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Path: utzoo!linus!decvax!genrad!panda!talcott!harvard!seismo!brl-tgr!tgr!brad@DCA-EUR.ARPA
From: brad@DCA-EUR.ARPA
Newsgroups: net.micro.cpm
Subject: 8251 USART
Message-ID: <1174@brl-tgr.ARPA>
Date: Thu, 2-Jan-86 09:42:30 EST
Article-I.D.: brl-tgr.1174
Posted: Thu Jan  2 09:42:30 1986
Date-Received: Fri, 3-Jan-86 05:24:38 EST
Sender: news@brl-tgr.ARPA
Lines: 21

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