 298

This appendix gives a short description of each module's registers. For
more information, please refer to the STARTECH 16C550 UART chip
data book.
All registers are one byte. Bit 0 is the least significant bit, and bit 7 is
the most significant bit. The address of each register is specified as an
offset from the port base address (BASE), COM1 is 3F8h and COM2
is 2F8h.
DLAB is the "Divisor Latch Access Bit", bit 7 of BASE+3.
BASE+0 Receiver buffer register when DLAB=0 and the operation
is a read.
BASE+0 Transmitter holding register when DLAB=0 and the
operation is write.
BASE+0 Divisor latch bits 0 - 7 when DLAB=1
BASE+1 Divisor latch bits 8-15 when DLAB=1.
Bytes BASE+0 and BASE+1 together form a 16-bit number, the divisor,
which determines the baud rate. Set the divisor as follows: