From: utzoo!decvax!cca!rconn@BRL@sri-unix Newsgroups: net.micro Title: Re: 6850 SIO ACIA Article-I.D.: sri-unix.3293 Posted: Wed Sep 15 21:08:31 1982 Received: Thu Sep 16 03:02:08 1982 From: Rick Conn"An Introduction to Microcomputers" Vol 2 and 3 by Os- borne is a very good source for answers to questions such as yours. I can really recommend it; among other things, it covers the 8080, 8085, Z80, 6800, 6502, and other microprocessor chips AND the support chips (6850 for ex) that can be used with these microprocessors. The answer to your question is in Vol 2, page 9-59: 6850 ACIA control register bits are (bit 7=high-order, bit 0=low ord- er): Bit 7: 0=disable receive interrupt logic 1=enable receive interrupt logic Bits 6&5: 00=-RTS low, disable xmit interrupt logic 01=-RTS low, enable xmit int logic 10=-RTS high, disable xmit int logic 11=-RTS high, enable xmit int logic Bits 43&2: 000=7 data bits, even parity, 2 stop bits 001=7,odd,2stop 010=7,even,1stop 011=7,odd,1stop 100=8,no parity,2stop 101=8,no,1stop 110=8,even,1stop 111=8,odd,1stop Bits 1&0: 00=/1 clock 01=/16 clock 10=/64 clock 11=Master Reset So, your sequence of 3H followed by 91H is: 03H = Master Reset 91H = Enable Recv Int Logic, -RTS Low & Disable Xmit Int Logic, 8 Data Bits & No Parity & 2 Stop Bits, /16 Clock Hope this helps. Rick