Date   : Thu, 02 Jan 1986 14:11:54 GMT
From   : brad@DCA-EUR.ARPA
Subject: 8251 USART

It has been 8 years since I worked on the BIOS for my
NorthStar Horizon.  I just dug out the code and found that
you have to send the 8251 both a Mode Word to tell it either
Async or Sync and a Command word.  The code for my Horizon
was:
* Send Mode Word for Asynchronous Mode
*
       MVI     A,0CEH  ; 2 Stops, 16XClock, 8 Bits, No Parity
       OUT     3       ; Send to 8251 (This different from Data Port)

* Set up Control Word for 8251
*      
* RTS = Request To Send, ER = Error Reset
* RXE = Receive Enable, DTR = Data Term Ready
* TXEN = Transmit Enable
*
       MVI     A,37H   ; CMD: RTS, ER, RXE, DTR, TXEN
       OUT     3       ; Output to 8251

Hope this helps.
Brad