Manual LPCI-COM-8SM
17
Address Map
The core of the UART function is supplied by the EXAR XR16L788 chip.
This chip is 16550 and 16750 compatible, but has extra features that require communicating
with an additional 8 registers per channel. For instance, it is necessary to set the “Auto RTS”
function. (EXAR refers to this function as “Auto RS485" in their literature). The required writing
to the UART is done automatically when the card is rebooted.
In order to utilize standard software that communicates only with the first 8 registers of each
channel, addresses are remapped on the card.
The 8 UARTS occupy the first 64 addresses.
The interrupt status register is located at base + 40h.
Baud Rates
On the card, the UART clock frequency is 1.8432 MHz. Below is a table of popular divisor
frequencies.
Baud Rate Divisor x1 Divisor x8
Max Diff. Cable Length*
921600 -
1
250
ft
460800 -
2
550
ft
230400 -
4
1400
ft
153600 -
6
2500
ft
115200 1
8
3000
ft
57600 2
16
4000
ft
38400 3
24
4000
ft
28800 4
32
4000
ft
19200 6
48
4000
ft
14400 8
64
4000
ft
9600
12
96 – Most Common 4000 ft
4800 24 192
4000
ft
2400 48 384
4000
ft
1200 96 768
4000
ft
*These are theoretical maximums based on typical conditions and good quality cables, based
on the EIA 485 and EIA 422 standard for balanced differential drivers. The maximum cable
length allowable for RS-232 communications is 50 feet due to single-ended line driver signaling.
Table 5-1:
Baud Rate Divisor Values
