Date   : Thu, 02 Jan 1986 20:56:28 GMT
From   : Tom Gillispie <tom%pedsgo.uucp@BRL.ARPA>
Subject: Re: Help needed with 8251 USART

Organization : Concurrent Computer Corp. (a P-E company), Tinton Falls, NJ
Keywords: 


I have shared your frustration with programming the 8251.  Maybe I can help
some.  First, you must 'kick' the 8251 into a mode where it will recognize
and accept the control byte.  If the board you are using has a hardware
reset facility of some type, that may work.  There is a pin on the 8251
that will physically reset the chip, and put it in a mode where it is
waiting for the control byte.  If your board doesnt have that (many dont),
you must send the 8251 several 'invalid' control bytes, which will get it
into a state where it is expecting the control byte.  I have found that
outputting several bytes of 0 will work.  I have seen some code use 2
zeros, and some use 3 zeros, it may not matter.  Once that has been done,
you set up the chip the way it states in the manual.  Your code looks good,
just add the outputting of serveral bytes of zero to the command port
first. 

Second, I have found several things with the 8251.  Some of the older
ceramic-body 8251 chips are not to be trusted.  I was told and found to be
true that you should stick to the plastic parts.  My ceramic 8251s were at
least 5 years old, so maybe newer ceramic parts are ok.  Also, I found that
some of my 8251s would not work correctly (sorry, I cant remember what
wasnt correct) when you ran them with NO PARITY.

Hope this help.

Tom